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Biederman" , "James E.J. Bottomley" , Michael Kerrisk , Alejandro Colomar Cc: Peter Collingbourne , Linux ARM , Kevin Brodsky , Andrey Konovalov , linux-api@vger.kernel.org, Helge Deller , David Spickett , linux-man@vger.kernel.org X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210712_183904_633133_2DCF1826 X-CRM114-Status: GOOD ( 20.41 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Signed-off-by: Peter Collingbourne --- This feature landed back in 5.11, but the manpage update seems to have fallen through the cracks. Here's a v3 with the introducing version specified and with one formatting nit fixed. v3: - s/5.x/5.11/g - s/.IR/.I/ in one location 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 57ad6418c..6b90982d4 100644 --- a/man2/sigaction.2 +++ b/man2/sigaction.2 @@ -261,6 +261,44 @@ 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 +.I 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 5.11)" +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 5.11 +must use +.B SA_UNSUPPORTED +to probe for support. .SS The siginfo_t argument to a SA_SIGINFO handler When the .B SA_SIGINFO @@ -846,6 +884,93 @@ 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 5.11 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 5.11. +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 +#include +#include + +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