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signal: define the SA_UNSUPPORTED bit in sa_flags

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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>
This commit is contained in:
Peter Collingbourne 2020-11-16 19:17:25 -08:00 committed by Eric W. Biederman
parent 7da5082a2f
commit a54f0dfda7
2 changed files with 13 additions and 0 deletions
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7
include/uapi/asm-generic/signal-defs.h
View File

@ -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 */

6
kernel/signal.c
View File

@ -3985,6 +3985,12 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
if (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