Re: [RFC] Get siginfo from unreaped task

On Sat, Feb 12, 2022 at 06:32:08PM -0800, Andy Lutomirski wrote:

quoted

On Feb 12, 2022, at 3:24 AM, Robert Święcki [off-list ref] wrote:

sob., 12 lut 2022 o 05:28 Kees Cook [off-list ref] napisał(a):

quoted

Make siginfo available through PTRACE_GETSIGINFO after process death,
without needing to have already used PTRACE_ATTACH. Uses 48 more bytes
in task_struct, though I bet there might be somewhere else we could
stash a copy of it?

An alternative way of accessing this info could be abusing the
waitid() interface, with some additional, custom to Linux, flag

waitid(P_ALL, 0, &si, __WCHILDSIGINFO);

which would change what is put into si.

But maybe ptrace() is better, because it's mostly incompatible with
other OSes anyway on the behavior/flag level, while waitd() seems to
be POSIX/BSD standard, even if Linux specifies some additional flags.

I had a kind of opposite thought, which is that it would be very nice
to be able to get all the waitid() data without reaping a process or
even necessarily being its parent.  Maybe these can be combined?  A
new waitid() option like you’re suggesting could add siginfo (and
might need permissions).  And we could have a different waitid() flag
that says “maybe not my child, don’t reap” (and also needs
permissions).

Although the “don’t reap” thing is fundamentally racy. What a sane
process manager actually wants is an interface to read all this info
from a pidfd, which means it all needs to get stuck in struct pid. And

/me briefly pops out from vacation

Agreed and not just siginfo I would expect(?). We already came to that
conclusion when we first introduced them.

task_struct needs a completion or wait queue so you can actually wait
for a pidfd to exit (unless someone already did this — I had patches a
while back).  And this would be awesome.

Currently, you can wait for a pidfd to exit via polling and you can use
a pidfd to pass it to waitid(P_PIDFD, pidfd, ...).

/me pops back into vacation