Commit a54f0dfd authored by Peter Collingbourne's avatar Peter Collingbourne Committed by Eric W. Biederman
Browse files

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: default avatarPeter Collingbourne <pcc@google.com>
Reviewed-by: default avatarDave 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: default avatarEric W. Biederman <ebiederm@xmission.com>
parent 7da5082a

include/uapi/asm-generic/signal-defs.h

+7 −0
Original line number Diff line number Diff line
@@ -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 */

kernel/signal.c

+6 −0
Original line number Diff line number Diff line
@@ -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