On Mon, Aug 17, 2020 at 08:33:49PM -0700, Peter Collingbourne wrote: > 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. > > 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 91000b6b97e0..c30a9c1a77b2 100644 > --- a/include/uapi/asm-generic/signal-defs.h > +++ b/include/uapi/asm-generic/signal-defs.h > @@ -13,6 +13,12 @@ > * SA_RESETHAND clears the handler when the signal is delivered. > * SA_NOCLDWAIT flag on SIGCHLD to inhibit zombies. > * SA_NODEFER prevents the current signal from being masked in the handler. > + * 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 @@ > * The following bits are used in architecture-specific SA_* definitions and > * should be avoided for new generic flags: 3, 4, 5, 6, 7, 8, 9, 16, 24, 25, 26. > */ > +#define SA_UNSUPPORTED 0x00000400 This concept confused me a bit initially, since in a sense this flag is supported, just with a rather peculiar meaning. Since the main (only) purpose of this bit will be to check whether SA_XFLAGS is actually supported, I wonder whether it makes sense to weld the two together, say: #define SA_REQUEST_XFLAGS 0x00000c00 #define SA_XFLAGS_MASK 0x00000c00 #define SA_HAVE_XFLAGS 0x00000800 This is a departure from the current style of definitions though. sa.sa_flags |= SA_REQUEST_XFLAGS; sigaction(..., &sa, &sa); if ((sa.sa_flags & SA_XFLAGS_MASK) == SA_HAVE_XFLAGS) /* xflags available */ This would require some juggling of the way SA_UAPI_FLAGS works though. Maybe not worth it, so long as the semantics get clearly documented. [...] Cheers ---Dave
