"Theodore Ts'o" <tytso@xxxxxxx> writes:
On Tue, Jan 04, 2022 at 12:00:34PM -0600, Eric W. Biederman wrote:
I dug through the previous conversations and there is a little debate
about what makes sense for SIGPWR to do by default. Alan Cox remembered
SIGPWR was sent when the power was restored, so ignoring SIGPWR by
default made sense. Ted Tso pointed out a different scenario where it
was reasonable for SIGPWR to be a terminating signal.
So far no one has actually found any applications that will regress if
SIGPWR becomes ignored by default. Furthermore on linux SIGPWR is only
defined to be sent to init, and init ignores all signals by default so
in practice SIGPWR is ignored by the only process that receives it
currently.
As it turns out, systemd does *not* ignore SIGPWR. Instead, it will
initiate the sigpwr target. From the systemd.special man page:
sigpwr.target
A special target that is started when systemd receives the
SIGPWR process signal, which is normally sent by the kernel
or UPS daemons when power fails.
And child processes of systemd are not ignoring SIGPWR. Instead, they
are getting terminated.
<tytso@cwcc>
41% /bin/sleep 50 &
[1] 180671
<tytso@cwcc>
42% kill -PWR 180671
[1]+ Power failure /bin/sleep 50
That is all as expected, and does not demonstrate a regression would
happen if SIGPWR were to treat SIG_DFL as SIG_IGN, as SIGWINCH, SIGCONT,
SIGCHLD, SIGURG do. It does show there is the possibility of problems.
The practical question is does anything send SIGPWR to anything besides
init, and expect the process to handle SIGPWR or terminate?
Possibly easier to implement (if people desire) is to simply send
SIGCONT with an si_code that indicates someone pressed the VSTATUS
key. We have a per signal 32bit si_code space so that should
be comparatively easy.
I claim, though, that we could implement VSTATUS without implenting
the SIGINFO part of the feature.
I agree that is the place to start. And if we aren't going to use
SIGINFO perhaps we could have an equally good notification method
if anyone wants one. Say call an ioctl and get an fd that can
be read when a VSTATUS request comes in.
SIGINFO vs SIGCONT vs a fd vs something else is something we can sort
out when people get interested in modifying userspace.
Eric
