[[ Removed some CCs because gmail didn't allow me to send it with so many CCs: Kevin B., Andrey K., Helge D., David S. ]] On 11/18/20 12:54 AM, Peter Collingbourne wrote: > Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx> > --- > These features are implemented in this patch series: > https://lore.kernel.org/linux-arm-kernel/cover.1605235762.git.pcc@xxxxxxxxxx/ > which is still under review, so the patch should not be applied > yet. > > Alejandro, thanks for the review. Since the patch was almost > rewritten I didn't base this on your patch, instead I tried to > use the correct formatting in this patch. Hi Peter, Fine. See below a small fix. Cheers, Alex > > v2: > - fix formatting > - address feedback from Dave > > man2/sigaction.2 | 125 +++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 125 insertions(+) > > diff --git a/man2/sigaction.2 b/man2/sigaction.2 > index 6a8142324..0e4236a43 100644 > --- a/man2/sigaction.2 > +++ b/man2/sigaction.2 > @@ -250,6 +250,44 @@ This flag is meaningful only when establishing a signal handler. > .\" .I sa_sigaction > .\" field was added in Linux 2.1.86.) > .\" > +.TP > +.BR SA_UNSUPPORTED > +Used to dynamically probe for flag bit support. > +.IP > +If an attempt to register a handler succeeds with this flag set in > +.I act->sa_flags > +alongside other flags that are potentially unsupported by the kernel, > +and an immediately subsequent > +.BR sigaction () > +call specifying the same signal number n and with non-NULL > +.I oldact > +yields > +.B SA_UNSUPPORTED > +.I clear > +in > +.IR oldact->sa_flags , > +then > +.IR oldact->sa_flags > +may be used as a bitmask > +describing which of the potentially unsupported flags are, > +in fact, supported. > +See the section "Dynamically probing for flag bit support" > +below for more details. > +.TP > +.BR SA_EXPOSE_TAGBITS " (since Linux 5.x)" > +Normally, when delivering a signal, > +an architecture-specific set of tag bits are cleared from the > +.I si_addr > +field of > +.IR siginfo_t . > +If this flag is set, > +an architecture-specific subset of the tag bits will be preserved in > +.IR si_addr . > +.IP > +Programs that need to be compatible with Linux versions older than 5.x > +must use > +.B SA_UNSUPPORTED > +to probe for support. > .SS The siginfo_t argument to a SA_SIGINFO handler > When the > .B SA_SIGINFO > @@ -833,6 +871,93 @@ Triggered by a > .BR seccomp (2) > filter rule. > .RE > +.SS Dynamically probing for flag bit support > +The > +.BR sigaction () > +call on Linux accepts unknown bits set in > +.I act->sa_flags > +without error. > +The behavior of the kernel starting with Linux 5.x is that a second > +.BR sigaction () > +will clear unknown bits from > +.IR oldact->sa_flags . > +However, historically, a second > +.BR sigaction () > +call would typically leave those bits set in > +.IR oldact->sa_flags . > +.PP > +This means that support for new flags cannot be detected > +simply by testing for a flag in > +.IR sa_flags , > +and a program must test that > +.B SA_UNSUPPORTED > +has been cleared before relying on the contents of > +.IR sa_flags . > +.PP > +Since the behavior of the signal handler cannot be guaranteed > +unless the check passes, > +it is wise to either block the affected signal > +while registering the handler and performing the check in this case, > +or where this is not possible, > +for example if the signal is synchronous, to issue the second > +.BR sigaction () > +in the signal handler itself. > +.PP > +In kernels that do not support a specific flag, > +the kernel's behavior is as if the flag was not set, > +even if the flag was set in > +.IR act->sa_flags . > +.PP > +The flags > +.BR SA_NOCLDSTOP , > +.BR SA_NOCLDWAIT , > +.BR SA_SIGINFO , > +.BR SA_ONSTACK , > +.BR SA_RESTART , > +.BR SA_NODEFER , > +.BR SA_RESETHAND , > +and, if defined by the architecture, > +.B SA_RESTORER > +may not be reliably probed for using this mechanism, > +because they were introduced before Linux 5.x. > +However, in general, programs may assume that these flags are supported, > +since they have all been supported since Linux 2.6, > +which was released in the year 2003. > +.PP > +The following example program exits with status 0 if > +.B SA_EXPOSE_TAGBITS > +is determined to be supported, and 1 otherwise. > +.PP > +.EX > +#include <signal.h> > +#include <stdio.h> > +#include <unistd.h> > + > +void handler(int signo, siginfo_t *info, void *context) { > + struct sigaction oldact; > + if (sigaction(SIGSEGV, 0, &oldact) == 0 && > + !(oldact.sa_flags & SA_UNSUPPORTED) && > + (oldact.sa_flags & SA_EXPOSE_TAGBITS)) { > + _exit(0); > + } else { > + _exit(1); > + } > +} > + > +int main(void) { > + struct sigaction act = {}; > + act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS; > + act.sa_sigaction = handler; > + if (sigaction(SIGSEGV, &act, 0) != 0) { > + perror("sigaction"); > + return 1; > + } > + > + /* Force a SIGSEGV. */ > + *(volatile int *)0 = 0; > + return 1; > +} > +.EE > .SH RETURN VALUE > .BR sigaction () > returns 0 on success; on error, \-1 is returned, and >
