Re: [patch] Fix Linux attach to signalled/stopped processes

Jan Kratochvil
2007-06-26T21:04:09Z

[-- Attachment #1: Type: text/plain, Size: 1251 bytes --]

On Fri, 15 Jun 2007 20:01:59 +0200, Mark Kettenis wrote:
> > Date: Wed, 6 Jun 2007 16:34:32 +0200
> > From: Jan Kratochvil <jan.kratochvil@redhat.com>
> >
> > The patch changes the functionality of TO_ATTACH. The former
> > functionality was too UNIX centric. I was reading all the TO_ATTACH
> > OS-flavor implementations and I believe the non-"inf-ptrace.c" ones
> > do not need update. The code was tested only on Linux kernel,
> > though.
>
> I disagree here. You're adding all sorts of goo to work around
> Linux-specific problems. Please put this stuff in linux-nat.c

While that "/proc/PID/status" reading part looks definitely Linux specific
I would guess that the first waitpid(2) after PTRACE_ATTACH may not return
SIGSTOP even on other ptrace(2)-using OSes (BSD?).

Attaching a testcase for a possible non-Linux kernel test which:
Should print approx. 1x '.' per second (on each caught pending SIGALRM signal).
At least Linux 2.6.22-rc5.x86_64 prints '!' which looks as a kernel bug to me.

How does currently GDB behave there while attaching to a SIGSTOPped process?
This waitpid(2)-after-ptrace(2) signals redelivery looks as ptrace(2) specific
and not Linux specific to me. And so it would belong to inf-ptrace.c.

Regards,
Jan

[-- Attachment #2: ptrace-test.c --]
[-- Type: text/plain, Size: 2926 bytes --]

/* This testcase is part of GDB, the GNU debugger.

Copyright 2007 Free Software Foundation, Inc.

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */

#define _GNU_SOURCE
#include <signal.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <assert.h>
#include <fcntl.h>

static void action(int sig, siginfo_t * info, void *uc)
{
raise (SIGALRM);
}

static void loop (void)
{
struct sigaction act;

memset (&act, 0, sizeof(struct sigaction));
act.sa_sigaction = action;
act.sa_flags = SA_RESTART;
sigaction (SIGALRM, &act, 0);

raise (SIGALRM);

putchar ('!');

for (;;)
pause ();
/* NOTREACHED */
abort ();
}

static pid_t child;

static void
cleanup (void)
{
kill (child, SIGKILL);
}

static void
handler (int signo)
{
cleanup ();
}

int main (void)
{
void (*handler_orig) (int signo);

setbuf (stdout, NULL);

child = fork ();
switch (child)
{
case -1:
abort ();
case 0:
loop ();
/* NOTREACHED */
abort ();
default:
break;
}

atexit (cleanup);
handler_orig = signal (SIGABRT, handler);
assert (handler_orig == SIG_DFL);

for (;;)
{
errno = 0;
ptrace (PTRACE_ATTACH, child, NULL, NULL);
assert_perror (errno);
unsigned long sig;

/* Deliver one SIGSTOP just for sure.
If the process was already stopped AND some other process (like shell)
has already waited for it we would get stuck in waitpid (). */
sig = SIGSTOP;
do
{
pid_t got_pid;
int status;

errno = 0;
ptrace (PT_CONTINUE, child, (void *) 1UL, (void *) sig);
/* For unstopped processes the preventive signal may ESRCH. */
if (sig != SIGSTOP)
{
assert_perror (errno);
putchar ('.');
}

got_pid = waitpid (child, &status, 0);
assert (got_pid == child);

/* Check if the thread has exited. */
assert (!WIFEXITED (status));
assert (!WIFSIGNALED (status));

assert (WIFSTOPPED (status));
sig = WSTOPSIG (status);
assert (sig != 0);
}
while (sig != SIGSTOP);

errno = 0;
ptrace (PTRACE_DETACH, child, (void *) 1UL, (void *) 0UL);
assert_perror (errno);
}

return 0;
}