Adds a new ioctl32 syscall for backwards compatibility layers

From: Ryan Houdek <redacted>

Problem presented:
A backwards compatibility layer that allows running x86-64 and x86
processes inside of an AArch64 process.
  - CPU is emulated
  - Syscall interface is mostly passthrough
  - Some syscalls require patching or emulation depending on behaviour
  - Not viable from the emulator design to use an AArch32 host process

x86-64 and x86 userspace emulator source:
https://github.com/FEX-Emu/FEX
Usage of ioctl32 is currently in a downstream fork. This will be the
first user of the syscall.

Cross documentation:
https://github.com/FEX-Emu/FEX/wiki/32Bit-x86-Woes#ioctl---54

ioctls are opaque from the emulator perspective and the data wants to be
passed through a syscall as unimpeded as possible.
Sadly due to ioctl struct differences between x86 and x86-64, we need a
syscall that exposes the compatibility ioctl handler to userspace in a
64bit process.

This is necessary behaves of the behaviour differences that occur
between an x86 process doing an ioctl and an x86-64 process doing an
ioctl.

Both of which are captured and passed through the AArch64 ioctl space.
This is implementing a new ioctl32 syscall that allows us to pass 32bit
x86 ioctls through to the kernel with zero or minimal manipulation.

The only supported hosts where we care about this currently is AArch64
and x86-64 (For testing purposes).
PPC64LE, MIPS64LE, and RISC-V64 might be interesting to support in the
future; But I don't have any platforms that get anywhere near Cortex-A77
performance in those architectures. Nor do I have the time to bring up
the emulator on them.
x86-64 can get to the compatibility ioctl through the int $0x80 handler.

66 more lines

diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 86a9d7b3eabe..949788f5ba40 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h

@@ -38,7 +38,7 @@
 #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
 #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
-#define __NR_compat_syscalls		442
+#define __NR_compat_syscalls		443
 #endif
 
 #define __ARCH_WANT_SYS_CLONE

diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index cccfbbefbf95..35e3bc83dbdc 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h

@@ -891,6 +891,8 @@ __SYSCALL(__NR_faccessat2, sys_faccessat2)
 __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SYSCALL(__NR_epoll_pwait2, compat_sys_epoll_pwait2)
+#define __NR_ioctl32 442
+__SYSCALL(__NR_ioctl32, compat_sys_ioctl)
 
 /*
  * Please add new compat syscalls above this comment and update

diff --git a/fs/ioctl.c b/fs/ioctl.c
index 4e6cc0a7d69c..116b9bca8c07 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c

@@ -790,8 +790,8 @@ long compat_ptr_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 }
 EXPORT_SYMBOL(compat_ptr_ioctl);
 
-COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
-		       compat_ulong_t, arg)
+long do_ioctl32(unsigned int fd, unsigned int cmd,
+			compat_ulong_t arg)
 {
 	struct fd f = fdget(fd);
 	int error;

@@ -850,4 +850,16 @@ COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
 
 	return error;
 }
+
+COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
+			compat_ulong_t, arg)
+{
+	return do_ioctl32(fd, cmd, arg);
+}
+
+SYSCALL_DEFINE3(ioctl32, unsigned int, fd, unsigned int, cmd,
+			compat_ulong_t, arg)
+{
+	return do_ioctl32(fd, cmd, arg);
+}
 #endif

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index f3929aff39cf..470f928831eb 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -386,6 +386,8 @@ asmlinkage long sys_inotify_rm_watch(int fd, __s32 wd);
 /* fs/ioctl.c */
 asmlinkage long sys_ioctl(unsigned int fd, unsigned int cmd,
 				unsigned long arg);
+asmlinkage long sys_ioctl32(unsigned int fd, unsigned int cmd,
+				compat_ulong_t arg);
 
 /* fs/ioprio.c */
 asmlinkage long sys_ioprio_set(int which, int who, int ioprio);

diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 728752917785..18279e5b7b4f 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h

@@ -862,8 +862,15 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)
 
+#define __NR_ioctl32 442
+#ifdef CONFIG_COMPAT
+__SC_COMP(__NR_ioctl32, sys_ioctl32, compat_sys_ioctl)
+#else
+__SC_COMP(__NR_ioctl32, sys_ni_syscall, sys_ni_syscall)
+#endif
+
 #undef __NR_syscalls
-#define __NR_syscalls 442
+#define __NR_syscalls 443
 
 /*
  * 32 bit systems traditionally used different

diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 19aa806890d5..5a2f25eb341c 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c

@@ -302,6 +302,9 @@ COND_SYSCALL(recvmmsg_time32);
 COND_SYSCALL_COMPAT(recvmmsg_time32);
 COND_SYSCALL_COMPAT(recvmmsg_time64);
 
+COND_SYSCALL(ioctl32);
+COND_SYSCALL_COMPAT(ioctl32);
+
 /*
  * Architecture specific syscalls: see further below
  */

diff --git a/tools/include/uapi/asm-generic/unistd.h b/tools/include/uapi/asm-generic/unistd.h
index 728752917785..18279e5b7b4f 100644
--- a/tools/include/uapi/asm-generic/unistd.h
+++ b/tools/include/uapi/asm-generic/unistd.h

@@ -862,8 +862,15 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)
 
+#define __NR_ioctl32 442
+#ifdef CONFIG_COMPAT
+__SC_COMP(__NR_ioctl32, sys_ioctl32, compat_sys_ioctl)
+#else
+__SC_COMP(__NR_ioctl32, sys_ni_syscall, sys_ni_syscall)
+#endif
+
 #undef __NR_syscalls
-#define __NR_syscalls 442
+#define __NR_syscalls 443
 
 /*
  * 32 bit systems traditionally used different

-- 
2.27.0

On Wed, Jan 6, 2021 at 7:48 AM [off-list ref] wrote:

From: Ryan Houdek <redacted>

...

