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

[Jan Kratochvil <jan.kratochvil@redhat.com>]

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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 --]
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/* 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;
}