On Wed, Aug 19, 2020 at 3:39 AM Dave Martin [off-list ref] wrote:

On Mon, Aug 17, 2020 at 08:33:48PM -0700, Peter Collingbourne wrote:

Nit: please say what the patch does.  Subject line should summarise
what is done, but should not add new information that is not present in
the description proper.

(Same for all the other patches.)

Will do.

quoted

This allows userspace to detect missing support for flag bits and
allows the kernel to use non-uapi bits internally, as we are already
doing in arch/x86 for two flag bits. Now that this change is in
place, we no longer need the code in arch/x86 that was hiding these
bits from userspace, so remove it.

Signed-off-by: Peter Collingbourne <redacted>
---
View this change in Gerrit: https://linux-review.googlesource.com/q/I35aab6f5be932505d90f3b3450c083b4db1eca86

 arch/arm/include/asm/signal.h    |  4 ++++
 arch/parisc/include/asm/signal.h |  4 ++++
 arch/x86/kernel/signal_compat.c  |  7 -------
 include/linux/signal_types.h     | 12 ++++++++++++
 kernel/signal.c                  | 10 ++++++++++
 5 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/arch/arm/include/asm/signal.h b/arch/arm/include/asm/signal.h
index 65530a042009..d1070a783993 100644
--- a/arch/arm/include/asm/signal.h
+++ b/arch/arm/include/asm/signal.h

@@ -17,6 +17,10 @@ typedef struct {
      unsigned long sig[_NSIG_WORDS];
 } sigset_t;

+#define SA_UAPI_FLAGS                                                          \
+     (SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO | SA_THIRTYTWO |             \
+      SA_RESTORER | SA_ONSTACK | SA_RESTART | SA_NODEFER | SA_RESETHAND)
+

I wonder whether all these per-arch definitions will tend to bitrot when
people add new common flags.

Can we have a common definition for the common bits, and just add the
extra arch-specific ones here?

I think so. We could have something like:

#define ARCH_UAPI_SA_FLAGS SA_THIRTYTWO

here. Then in signal_types.h we can do:

#ifndef ARCH_UAPI_SA_FLAGS
#define ARCH_UAPI_SA_FLAGS 0
#endif

#define UAPI_SA_FLAGS (... | ARCH_UAPI_SA_FLAGS)

I'll do that in v10.

Also, I wonder whether we should avoid the "SA_" prefix here.  Maybe
UAPI_SA_FLAGS?

Sounds good to me.

quoted

 #define __ARCH_HAS_SA_RESTORER

 #include <asm/sigcontext.h>

diff --git a/arch/parisc/include/asm/signal.h b/arch/parisc/include/asm/signal.h
index 715c96ba2ec8..ad06e14f6e8a 100644
--- a/arch/parisc/include/asm/signal.h
+++ b/arch/parisc/include/asm/signal.h

@@ -21,6 +21,10 @@ typedef struct {
      unsigned long sig[_NSIG_WORDS];
 } sigset_t;

+#define SA_UAPI_FLAGS                                                          \
+     (SA_ONSTACK | SA_RESETHAND | SA_NOCLDSTOP | SA_SIGINFO | SA_NODEFER |  \
+      SA_RESTART | SA_NOCLDWAIT | _SA_SIGGFAULT)
+
 #include <asm/sigcontext.h>

 #endif /* !__ASSEMBLY */

diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
index 9ccbf0576cd0..c599013ae8cb 100644
--- a/arch/x86/kernel/signal_compat.c
+++ b/arch/x86/kernel/signal_compat.c

@@ -165,16 +165,9 @@ void sigaction_compat_abi(struct k_sigaction *act, struct k_sigaction *oact)
 {
      signal_compat_build_tests();

-     /* Don't leak in-kernel non-uapi flags to user-space */
-     if (oact)
-             oact->sa.sa_flags &= ~(SA_IA32_ABI | SA_X32_ABI);
-
      if (!act)
              return;

-     /* Don't let flags to be set from userspace */
-     act->sa.sa_flags &= ~(SA_IA32_ABI | SA_X32_ABI);
-
      if (in_ia32_syscall())
              act->sa.sa_flags |= SA_IA32_ABI;
      if (in_x32_syscall())

diff --git a/include/linux/signal_types.h b/include/linux/signal_types.h
index f8a90ae9c6ec..e792f29b5846 100644
--- a/include/linux/signal_types.h
+++ b/include/linux/signal_types.h

@@ -68,4 +68,16 @@ struct ksignal {
      int sig;
 };

+#ifndef SA_UAPI_FLAGS
+#ifdef SA_RESTORER
+#define SA_UAPI_FLAGS                                                          \
+     (SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO | SA_ONSTACK | SA_RESTART |  \
+      SA_NODEFER | SA_RESETHAND | SA_RESTORER)
+#else
+#define SA_UAPI_FLAGS                                                          \
+     (SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO | SA_ONSTACK | SA_RESTART |  \
+      SA_NODEFER | SA_RESETHAND)
+#endif
+#endif
+
 #endif /* _LINUX_SIGNAL_TYPES_H */

diff --git a/kernel/signal.c b/kernel/signal.c
index 42b67d2cea37..348b7981f1ff 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c

@@ -3984,6 +3984,16 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
      if (oact)
              *oact = *k;

+     /*
+      * Clear unknown flag bits in order to allow userspace to detect missing
+      * support for flag bits and to allow the kernel to use non-uapi bits
+      * internally.
+      */
+     if (act)
+             act->sa.sa_flags &= SA_UAPI_FLAGS;
+     if (oact)
+             oact->sa.sa_flags &= SA_UAPI_FLAGS;
+

Seems reasonable.

Thanks. I also decided to check how other operating systems handle
unknown flag bits in sigaction.sa_flags. It looks like OpenBSD and
illumos also accept unknown bits but (implicitly, as a result of using
a different internal representation) end up clearing them in oldact:

https://github.com/openbsd/src/blob/f634a6a4b5bf832e9c1de77f7894ae2625e74484/sys/kern/kern_sig.c#L278
https://github.com/illumos/illumos-gate/blob/76f19f5fdc974fe5be5c82a556e43a4df93f1de1/usr/src/uts/common/syscall/sigaction.c#L86

and FreeBSD and NetBSD fail the syscall if unknown bits are set:

https://github.com/freebsd/freebsd/blob/eded70c37057857c6e23fae51f86b8f8f43cd2d0/sys/kern/kern_sig.c#L699
https://github.com/NetBSD/src/blob/3365779becdcedfca206091a645a0e8e22b2946e/sys/kern/sys_sig.c#L473

So there is some precedent for doing what we're planning to do here,
which makes it yet more likely that we'll be okay doing this.

Peter

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