This does not solve the following problems:
1) compat_alloc_user_space inside ioctl
2) ioctls that check task mode instead of entry point for behaviour
3) ioctls allocating memory
4) struct packing problems between architectures

Workarounds for the problems presented:
1a) Do a stack pivot to the lower 32bits from userspace
  - Forces host 64bit process to have its thread stacks to live in 32bit
  space. Not ideal.
  - Only do a stack pivot on ioctl to save previous 32bit VA space
1b) Teach kernel that compat_alloc_userspace can return a 64bit pointer
  - x86-64 truncates stack from this function
  - AArch64 returns the full stack pointer
  - Only ~29 users. Validating all of them support a 64bit stack is
  trivial?

I've almost completed the removal of compat_alloc_user_space(),
that should no longer be a concern when the syscall gets added.

2a) Any application using these can be checked for compatibility in
userspace and put on a block list.
2b) Fix any ioctls doing broken behaviour based on task mode rather than
ioctl entry point

What the ioctls() actually check is 'in_compat_syscall()', which is not
the mode of the task but the type of syscall. There is actually a general
trend to use this helper more rather than less, and I think the only
way forward here is to ensure that this returns true when entering
through the new syscall number.

For x86, this has another complication, as some ioctls also need to
check whether they are in an ia32 task (with packed u64 and 32-bit
__kernel_old_time_t) or an x32 task (with aligned u64 and 64-bit
__kernel_old_time_t). If the new syscall gets wired up on x86 as well,
you'd need to decide which of the two behaviors you want.

3a) Userspace consumes all VA space above 32bit. Forcing allocations to
occur in lower 32bits
  - This is the current implementation
3b) Ensure any allocation in the ioctl handles ioctl entrypoint rather
than just allow generic memory allocations in full VA space
  - This is hard to guarantee

What kind of allocation do you mean here? Can you give an example of
an ioctl that does this?

4a) Blocklist any application using ioctls that have different struct
packing across the boundary
  - Can happen when struct packing of 32bit x86 application goes down
  the aarch64 compat_ioctl path
  - Userspace is a AArch64 process passing 32bit x86 ioctl structures
  through the compat_ioctl path which is typically for AArch32 processes
  - None currently identified
4b) Work with upstream kernel and userspace projects to evaluate and fix
  - Identify the problem ioctls
  - Implement a new ioctl with more sane struct packing that matches
  cross-arch
  - Implement new ioctl while maintaining backwards compatibility with
  previous ioctl handler
  - Change upstream project to use the new compatibility ioctl
  - ioctl deprecation will be case by case per device and project
4b) Userspace implements a full ioctl emulation layer
  - Parses the full ioctl tree
  - Either passes through ioctls that it doesn't understand or
  transforms ioctls that it knows are trouble
  - Has the downside that it can still run in to edge cases that will
  fail
  - Performance of additional tracking is a concern
  - Prone to failure keeping the kernel ioctl and userspace ioctl
  handling in sync
  - Really want to have it in the kernel space as much as possible

I think there are only a few ioctls that are affected, and you can
probably get a list from qemu, which emulates them in user space
already. Doing that transformation should not be all that hard
in the end.

If we want to do this in the kernel, this probably requires changes
to the syscall calling convention. Adding a flag to pick a particular
style of ioctl arguments would work, and this could enable the
case of emulating arm32 ioctls on x86-64 hosts.

I'm not sure why you want this in 32-bit processes, can't they just call
the normal ioctl() function?

These two look identical to me, I don't think you need to add a wrapper
here at all, but can just use the normal compat_sys_ioctl entry point
unless you want to add a 'flags' argument to control the struct padding.

(already mentioned on IRC)

If you add it here, the same number should be assigned across all architectures,
or at least a comment added to declare the number as reserved, to keep
the following syscalls in sync.

        Arnd

I encountered a similar problem when writing the Tango binary
translator (https://www.amanieusystems.com/). Tango allows AArch32
programs to run on AArch64 CPUs that don't support AArch32 (e.g.
ThunderX). The technology has been licensed to several customers who
are primarily using it to run Android instances on cloud servers.

Doing compat system call translation entirely in user doesn't work out
for several reasons:
- As Ryan stated, it is impractical and error-prone to manually
translate all ioctls in user space. It would be much better to reuse
the existing compat layer in the kernel. This is even worse if you
take out-of-tree drivers (which have their own ioctls) into account.
- There are quite a few system calls which create VM mappings: the
usual suspects (mmap, etc) but also io_setup and some ioctls (e.g. in
the out-of-tree Mali GPU drivers).
- Seccomp filters simply don't work. They could be emulated in user
space but this is insecure since the translator is not designed to be
a secure sandbox. They also don't propagate across execve.
- Some syscalls rely on information that is only known to the kernel.
For example, a hugetlbfs fd can only be mapped at a huge page
boundary, but the translator cannot know this when selecting a virtual
address in the low 4GB for the mapping.

