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sigaltstack.2: Document SS_AUTODISARM flag added in Linux 4.7 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2017-05-22 22:29:00 +02:00
parent e4631a3524
commit cfdc176b44
1 changed files with 49 additions and 4 deletions
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53
man2/sigaltstack.2
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@ -1,5 +1,5 @@
'\" t
.\" Copyright (c) 2001, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Copyright (c) 2001, 2017 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
@ -103,7 +103,27 @@ To establish a new alternate signal stack,
the fields of this structure are set as follows:
.TP
.I ss.ss_flags
This field is set to zero.
This field contains either 0, or the following flag:
.RS
.TP
.BR SS_AUTODISARM " (since Linux 4.7)"
.\" commit 2a74213838104a41588d86fd5e8d344972891ace
.\" See tools/testing/selftests/sigaltstack/sas.c in kernel sources
Clear the alternate signal stack settings on entry to the signal handler.
When the signal handler returns,
the previous alternate signal stack settings are restored.
.IP
This flag was added in order make it safe
to switch away from the signal handler with
.BR swapcontext (3).
Without this flag, a subsequently handled signal will corrupt
the state of the switched-away signal handler.
On kernels where this flag is not supported,
.BR sigaltstack ()
fails with the error
.BR EINVAL
when this flag is supplied.
.RE
.TP
.I ss.ss_sp
This field specifies the starting address of the stack.
@ -120,8 +140,10 @@ size required to execute a signal handler.
.PP
To disable an existing stack, specify \fIss.ss_flags\fP
as \fBSS_DISABLE\fP.
In this case, the remaining fields
in \fIss\fP are ignored.
In this case, the kernel ignores any other flags in
.IR ss.ss_flags
and the remaining fields
in \fIss\fP.
.PP
If \fIold_ss\fP is not NULL, then it is used to return information about
the alternate signal stack which was in effect prior to the
@ -139,6 +161,25 @@ currently executing on it.)
.TP
.B SS_DISABLE
The alternate signal stack is currently disabled.
.IP
Alternatively, this value is returned if the process is currently
executing on an alternate signal stack that was established using the
.B SS_AUTODISARM
flag.
In this case, it is safe to switch away from the signal handler with
.BR swapcontext (3).
It is also possible to set up a different alternative signal stack
using a further call to
.BR sigaltstack ().
.\" FIXME Was it intended that one can set up a different alternative
.\" signal stack in this scenario? (In passing, if one does this, the
.\" sigaltstack(NULL, &old_ss) now returns old_ss.ss_flags==SS_AUTODISARM
.\" rather than old_ss.ss_flags==SS_DISABLE. The API design here seems
.\" confusing...
.TP
.B SS_AUTODISARM
The alternate signal stack has been marked to be autodisarmed
as described above.
.PP
By specifying
.I ss
@ -184,6 +225,10 @@ T} Thread safety MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2009, SUSv2, SVr4.
The
.B SS_AUTODISARM
flag is a Linux extension.
.SH NOTES
The most common usage of an alternate signal stack is to handle the
.B SIGSEGV