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

[Walt Drummond]

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 <tytso at mit.edu> wrote:
>
> On Tue, Jan 04, 2022 at 04:05:26PM -0600, Eric W. Biederman wrote:
> >
> > 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.
>
> > > 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