On Thursday 17 December 2015 18:27:53 Catalin Marinas wrote:

On Wed, Dec 16, 2015 at 12:42:38AM +0300, Yury Norov wrote:

quoted

+#define compat_sys_lookup_dcookie      sys_lookup_dcookie
+#define compat_sys_pread64             sys_pread64
+#define compat_sys_pwrite64            sys_pwrite64
+#define compat_sys_readahead           sys_readahead
+#define compat_sys_shmat               sys_shmat

I wonder whether we need wrappers (actually, not only for these but
sys_read etc.). These functions take either a pointer or a size_t
argument which are 32-bit with ILP32 but treated as 64-bit by an LP64
kernel. Can we guarantee that user space zeros the top 32-bit of the
arguments passed here?

I'm pretty sure that is safe. I haven't read the calling conventions
specification for arm64 ilp32, but usually all function arguments are
passed as 64-bit registers with proper sign-extend or zero-extend.

Most other syscalls rely on this behavior too, not just the ones that
are being modified here.

With compat/AArch32, this is guaranteed by the kernel since EL0 won't be
able to touch the top part but here I'm not entirely sure. As long as
user space used Wn registers for 32-bit types, we are probably fine (the
architecture guarantees the top 32-bit zeroing following a MOV, LDR etc.
instruction into a Wn register). We just need to mention this in the ABI
document (ilp32.txt).

I think the aarch32 case is actually the hard one, because it has to
worry about explicitly sign-extending 32-bit arguments (signed int or
signed long) that might be negative, e.g. user space passes -1 
as 0xffffffff, which the kernel entry turns into 0x00000000ffffffff
when it should use 0xffffffffffffffff. The COMPAT_SYSCALL_DEFINEx
macros take care of this.

quoted

+
+#define compat_sys_open_by_handle_at   sys_open_by_handle_at
+#define compat_sys_openat              sys_openat

So using sys_openat() forces O_LARGEFILE and we don't have a problem
with (f)truncate. We may have an issue with AArch32 compat though.

aarch32 uses the correct compat functions in asm/unistd32.h

	Arnd