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From:   cambda@linux.alibaba.com
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Subject: Syscall kill() can send signal to thread ID
Message-Id: <69E17223-F0CA-4A4C-AAD7-065D6E6266D9@linux.alibaba.com>
Date:   Thu, 22 Sep 2022 17:11:12 +0800
Cc:     linux-api@vger.kernel.org, Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
        Dust Li <dust.li@linux.alibaba.com>,
        Tony Lu <tonylu@linux.alibaba.com>,
        Cambda Zhu <cambda@linux.alibaba.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel@vger.kernel.org
Precedence: bulk

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.

Regards,
Cambda