Re: [RFC PATCH 0/8] signals: Support more than 64 signals

The only standard tools that support SIGINFO are sleep, dd and ping,
(and kill, for obvious reasons) so it's not like there's a vast hole
in the tooling or something, nor is there a large legacy software base
just waiting for SIGINFO to appear.   So while I very much enjoyed
figuring out how to make SIGINFO work ...

I'll have the VSTATUS patch out in a little bit.

I also think there might be some merit in consolidating the 10
'sigsetsize != sizeof(sigset_t)' checks in a macro and adding comments
that wave people off on trying to do what I did.  If that would be
useful, happy to provide the patch.

On Tue, Jan 4, 2022 at 2:23 PM Theodore Ts'o [off-list ref] wrote:

On Tue, Jan 04, 2022 at 04:05:26PM -0600, Eric W. Biederman wrote:

quoted

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?

So if I *cared* about SIGINFO, what I'd do is ask the systemd
developers and users list if there are any users of the sigpwr.target
feature that they know of.  And I'd also download all of the open
source UPS monitoring applications (and perhaps documentation of
closed-source UPS applications, such as for example APC's program) and
see if any of them are trying to send the SIGPWR signal.

I don't personally think it's worth the effort to do that research,
but maybe other people care enough to do the work.

quoted

quoted

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.

Once VSTATUS support lands in the kernel, we can wait and see if there
is anyone who shows up wanting the SIGINFO functionality.  Certainly
we have no shortage of userspace notification interfaces in Linux.  :-)

                                              - Ted