--- 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. man2/sigaction.2 | 62 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 62 insertions(+) diff --git a/man2/sigaction.2 b/man2/sigaction.2 index 6a8142324..82fb69e26 100644 --- a/man2/sigaction.2 +++ b/man2/sigaction.2 @@ -250,6 +250,19 @@ 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 " (since Linux 5.x)" +This flag bit will never be supported by the kernel. It is used as +part of the flag support detection protocol described below. +.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, the tag bits will be preserved in +.IR si_addr . .SS The siginfo_t argument to a SA_SIGINFO handler When the .B SA_SIGINFO @@ -833,6 +846,55 @@ Triggered by a .BR seccomp (2) filter rule. .RE +.SS Detecting flag support in sa_flags +The Linux kernel supports a mechanism for programs to detect kernel +support for +.B SA_* +flags in +.IR sa_flags . +This mechanism is quite subtle for backwards compatibility reasons +related to the historical behavior of the kernel. + +Starting with Linux 5.x, the kernel will clear any unrecognized bits +from the +.I sa_flags +value returned via +.I oldact +if those bits were set when the signal handler was originally installed. +Therefore, a program that only needs to be compatible with Linux 5.x +and above may test for flag bit support by issuing a second call to +.BR sigaction () +and testing whether the bit remains set in +.IR oldact.sa_flags . + +Prior to Linux 5.x, unrecognized flag bits were preserved in +.I oldact.sa_flags +so this protocol on its own would not be sufficient to allow a +userspace program to test for flag bit support if it needs to be +compatible with kernel versions older than 5.x. Therefore, the +.B SA_UNSUPPORTED +flag bit was defined, which the kernel will always consider to be +unknown. A userspace program that sets this flag bit in +.I act.sa_flags +and finds that it has been cleared in +.I oldact.sa_flags +in a subsequent call to +.BR sigaction () +may trust that any other unknown flag bits have been cleared. + +A reasonably modern program may trust that the flags +.BR SA_NOCLDSTOP , +.BR SA_NOCLDWAIT , +.BR SA_SIGINFO , +.BR SA_ONSTACK , +.BR SA_RESTART , +.BR SA_NODEFER , +.B SA_RESETHAND +and, if defined by the architecture, +.B SA_RESTORER +are supported by the kernel, without relying on the flag bit support +detection protocol, since these flags have all been supported +since Linux 2.6. .SH RETURN VALUE .BR sigaction () returns 0 on success; on error, \-1 is returned, and -- 2.29.2.299.gdc1121823c-goog
