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

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Message-ID: <062228e73ef444dcafbcb6056bd6c37c@AcuMS.aculab.com>
Date:   Fri, 23 Sep 2022 11:24:18 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'cambda@...ux.alibaba.com'" <cambda@...ux.alibaba.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-api@...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

...
> And yes, I'm tracking a bug. A service monitor, like systemd or
> some watchdog, uses kill() to check if a pid is valid or not:
>   1. Store service pid into cache.
>   2. Check if pid in cache is valid by kill(pid, 0).
>   3. Check if pid in cache is the service to watch.
> 
> So if kill(pid, 0) returns success but no process info shows on 'ps'
> command, the service monitor could be confused. The monitor could
> check if pid is tid, but this means the odd behavior would be used
> intentionally. And this workaround may be unsafe on other OS?

That looks pretty broken to me.
On Linux a pid can be reused immediately a process exits.
So there is really no guarantee that the pid is the one you want.
IIRC there are some recent changes that mean opening /proc/<pid>
will stop the pid being reused - allowing checks before sending a signal.
(Netbsd won't reuse a pid for a reasonable number of forks
and then uses a semi-random pid allocator.
Don't know whether any other 'bsd picked up that change.)

Also using signals in multi-threaded programs is pretty much
non-portable.

	David

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