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

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Message-ID: <87k05v5sqn.fsf@email.froward.int.ebiederm.org>
Date:   Thu, 22 Sep 2022 10:33:20 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     cambda@...ux.alibaba.com
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Dust Li <dust.li@...ux.alibaba.com>,
        Tony Lu <tonylu@...ux.alibaba.com>
Subject: Re: Syscall kill() can send signal to thread ID

cambda@...ux.alibaba.com writes:

> I found syscall kill() can send signal to a thread id, which is
> not the TGID. But the Linux manual page kill(2) said:
>
> "The kill() system call can be used to send any signal to any
> process group or process."
>
> And the Linux manual page tkill(2) said:
>
> "tgkill() sends the signal sig to the thread with the thread ID
> tid in the thread group tgid.  (By contrast, kill(2) can be used
> to send a signal only to a process (i.e., thread group) as a
> whole, and the signal will be delivered to an arbitrary thread
> within that process.)"
>
> I don't know whether the meaning of this 'process' should be
> the TGID? Because I found kill(tid, 0) will return ESRCH on FreeBSD,
> while Linux sends signal to the thread group that the thread belongs
> to.
>
> If this is as expected, should we add a notice to the Linux manual
> page? Because it's a syscall and the pids not equal to tgid are not
> listed under /proc. This may be a little confusing, I guess.

How did you come across this?  Were you just experimenting?

I am wondering if you were tracking a bug, or a portability problem
or something else.  If the current behavior is causing problems in
some way instead of just being a detail that no one really cares about
either way it would be worth considering if we want to maintain the
current behavior.

Eric