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@xxxxxxxxxx> Reviewed-by: Dave Martin <Dave.Martin@xxxxxxx> Link: https://linux-review.googlesource.com/id/Ic2501ad150a3a79c1cf27fb8c99be342e9dffbcb --- v11: - clarify the commit message include/uapi/asm-generic/signal-defs.h | 7 +++++++ kernel/signal.c | 6 ++++++ 2 files changed, 13 insertions(+) diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h index 493953fe319b..0126ebda4d31 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. @@ -42,6 +48,7 @@ #ifndef SA_RESETHAND #define SA_RESETHAND 0x80000000 #endif +#define SA_UNSUPPORTED 0x00000400 #define SA_NOMASK SA_NODEFER #define SA_ONESHOT SA_RESETHAND diff --git a/kernel/signal.c b/kernel/signal.c index 2ef8c238679e..018c19f6cf66 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -3984,6 +3984,12 @@ 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 -- 2.29.0.rc1.297.gfa9743e501-goog
