diff options
| author | Peter Collingbourne <pcc@google.com> | 2020-11-16 19:17:25 -0800 |
|---|---|---|
| committer | Eric W. Biederman <ebiederm@xmission.com> | 2020-11-23 10:31:06 -0600 |
| commit | a54f0dfda754c5cecc89a14dab68a3edc1e497b5 (patch) | |
| tree | a8899e5a502ef6a262acbbf073d155c6c87c892d | |
| parent | 7da5082a2f9a1d16eded00c204fdb52a855c2bb2 (diff) | |
signal: define the SA_UNSUPPORTED bit in sa_flags
Define a sa_flags bit, SA_UNSUPPORTED, which will never be supported
in the uapi. The purpose of this flag bit is to allow userspace to
distinguish an old kernel that does not clear unknown sa_flags bits
from a kernel that supports every flag bit.
In other words, if userspace does something like:
act.sa_flags |= SA_UNSUPPORTED;
sigaction(SIGSEGV, &act, 0);
sigaction(SIGSEGV, 0, &oldact);
and finds that SA_UNSUPPORTED remains set in oldact.sa_flags, it means
that the kernel cannot be trusted to have cleared unknown flag bits
from sa_flags, so no assumptions about flag bit support can be made.
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Link: https://linux-review.googlesource.com/id/Ic2501ad150a3a79c1cf27fb8c99be342e9dffbcb
Link: https://lkml.kernel.org/r/bda7ddff8895a9bc4ffc5f3cf3d4d37a32118077.1605582887.git.pcc@google.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
| -rw-r--r-- | include/uapi/asm-generic/signal-defs.h | 7 | ||||
| -rw-r--r-- | kernel/signal.c | 6 |
2 files changed, 13 insertions, 0 deletions
diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h index 44f070982752..c790f67304ba 100644 --- a/include/uapi/asm-generic/signal-defs.h +++ b/include/uapi/asm-generic/signal-defs.h @@ -14,6 +14,12 @@ * SA_RESTART flag to get restarting signals (which were the default long ago) * SA_NODEFER prevents the current signal from being masked in the handler. * SA_RESETHAND clears the handler when the signal is delivered. + * SA_UNSUPPORTED is a flag bit that will never be supported. Kernels from + * before the introduction of SA_UNSUPPORTED did not clear unknown bits from + * sa_flags when read using the oldact argument to sigaction and rt_sigaction, + * so this bit allows flag bit support to be detected from userspace while + * allowing an old kernel to be distinguished from a kernel that supports every + * flag bit. * * SA_ONESHOT and SA_NOMASK are the historical Linux names for the Single * Unix names RESETHAND and NODEFER respectively. @@ -34,6 +40,7 @@ /* 0x00000080 used on parisc */ /* 0x00000100 used on sparc */ /* 0x00000200 used on sparc */ +#define SA_UNSUPPORTED 0x00000400 /* 0x00010000 used on mips */ /* 0x01000000 used on x86 */ /* 0x02000000 used on x86 */ diff --git a/kernel/signal.c b/kernel/signal.c index 8f5bd12ee41b..8f34819e80de 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -3986,6 +3986,12 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact) *oact = *k; /* + * Make sure that we never accidentally claim to support SA_UNSUPPORTED, + * e.g. by having an architecture use the bit in their uapi. + */ + BUILD_BUG_ON(UAPI_SA_FLAGS & SA_UNSUPPORTED); + + /* * 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. |