The solution that we ended up with was to allow AArch64 processes to
issue AArch32 syscalls by using a special system call number. This has
several effects for the duration of the syscall:
- The compat syscall table is used instead of the primary one.
- is_compat_task/in_compat_syscall return true.
- get_unmapped_area returns addresses below 4G and uses a separate
mmap_base. The separate mmap_base is needed to allow 32-bit
applications to benefit from address space randomization. Tango still
uses normal mmap syscalls for private memory allocations that are not
visible to the translated 32-bit process.
- KSTK_EIP and KSTK_ESP return the values of x13/x15 instead of pc/sp.
This is necessary for correct functioning seccomp filters and SELinux
checks respectively. Tango will set x13 and x15 to the correct values
when issuing a 32-bit syscall.
- System call restart after a signal sets things up so that
restart_syscall is executed as a 32-bit syscall after the signal.

15 more lines

From: Ryan Houdek <redacted>

Problem presented:
A backwards compatibility layer that allows running x86-64 and x86
processes inside of an AArch64 process.
  - CPU is emulated
  - Syscall interface is mostly passthrough
  - Some syscalls require patching or emulation depending on behaviour
  - Not viable from the emulator design to use an AArch32 host process

x86-64 and x86 userspace emulator source:
https://github.com/FEX-Emu/FEX
Usage of ioctl32 is currently in a downstream fork. This will be the
first user of the syscall.

Cross documentation:
https://github.com/FEX-Emu/FEX/wiki/32Bit-x86-Woes#ioctl---54

ioctls are opaque from the emulator perspective and the data wants to be
passed through a syscall as unimpeded as possible.
Sadly due to ioctl struct differences between x86 and x86-64, we need a
syscall that exposes the compatibility ioctl handler to userspace in a
64bit process.

This is necessary behaves of the behaviour differences that occur
between an x86 process doing an ioctl and an x86-64 process doing an
ioctl.

Both of which are captured and passed through the AArch64 ioctl space.
This is implementing a new ioctl32 syscall that allows us to pass 32bit
x86 ioctls through to the kernel with zero or minimal manipulation.

The only supported hosts where we care about this currently is AArch64
and x86-64 (For testing purposes).
PPC64LE, MIPS64LE, and RISC-V64 might be interesting to support in the
future; But I don't have any platforms that get anywhere near Cortex-A77
performance in those architectures. Nor do I have the time to bring up
the emulator on them.
x86-64 can get to the compatibility ioctl through the int $0x80 handler.

197 more quoted lines

On Wed, Jan 6, 2021 at 12:49 AM Arnd Bergmann [off-list ref] wrote:

On Wed, Jan 6, 2021 at 7:48 AM [off-list ref] wrote:

quoted

From: Ryan Houdek <redacted>

...

quoted

This does not solve the following problems:
1) compat_alloc_user_space inside ioctl
2) ioctls that check task mode instead of entry point for behaviour
3) ioctls allocating memory
4) struct packing problems between architectures

Workarounds for the problems presented:
1a) Do a stack pivot to the lower 32bits from userspace
  - Forces host 64bit process to have its thread stacks to live in 32bit
  space. Not ideal.
  - Only do a stack pivot on ioctl to save previous 32bit VA space
1b) Teach kernel that compat_alloc_userspace can return a 64bit pointer
  - x86-64 truncates stack from this function
  - AArch64 returns the full stack pointer
  - Only ~29 users. Validating all of them support a 64bit stack is
  trivial?

I've almost completed the removal of compat_alloc_user_space(),
that should no longer be a concern when the syscall gets added.

quoted

2a) Any application using these can be checked for compatibility in
userspace and put on a block list.
2b) Fix any ioctls doing broken behaviour based on task mode rather than
ioctl entry point

What the ioctls() actually check is 'in_compat_syscall()', which is not
the mode of the task but the type of syscall. There is actually a general
trend to use this helper more rather than less, and I think the only
way forward here is to ensure that this returns true when entering
through the new syscall number.

For x86, this has another complication, as some ioctls also need to
check whether they are in an ia32 task (with packed u64 and 32-bit
__kernel_old_time_t) or an x32 task (with aligned u64 and 64-bit
__kernel_old_time_t). If the new syscall gets wired up on x86 as well,
you'd need to decide which of the two behaviors you want.

I can have a follow-up patch that makes this do ni_syscall on x86_64
since we can go through the int 0x80 handler, or x32 handler path and
choose whichever one there.

quoted

3a) Userspace consumes all VA space above 32bit. Forcing allocations to
occur in lower 32bits
  - This is the current implementation
3b) Ensure any allocation in the ioctl handles ioctl entrypoint rather
than just allow generic memory allocations in full VA space
  - This is hard to guarantee

What kind of allocation do you mean here? Can you give an example of
an ioctl that does this?

My concern here would be something like DRM allocating memory and
returning a pointer to userspace that ends up in 64bit space.
I can see something like `drm_get_unmapped_area` calls in to
`current->mm->get_unmapped_area` which I believe only ends up falling
down TASK_SIZE checks.
Which could potentially return pointers in the 64bit address space
range in this case. Theoretically can be resolved either by thieving
the full 64bit VA range, or doing something like the Tango layer
patches that on syscall entry changes the syscall to a "compat"
syscall.
compat syscall flag like Tango might be nicer here?

quoted

4a) Blocklist any application using ioctls that have different struct
packing across the boundary
  - Can happen when struct packing of 32bit x86 application goes down
  the aarch64 compat_ioctl path
  - Userspace is a AArch64 process passing 32bit x86 ioctl structures
  through the compat_ioctl path which is typically for AArch32 processes
  - None currently identified
4b) Work with upstream kernel and userspace projects to evaluate and fix
  - Identify the problem ioctls
  - Implement a new ioctl with more sane struct packing that matches
  cross-arch
  - Implement new ioctl while maintaining backwards compatibility with
  previous ioctl handler
  - Change upstream project to use the new compatibility ioctl
  - ioctl deprecation will be case by case per device and project
4b) Userspace implements a full ioctl emulation layer
  - Parses the full ioctl tree
  - Either passes through ioctls that it doesn't understand or
  transforms ioctls that it knows are trouble
  - Has the downside that it can still run in to edge cases that will
  fail
  - Performance of additional tracking is a concern
  - Prone to failure keeping the kernel ioctl and userspace ioctl
  handling in sync
  - Really want to have it in the kernel space as much as possible

I think there are only a few ioctls that are affected, and you can
probably get a list from qemu, which emulates them in user space
already. Doing that transformation should not be all that hard
in the end.

