Peter Collingbourne <pcc@xxxxxxxxxx> writes: > 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 Why this value and why not in numerical order with the other flags? At the very least not being in order with the other bits makes it a little easier to overlook it and define something at that position. Eric > #define SA_NOMASK SA_NODEFER > #define SA_ONESHOT SA_RESETHAND > diff --git a/kernel/signal.c b/kernel/signal.c > index 8f5bd12ee41b..8f34819e80de 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -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
