linux-kernel - Re: ptrace group stop signal number not reset before PTRACE

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Date:	Thu, 18 Aug 2016 16:37:50 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Keno Fischer <keno@...iacomputing.com>
Cc:	roland@...k.frob.com, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>
Subject: Re: ptrace group stop signal number not reset before
	PTRACE_INTERRUPT is delivered?

On 08/17, Keno Fischer wrote:
>
> In this test case, the last
> group-stop (after PTRACE_INTERRUPT) is delivered with a
> WSTOPSIG(status) of SIGTTIN, which was the signr of the previous group
> stop. From reading the man-page, I would have expected SIGTRAP.

Me too ;)

> Now, I
> understand that if there is another stop pending, PTRACE_INTERRUPT
> will simply piggy-backs off that one, but I don't believe that is
> happening in this case.

Yes, thanks. This is wrong. We need to remove SIGTTIN from jobctl.
The problem, I am not sure when... I'll try to think.

Thanks!

> #include <assert.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <signal.h>
> #include <unistd.h>
> #include <sys/ptrace.h>
> #include <sys/wait.h>
> #include <linux/ptrace.h>
> 
> int main() {
>     pid_t child, ret;
>     int err;
>     int status;
>     if (0 == (child = fork())) {
>        kill(getpid(), SIGSTOP);
>        kill(getpid(), SIGSTOP);
>        kill(getpid(), SIGTTIN);
>        sleep(1000);
>        exit(0);
>     }
>     ret = waitpid(child, &status, WSTOPPED);
>     assert(ret == child);
>     err = ptrace(PTRACE_SEIZE, child, NULL, NULL);
>     assert(err == 0);
>     err = ptrace(PTRACE_CONT, child, NULL, NULL);
>     assert(err == 0);
>     // Should now hit SIGSTOP signal-stop
>     ret = waitpid(child, &status, 0);
>     assert(ret == child && WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);
>     err = ptrace(PTRACE_CONT, child, NULL, (void*)SIGSTOP);
>     assert(err == 0);
>     // Should now hit SIGSTOP group-stop
>     ret = waitpid(child, &status, 0);
>     assert(ret == child && (status>>16 == PTRACE_EVENT_STOP) &&
> WSTOPSIG(status) == SIGSTOP);
>     err = ptrace(PTRACE_CONT, child, NULL, NULL);
>     assert(err == 0);
>     // Should now hit SIGTTIN signal-stop
>     ret = waitpid(child, &status, 0);
>     assert(ret == child && WIFSTOPPED(status) && WSTOPSIG(status) == SIGTTIN);
>     err = ptrace(PTRACE_CONT, child, NULL, (void*)SIGTTIN);
>     assert(err == 0);
>     // Should now hit SIGTTIN group-stop
>     ret = waitpid(child, &status, 0);
>     assert(ret == child && (status>>16 == PTRACE_EVENT_STOP) &&
> WSTOPSIG(status) == SIGTTIN);
>     err = ptrace(PTRACE_CONT, child, NULL, NULL);
>     assert(err == 0);
>     // Now interrupt it
>     err = ptrace(PTRACE_INTERRUPT, child, NULL, NULL);
>     assert(err == 0);
>     // Should now hit interrupt group-stop
>     ret = waitpid(child, &status, 0);
>     printf("Interrupt group-stop delivered with signal %d\n", WSTOPSIG(status));
>     assert(ret == child && (status>>16 == PTRACE_EVENT_STOP) &&
> WSTOPSIG(status) == SIGTRAP);
>     exit(0);
> }
> ```