If we want to do this in the kernel, this probably requires changes
to the syscall calling convention. Adding a flag to pick a particular
style of ioctl arguments would work, and this could enable the
case of emulating arm32 ioctls on x86-64 hosts.

quoted

diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 86a9d7b3eabe..949788f5ba40 100644
--- a/arch/arm64/include/asm/unistd.h

25 more quoted lines

I'll have a follow up patch that on 32bit hosts this is a ni_syscall instead

quoted

 }
+
+COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
+                       compat_ulong_t, arg)
+{
+       return do_ioctl32(fd, cmd, arg);
+}
+
+SYSCALL_DEFINE3(ioctl32, unsigned int, fd, unsigned int, cmd,
+                       compat_ulong_t, arg)
+{
+       return do_ioctl32(fd, cmd, arg);
+}

These two look identical to me, I don't think you need to add a wrapper
here at all, but can just use the normal compat_sys_ioctl entry point
unless you want to add a 'flags' argument to control the struct padding.

I tried having the dispatch table call directly in to the COMPAT one
and the way things were lining up weren't allowing me to do this.
Since this is a bit unique in how it operates, I'm not quite sure if
there is another example I could pull from for this.

quoted

diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 728752917785..18279e5b7b4f 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h

@@ -862,8 +862,15 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)

+#define __NR_ioctl32 442
+#ifdef CONFIG_COMPAT
+__SC_COMP(__NR_ioctl32, sys_ioctl32, compat_sys_ioctl)
+#else
+__SC_COMP(__NR_ioctl32, sys_ni_syscall, sys_ni_syscall)
+#endif
+
 #undef __NR_syscalls
-#define __NR_syscalls 442
+#define __NR_syscalls 443

(already mentioned on IRC)

If you add it here, the same number should be assigned across all architectures,
or at least a comment added to declare the number as reserved, to keep
the following syscalls in sync.

I can have a followup patch that adds this to all the backends.

        Arnd

From: Ryan Houdek <redacted>

Problem presented:
A backwards compatibility layer that allows running x86-64 and x86
processes inside of an AArch64 process.
  - CPU is emulated
  - Syscall interface is mostly passthrough
  - Some syscalls require patching or emulation depending on behaviour
  - Not viable from the emulator design to use an AArch32 host process

x86-64 and x86 userspace emulator source:
https://github.com/FEX-Emu/FEX
Usage of ioctl32 is currently in a downstream fork. This will be the
first user of the syscall.

Cross documentation:
https://github.com/FEX-Emu/FEX/wiki/32Bit-x86-Woes#ioctl---54

ioctls are opaque from the emulator perspective and the data wants to be
passed through a syscall as unimpeded as possible.
Sadly due to ioctl struct differences between x86 and x86-64, we need a
syscall that exposes the compatibility ioctl handler to userspace in a
64bit process.

This is necessary behaves of the behaviour differences that occur
between an x86 process doing an ioctl and an x86-64 process doing an
ioctl.

Both of which are captured and passed through the AArch64 ioctl space.
This is implementing a new ioctl32 syscall that allows us to pass 32bit
x86 ioctls through to the kernel with zero or minimal manipulation.

The only supported hosts where we care about this currently is AArch64
and x86-64 (For testing purposes).
PPC64LE, MIPS64LE, and RISC-V64 might be interesting to support in the
future; But I don't have any platforms that get anywhere near Cortex-A77
performance in those architectures. Nor do I have the time to bring up
the emulator on them.
x86-64 can get to the compatibility ioctl through the int $0x80 handler.

82 more lines

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index a6617067dbe6..81e70fd241d7 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl

@@ -481,3 +481,4 @@
 549	common	faccessat2			sys_faccessat2
 550	common	process_madvise			sys_process_madvise
 551	common	epoll_pwait2			sys_epoll_pwait2
+552	common	ioctl32			sys_ni_syscall

diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index 20e1170e2e0a..98fbf1af1169 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl

@@ -455,3 +455,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 86a9d7b3eabe..949788f5ba40 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h

@@ -38,7 +38,7 @@
 #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
 #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
-#define __NR_compat_syscalls		442
+#define __NR_compat_syscalls		443
 #endif
 
 #define __ARCH_WANT_SYS_CLONE

diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index cccfbbefbf95..35e3bc83dbdc 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h

@@ -891,6 +891,8 @@ __SYSCALL(__NR_faccessat2, sys_faccessat2)
 __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SYSCALL(__NR_epoll_pwait2, compat_sys_epoll_pwait2)
+#define __NR_ioctl32 442
+__SYSCALL(__NR_ioctl32, compat_sys_ioctl)
 
 /*
  * Please add new compat syscalls above this comment and update

diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
index bfc00f2bd437..087fc9627357 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl

