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Date: Fri, 20 Mar 2015 17:25:48 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Pavel Labath <labath@google.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: A peculiarity in ptrace/waitpid behavior
Message-ID: <20150320162548.GA21069@redhat.com>
References: <CAJt8pk-+UGsmAzA8cTn3deWfSrDAy__Yh=bqi4_NRqJVhg63JQ@mail.gmail.com>
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Hi Pavel,

let me add lkml, we should not discuss this offlist.

On 03/20, Pavel Labath wrote:
>
> 1) we get a waitpid() notification that the tracee got SIGUSR1
> 2) we do a ptrace(GETSIGINFO) to get more info
> 3) eventually we decide to restart the tracee with PTRACE_CONT, passing it
> SIGUSR1
> 4) immediately after that we get another waitpid notification, again with
> SIGUSR1, even though the thread had received no additional signals
> 5) we again try to a GETSIGINFO, however this time it fails with ESRCH.
> Therefore, we assume that the thread has died

I found a similar bug by code inspection some time ago. I even have
a fix, but I need to think more... And I even wrote the test-case ;)
see below.

But so far I can't say if you hit the same problem or not. If you can
reproduce the problem, perhaps I can send you debugging patch?

Oleg.

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <sys/syscall.h>
#include <assert.h>

#define tkill(pid, sig)	\
	syscall(__NR_tkill, pid, sig)

void run_test(void)
{
	int pid, stat;

	pid = fork();
	if (!pid) {
		assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
		raise(SIGSTOP);
		assert(0);
	}

	assert(pid == wait(&stat) && stat == 0x137f);

	tkill(pid, SIGTRAP);	/* should not be reported */
	tkill(pid, SIGKILL);
	assert(pid == wait(&stat));
	if (stat == 0x9)
		return;

	printf("unexpected wait: stat=%x\n", stat);
	kill(0, SIGKILL);
}

int main(void)
{
	int i = 8; /* random */

	while (--i)
		if (!fork())
			break;

	for (;;)
		run_test();

	return 0;
}