Re: Syscall kill() can send signal to thread ID

[Eric W. Biederman]

Cambda Zhu <cambda@xxxxxxxxxxxxxxxxx> writes:

>> On Sep 23, 2022, at 15:53, Florian Weimer <fweimer@xxxxxxxxxx> wrote:
>> 
>>> I don't quite understand what you mean, sorry. But if kill() returns
>>> -ESRCH for tid which is not equal to tgid, kill() can only send signal
>>> to thread group via main thread id, that is what BSD did and manual
>>> said. It seems not odd?
>> 
>> It's still odd because there's one TID per process that's valid for
>> kill by accident.  That's all.

> As far as I know, there is no rule forbidding 'process ID'(TGID on Linux)
> equals to main thread ID, is it right?

There is an unfortunate guarantee that glibc depends upon that after
exec TGID == TID for the initial thread in a process.  I say unfortunate
because maintaining that guarantee when another thread in the process
calls exec is a bit painful.

> If one wants to send signal to a specific thread, tgkill() can do
> that. As far as I understand, the difference between kill() and
> tgkill() is whether the signal is set on shared_pending, whatever the
> ID is a process ID or a thread ID. For Linux, the main thread ID just
> equals to the process ID.

Correct.  kill and tgkill uses different signal queues.  Kill is global
to the destination process and tgkill is always thread local.

> So the meaning of kill(main_tid, sig) is sending signal to a process,
> of which the PID equals to the first argument. It's not odd, I think.

Yes, the oddity is the TGID and TID share the same value, nothing else.

Eric