@@ -362,3 +362,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	sys_ioctl32			sys_ioctl32

diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index 7fe4e45c864c..502b2f87ab60 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl

@@ -441,3 +441,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index a522adf194ab..e69be6c836d2 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl

@@ -447,3 +447,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index 0f03ad223f33..ba395218446f 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl

@@ -380,3 +380,4 @@
 439	n32	faccessat2			sys_faccessat2
 440	n32	process_madvise			sys_process_madvise
 441	n32	epoll_pwait2			compat_sys_epoll_pwait2
+442	n32	ioctl32			sys_ni_syscall

diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 91649690b52f..f42f939702e2 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl

@@ -356,3 +356,5 @@
 439	n64	faccessat2			sys_faccessat2
 440	n64	process_madvise			sys_process_madvise
 441	n64	epoll_pwait2			sys_epoll_pwait2
+441	n64	epoll_pwait2			sys_epoll_pwait2
+442	n64	ioctl32			sys_ioctl32

diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 4bad0c40aed6..b08ff6066f06 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl

@@ -429,3 +429,4 @@
 439	o32	faccessat2			sys_faccessat2
 440	o32	process_madvise			sys_process_madvise
 441	o32	epoll_pwait2			sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	o32	ioctl32			sys_ni_syscall

diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index 6bcc31966b44..84d2b88d92fa 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl

@@ -439,3 +439,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	64	ioctl32			sys_ioctl32

diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index f744eb5cba88..9f04d73cf649 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl

@@ -531,3 +531,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	64	sys_ioctl32				sys_ioctl32

diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index d443423495e5..2c90c0ecb5c7 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl

@@ -444,3 +444,4 @@
 439  common	faccessat2		sys_faccessat2			sys_faccessat2
 440  common	process_madvise		sys_process_madvise		sys_process_madvise
 441  common	epoll_pwait2		sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	64	sys_ioctl32			sys_ni_syscall

diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index 9df40ac0ebc0..1e02a13fa049 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl

@@ -444,3 +444,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index 40d8c7cd8298..f7d24678d0b1 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl

@@ -487,3 +487,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	64	sys_ioctl32			sys_ioctl32

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 874aeacde2dd..b1a3461e1e20 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl

@@ -446,3 +446,4 @@
 439	i386	faccessat2		sys_faccessat2
 440	i386	process_madvise		sys_process_madvise
 441	i386	epoll_pwait2		sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	i386	ioctl32		sys_ni_syscall

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 78672124d28b..0250a04df0df 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl

@@ -363,6 +363,7 @@
 439	common	faccessat2		sys_faccessat2
 440	common	process_madvise		sys_process_madvise
 441	common	epoll_pwait2		sys_epoll_pwait2
+442	64	ioctl32		sys_ioctl32
 
 #
 # Due to a historical design error, certain syscalls are numbered differently

diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 46116a28eeed..34b653b36b7b 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl

@@ -412,3 +412,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/fs/ioctl.c b/fs/ioctl.c
index 4e6cc0a7d69c..7b324a21a257 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c

@@ -790,8 +790,8 @@ long compat_ptr_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 }
 EXPORT_SYMBOL(compat_ptr_ioctl);
 
-COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
-		       compat_ulong_t, arg)
+long do_ioctl32(unsigned int fd, unsigned int cmd,
+			compat_ulong_t arg)
 {
 	struct fd f = fdget(fd);
 	int error;

@@ -850,4 +850,18 @@ COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
 
 	return error;
 }
+
+COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
+			compat_ulong_t, arg)
+{
+	return do_ioctl32(fd, cmd, arg);
+}
+
+#if BITS_PER_LONG == 64
+SYSCALL_DEFINE3(ioctl32, unsigned int, fd, unsigned int, cmd,
+			compat_ulong_t, arg)
+{
+	return do_ioctl32(fd, cmd, arg);
+}
+#endif
 #endif

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index f3929aff39cf..fb7bac17167a 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -386,6 +386,10 @@ asmlinkage long sys_inotify_rm_watch(int fd, __s32 wd);
 /* fs/ioctl.c */
 asmlinkage long sys_ioctl(unsigned int fd, unsigned int cmd,
 				unsigned long arg);
+#if defined(CONFIG_COMPAT) && BITS_PER_LONG == 64
+asmlinkage long sys_ioctl32(unsigned int fd, unsigned int cmd,
+				compat_ulong_t arg);
+#endif
 
 /* fs/ioprio.c */
 asmlinkage long sys_ioprio_set(int which, int who, int ioprio);

diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 728752917785..18279e5b7b4f 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h

@@ -862,8 +862,15 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)
 
+#define __NR_ioctl32 442
+#ifdef CONFIG_COMPAT
+__SC_COMP(__NR_ioctl32, sys_ioctl32, compat_sys_ioctl)
+#else
+__SC_COMP(__NR_ioctl32, sys_ni_syscall, sys_ni_syscall)
+#endif
+
 #undef __NR_syscalls
-#define __NR_syscalls 442
+#define __NR_syscalls 443
 
 /*
  * 32 bit systems traditionally used different

diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 19aa806890d5..5a2f25eb341c 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c

@@ -302,6 +302,9 @@ COND_SYSCALL(recvmmsg_time32);
 COND_SYSCALL_COMPAT(recvmmsg_time32);
 COND_SYSCALL_COMPAT(recvmmsg_time64);
 
