On Wed, Aug 19, 2020 at 05:23:25PM -0700, Peter Collingbourne wrote: > On Wed, Aug 19, 2020 at 7:51 AM Dave Martin <Dave.Martin@xxxxxxx> wrote: > > > > 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. > > Hmm. Maybe it should be named "SA_UNKNOWN" to mean that the bit will > always be "unknown" to the kernel in the sense that it shall be > treated in the same way as any other "unknown" bit. Then we can define > the kernel's behavior in terms of what happens if a bit is "known". I > don't know if this is just shuffling terms around though. At any rate, > this seems like a problem to be solved with documentation. > > > Since the main (only) purpose of this bit will be to check whether > > I wouldn't necessarily say that it is the only purpose. If another new > sa_flags bit were to be introduced in the future, SA_UN(whatever) > could be used to detect kernel support for that bit in the same way as > SA_XFLAGS. > > > 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. > > I'm not sure about this. I personally think that it would be clearer > to keep the flags orthogonal. Fair enough. I didn't think my approach was a whole lot better tbh. Cheers ---Dave
