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[122.61.176.117]) by smtp.gmail.com with ESMTPSA id l6sm17375048pff.74.2021.08.08.14.32.34 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 08 Aug 2021 14:32:36 -0700 (PDT) Cc: mtk.manpages@gmail.com, Peter Collingbourne , linux-man@vger.kernel.org Subject: Re: [PATCH 28/32] sigaction.2: Document SA_EXPOSE_TAGBITS and the flag support detection protocol To: Alejandro Colomar References: <20210728202008.3158-1-alx.manpages@gmail.com> <20210728202008.3158-29-alx.manpages@gmail.com> From: "Michael Kerrisk (man-pages)" Message-ID: Date: Sun, 8 Aug 2021 23:32:32 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210728202008.3158-29-alx.manpages@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Dropped, because reposted in a later series from Alex. On 7/28/21 10:20 PM, Alejandro Colomar wrote: > From: Peter Collingbourne > > Signed-off-by: Peter Collingbourne > Signed-off-by: Alejandro Colomar > --- > man2/sigaction.2 | 123 +++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 123 insertions(+) > > diff --git a/man2/sigaction.2 b/man2/sigaction.2 > index 57ad6418c..4bf6f095e 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 > +.B 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 +884,91 @@ 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 = {0}; > + act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS; > + act.sa_sigaction = handler; > + if (sigaction(SIGSEGV, &act, 0) != 0) { > + perror("sigaction"); > + return 1; > + } > + > + raise(SIGSEGV); > +} > +.EE > .SH RETURN VALUE > .BR sigaction () > returns 0 on success; on error, \-1 is returned, and > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/