+COND_SYSCALL(ioctl32);
+COND_SYSCALL_COMPAT(ioctl32);
+
 /*
  * Architecture specific syscalls: see further below
  */

diff --git a/tools/include/uapi/asm-generic/unistd.h b/tools/include/uapi/asm-generic/unistd.h
index 728752917785..18279e5b7b4f 100644
--- a/tools/include/uapi/asm-generic/unistd.h
+++ b/tools/include/uapi/asm-generic/unistd.h

@@ -862,8 +862,15 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)
 
+#define __NR_ioctl32 442
+#ifdef CONFIG_COMPAT
+__SC_COMP(__NR_ioctl32, sys_ioctl32, compat_sys_ioctl)
+#else
+__SC_COMP(__NR_ioctl32, sys_ni_syscall, sys_ni_syscall)
+#endif
+
 #undef __NR_syscalls
-#define __NR_syscalls 442
+#define __NR_syscalls 443
 
 /*
  * 32 bit systems traditionally used different

-- 
2.27.0

From: Ryan Houdek <redacted>

Problem presented:
A backwards compatibility layer that allows running x86-64 and x86
processes inside of an AArch64 process.
  - CPU is emulated
  - Syscall interface is mostly passthrough
  - Some syscalls require patching or emulation depending on behaviour
  - Not viable from the emulator design to use an AArch32 host process

x86-64 and x86 userspace emulator source:
https://github.com/FEX-Emu/FEX
Usage of ioctl32 is currently in a downstream fork. This will be the
first user of the syscall.

Cross documentation:
https://github.com/FEX-Emu/FEX/wiki/32Bit-x86-Woes#ioctl---54

ioctls are opaque from the emulator perspective and the data wants to be
passed through a syscall as unimpeded as possible.
Sadly due to ioctl struct differences between x86 and x86-64, we need a
syscall that exposes the compatibility ioctl handler to userspace in a
64bit process.

This is necessary behaves of the behaviour differences that occur
between an x86 process doing an ioctl and an x86-64 process doing an
ioctl.

Both of which are captured and passed through the AArch64 ioctl space.
This is implementing a new ioctl32 syscall that allows us to pass 32bit
x86 ioctls through to the kernel with zero or minimal manipulation.

The only supported hosts where we care about this currently is AArch64
and x86-64 (For testing purposes).
PPC64LE, MIPS64LE, and RISC-V64 might be interesting to support in the
future; But I don't have any platforms that get anywhere near Cortex-A77
performance in those architectures. Nor do I have the time to bring up
the emulator on them.
x86-64 can get to the compatibility ioctl through the int $0x80 handler.

88 more lines

diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index a6617067dbe6..81e70fd241d7 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl

@@ -481,3 +481,4 @@
 549	common	faccessat2			sys_faccessat2
 550	common	process_madvise			sys_process_madvise
 551	common	epoll_pwait2			sys_epoll_pwait2
+552	common	ioctl32			sys_ni_syscall

diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index 20e1170e2e0a..98fbf1af1169 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl

@@ -455,3 +455,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 86a9d7b3eabe..949788f5ba40 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h

@@ -38,7 +38,7 @@
 #define __ARM_NR_compat_set_tls		(__ARM_NR_COMPAT_BASE + 5)
 #define __ARM_NR_COMPAT_END		(__ARM_NR_COMPAT_BASE + 0x800)
 
-#define __NR_compat_syscalls		442
+#define __NR_compat_syscalls		443
 #endif
 
 #define __ARCH_WANT_SYS_CLONE

diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index cccfbbefbf95..35e3bc83dbdc 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h

@@ -891,6 +891,8 @@ __SYSCALL(__NR_faccessat2, sys_faccessat2)
 __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SYSCALL(__NR_epoll_pwait2, compat_sys_epoll_pwait2)
+#define __NR_ioctl32 442
+__SYSCALL(__NR_ioctl32, compat_sys_ioctl)
 
 /*
  * Please add new compat syscalls above this comment and update

diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
index bfc00f2bd437..087fc9627357 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl

@@ -362,3 +362,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	sys_ioctl32			sys_ioctl32

diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index 7fe4e45c864c..502b2f87ab60 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl

@@ -441,3 +441,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index a522adf194ab..e69be6c836d2 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl

@@ -447,3 +447,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index 0f03ad223f33..ba395218446f 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl

@@ -380,3 +380,4 @@
 439	n32	faccessat2			sys_faccessat2
 440	n32	process_madvise			sys_process_madvise
 441	n32	epoll_pwait2			compat_sys_epoll_pwait2
+442	n32	ioctl32			sys_ni_syscall

diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 91649690b52f..f42f939702e2 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl

@@ -356,3 +356,5 @@
 439	n64	faccessat2			sys_faccessat2
 440	n64	process_madvise			sys_process_madvise
 441	n64	epoll_pwait2			sys_epoll_pwait2
+441	n64	epoll_pwait2			sys_epoll_pwait2
+442	n64	ioctl32			sys_ioctl32

diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 4bad0c40aed6..b08ff6066f06 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl

@@ -429,3 +429,4 @@
 439	o32	faccessat2			sys_faccessat2
 440	o32	process_madvise			sys_process_madvise
 441	o32	epoll_pwait2			sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	o32	ioctl32			sys_ni_syscall

diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index 6bcc31966b44..84d2b88d92fa 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl

@@ -439,3 +439,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	64	ioctl32			sys_ioctl32

diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index f744eb5cba88..9f04d73cf649 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl

@@ -531,3 +531,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	64	sys_ioctl32				sys_ioctl32

diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index d443423495e5..2c90c0ecb5c7 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl

