On Fri, Aug 21, 2020 at 10:10:14PM -0700, Peter Collingbourne wrote: Nit: no statement of the chage being made (other than in the subject line). > This bit 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 finds that this bit 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. This isn't quite right? After a single sigaction() call, oact will contain the sa_flags for the previously registered handler. So a second sigaction() call would be needed to find out the newly effective sa_flags. > > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > --- > View this change in Gerrit: https://linux-review.googlesource.com/q/Ic2501ad150a3a79c1cf27fb8c99be342e9dffbcb > > 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 319628058a53..e853cbe8722d 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 I guess people may debate which bit is chosen, but your consolidation of these definitions should help to reduce the possibility of future collisions. This bit appears unused for now, so I guess I don't have a strong opinion. > #define SA_NOMASK SA_NODEFER > #define SA_ONESHOT SA_RESETHAND > diff --git a/kernel/signal.c b/kernel/signal.c > index f802c82c7bcc..c80e70bde11d 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); > + Seems reasonable. With the above rewording in the commit message to clarify that a second sigaction() is needed: Reviewed-by: Dave Martin <Dave.Martin@xxxxxxx> [...] Cheers ---Dave
