Re: [RFC] Get siginfo from unreaped task

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 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.