@@ -444,3 +444,4 @@
 439  common	faccessat2		sys_faccessat2			sys_faccessat2
 440  common	process_madvise		sys_process_madvise		sys_process_madvise
 441  common	epoll_pwait2		sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	64	sys_ioctl32			sys_ni_syscall

diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index 9df40ac0ebc0..1e02a13fa049 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl

@@ -444,3 +444,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index 40d8c7cd8298..f7d24678d0b1 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl

@@ -487,3 +487,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	64	sys_ioctl32			sys_ioctl32

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 874aeacde2dd..b1a3461e1e20 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl

@@ -446,3 +446,4 @@
 439	i386	faccessat2		sys_faccessat2
 440	i386	process_madvise		sys_process_madvise
 441	i386	epoll_pwait2		sys_epoll_pwait2		compat_sys_epoll_pwait2
+442	i386	ioctl32		sys_ni_syscall

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 78672124d28b..0250a04df0df 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl

@@ -363,6 +363,7 @@
 439	common	faccessat2		sys_faccessat2
 440	common	process_madvise		sys_process_madvise
 441	common	epoll_pwait2		sys_epoll_pwait2
+442	64	ioctl32		sys_ioctl32
 
 #
 # Due to a historical design error, certain syscalls are numbered differently

diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 46116a28eeed..34b653b36b7b 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl

@@ -412,3 +412,4 @@
 439	common	faccessat2			sys_faccessat2
 440	common	process_madvise			sys_process_madvise
 441	common	epoll_pwait2			sys_epoll_pwait2
+442	common	ioctl32			sys_ni_syscall

diff --git a/fs/ioctl.c b/fs/ioctl.c
index 4e6cc0a7d69c..7b324a21a257 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c

@@ -790,8 +790,8 @@ long compat_ptr_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 }
 EXPORT_SYMBOL(compat_ptr_ioctl);
 
-COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
-		       compat_ulong_t, arg)
+long do_ioctl32(unsigned int fd, unsigned int cmd,
+			compat_ulong_t arg)
 {
 	struct fd f = fdget(fd);
 	int error;

@@ -850,4 +850,18 @@ COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
 
 	return error;
 }
+
+COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
+			compat_ulong_t, arg)
+{
+	return do_ioctl32(fd, cmd, arg);
+}
+
+#if BITS_PER_LONG == 64
+SYSCALL_DEFINE3(ioctl32, unsigned int, fd, unsigned int, cmd,
+			compat_ulong_t, arg)
+{
+	return do_ioctl32(fd, cmd, arg);
+}
+#endif
 #endif

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index f3929aff39cf..fb7bac17167a 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -386,6 +386,10 @@ asmlinkage long sys_inotify_rm_watch(int fd, __s32 wd);
 /* fs/ioctl.c */
 asmlinkage long sys_ioctl(unsigned int fd, unsigned int cmd,
 				unsigned long arg);
+#if defined(CONFIG_COMPAT) && BITS_PER_LONG == 64
+asmlinkage long sys_ioctl32(unsigned int fd, unsigned int cmd,
+				compat_ulong_t arg);
+#endif
 
 /* fs/ioprio.c */
 asmlinkage long sys_ioprio_set(int which, int who, int ioprio);

diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 728752917785..18279e5b7b4f 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h

@@ -862,8 +862,15 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)
 
+#define __NR_ioctl32 442
+#ifdef CONFIG_COMPAT
+__SC_COMP(__NR_ioctl32, sys_ioctl32, compat_sys_ioctl)
+#else
+__SC_COMP(__NR_ioctl32, sys_ni_syscall, sys_ni_syscall)
+#endif
+
 #undef __NR_syscalls
-#define __NR_syscalls 442
+#define __NR_syscalls 443
 
 /*
  * 32 bit systems traditionally used different

diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 19aa806890d5..5a2f25eb341c 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c

@@ -302,6 +302,9 @@ COND_SYSCALL(recvmmsg_time32);
 COND_SYSCALL_COMPAT(recvmmsg_time32);
 COND_SYSCALL_COMPAT(recvmmsg_time64);
 
+COND_SYSCALL(ioctl32);
+COND_SYSCALL_COMPAT(ioctl32);
+
 /*
  * Architecture specific syscalls: see further below
  */

diff --git a/tools/include/uapi/asm-generic/unistd.h b/tools/include/uapi/asm-generic/unistd.h
index 728752917785..18279e5b7b4f 100644
--- a/tools/include/uapi/asm-generic/unistd.h
+++ b/tools/include/uapi/asm-generic/unistd.h

@@ -862,8 +862,15 @@ __SYSCALL(__NR_process_madvise, sys_process_madvise)
 #define __NR_epoll_pwait2 441
 __SC_COMP(__NR_epoll_pwait2, sys_epoll_pwait2, compat_sys_epoll_pwait2)
 
+#define __NR_ioctl32 442
+#ifdef CONFIG_COMPAT
+__SC_COMP(__NR_ioctl32, sys_ioctl32, compat_sys_ioctl)
+#else
+__SC_COMP(__NR_ioctl32, sys_ni_syscall, sys_ni_syscall)
+#endif
+
 #undef __NR_syscalls
-#define __NR_syscalls 442
+#define __NR_syscalls 443
 
 /*
  * 32 bit systems traditionally used different

-- 
2.27.0

From: <redacted>

Sent: 15 January 2021 07:03
Problem presented:
A backwards compatibility layer that allows running x86-64 and x86
processes inside of an AArch64 process.
  - CPU is emulated
  - Syscall interface is mostly passthrough
  - Some syscalls require patching or emulation depending on behaviour
  - Not viable from the emulator design to use an AArch32 host process

You are going to need to add all the x86 compatibility code into
your arm64 kernel.
This is likely to be different from the 32bit arm compatibility
because 64bit items are only aligned on 32bit boundaries.
The x86 x32 compatibility will be more like the 32bit arm 'compat'
code - I'm pretty sure arm32 64bit aligned 64bit data.

You'll then need to remember how the process entered the kernel
to work out which compatibility code to invoke.
This is what x86 does.
It allows a single process to do all three types of system call.

Trying to 'patch up' structures outside the kernel, or in the
syscall interface code will always cause grief somewhere.
The only sane place is in the code that uses the structures.
Which, for ioctls, means inside the driver that parses them.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

On Fri, Jan 15, 2021 at 9:01 PM David Laight [off-list ref] wrote:

From: <redacted>

quoted

Sent: 15 January 2021 07:03
Problem presented:
A backwards compatibility layer that allows running x86-64 and x86
processes inside of an AArch64 process.
  - CPU is emulated
  - Syscall interface is mostly passthrough
  - Some syscalls require patching or emulation depending on behaviour
  - Not viable from the emulator design to use an AArch32 host process

You are going to need to add all the x86 compatibility code into
your arm64 kernel.
This is likely to be different from the 32bit arm compatibility
because 64bit items are only aligned on 32bit boundaries.
The x86 x32 compatibility will be more like the 32bit arm 'compat'
code - I'm pretty sure arm32 64bit aligned 64bit data.

All other architectures that have both 32-bit and 64-bit variants
use the same alignment for all types, except for x86.

There are additional differences though, especially if one
were to try to generalize the interface to all architectures.
A subset of the issues includes

- x32 has 64-bit types in places of some types that are
  32 bit everywhere else (time_t, ino_t, off_t, clock_t, ...)

- m68k aligns struct members to at most 16 bits

- uid_t/gid_t/ino_t/dev_t/... are

You'll then need to remember how the process entered the kernel
to work out which compatibility code to invoke.
This is what x86 does.
It allows a single process to do all three types of system call.

Trying to 'patch up' structures outside the kernel, or in the
syscall interface code will always cause grief somewhere.
The only sane place is in the code that uses the structures.
Which, for ioctls, means inside the driver that parses them.

He's already doing the system call emulation for all the system
calls other than ioctl in user space though. In my experience,
there are actually fairly few ioctl commands that are different
between architectures -- most of them have no misaligned
or architecture-defined struct members at all.

Once you have conversion functions to deal with the 32/64-bit
interface differences and architecture specifics of sockets,
sysvipc, signals, stat, and input_event, handling the
x86-32 specific ioctl commands is comparably easy.

         Arnd

On Fri, Jan 15, 2021 at 11:17:09PM +0100, Arnd Bergmann wrote:

On Fri, Jan 15, 2021 at 9:01 PM David Laight [off-list ref] wrote:

quoted

From: <redacted>

quoted

Sent: 15 January 2021 07:03
Problem presented:
A backwards compatibility layer that allows running x86-64 and x86
processes inside of an AArch64 process.
  - CPU is emulated
  - Syscall interface is mostly passthrough
  - Some syscalls require patching or emulation depending on behaviour
  - Not viable from the emulator design to use an AArch32 host process

You are going to need to add all the x86 compatibility code into
your arm64 kernel.
This is likely to be different from the 32bit arm compatibility
because 64bit items are only aligned on 32bit boundaries.
The x86 x32 compatibility will be more like the 32bit arm 'compat'
code - I'm pretty sure arm32 64bit aligned 64bit data.

All other architectures that have both 32-bit and 64-bit variants
use the same alignment for all types, except for x86.

There are additional differences though, especially if one
were to try to generalize the interface to all architectures.
A subset of the issues includes

- x32 has 64-bit types in places of some types that are
  32 bit everywhere else (time_t, ino_t, off_t, clock_t, ...)

- m68k aligns struct members to at most 16 bits

- uid_t/gid_t/ino_t/dev_t/... are

quoted

You'll then need to remember how the process entered the kernel
to work out which compatibility code to invoke.
This is what x86 does.
It allows a single process to do all three types of system call.

Trying to 'patch up' structures outside the kernel, or in the

14 more quoted lines

Indeed, all of this should just be done in userspace. Note (as you of
course know, but others on CC probably don't) that we did this in musl
libc for the sake of being able to run a time64 userspace on a
pre-time64 kernel, with translation from the new time64 ioctl
structures to the versions needed by the old ioctls and back using a
fairly simple table:

https://git.musl-libc.org/cgit/musl/tree/src/misc/ioctl.c?id=v1.2.2

I imagine there's a fair bit more to be done for 32-/64-bit mismatch
in size/long/pointer types and different alignments, but the problem
is almost certainly tractable, and much easier than what they already
have to be doing for syscalls.

Rich

...

He's already doing the system call emulation for all the system
calls other than ioctl in user space though. In my experience,
there are actually fairly few ioctl commands that are different
between architectures -- most of them have no misaligned
or architecture-defined struct members at all.

Aren't there also some intractable issues with socket options?
IIRC the kernel code that tried to change them to 64bit was
horribly broken in some obscure cases.

Pushing the conversion down the stack not only identified the
issues, it also made them easier to fix.

If you change the kernel so a 64bit process can execute 32bit
system calls then a lot of the problems do go away.
This is probably easiest done by setting a high bit on the
system call number - as x86_64 does for x32 calls.

You still have to solve the different alignment of 64bit data
on i386.

Of course the system call numbers are different - but that is
just a lookup.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)