Re: [Regression] Segfault on native-extended-gdbserver + fork

[Sergio Durigan Junior <sergiodj@redhat.com>]

On Monday, January 29 2018, I wrote:

> On Monday, January 29 2018, Simon Marchi wrote:
>
>> On 2018-01-29 11:00, Pedro Alves wrote:
>>> On 01/28/2018 04:50 PM, Simon Marchi wrote:
>>>> On 2018-01-28 01:32, Sergio Durigan Junior wrote:
>>>> This means (I just tried it) that it won't show the "[Inferior %d
>>>> detached]\n" message in that case.  So what I would suggest is
>>>> putting
>>>>
>>>>   if (print_inferior_events)
>>>>     printf_unfiltered (_("[Inferior %d detached]\n"), pid);
>>>>
>>>> in its own function, called by both versions of detach_inferior for
>>>> now (bonus, it de-duplicates the printing of the message).  In the
>>>> multi-target branch, remote_target::follow_fork (renamed from
>>>> remote_follow_fork) can call this function in the case where we
>>>> don't have an inferior object.
>>>
>>> But why would we want to print that?  We will have already printed
>>>
>>>   "Detaching after fork from child process PID."
>>>
>>> from the common code.  When native debugging, in this scenario,
>>> we don't call detach_inferior either, right?  Can't see why
>>> we'd want to call it for remote.
>>
>> It's true that it's a bit of a lie to say "[Inferior PID detached]" if
>> there never actually was an inferior for that PID.  Since we never
>> print "[Inferior PID detached]" on native in that case, I am fine with
>> removing the call from remote.c.  Sergio, that would fix the crash you
>> found I think?
>
> I was also unsure about printing the message in this case, because
> there's no real detach happening.  I'm fine with not printing it.  And
> yes, removing the call to "detach_inferior" also fixes the problem.
>
> I'll prepare a patch.

Here's what I have.  WDYT?

I'll address Pedro's comment about changing the "[Inferior PID
detached]" output in another patch.

-- 
Sergio
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From 4a37d08ca6c1aec7f47e2278b0fe78a0038eb9ee Mon Sep 17 00:00:00 2001
From: Sergio Durigan Junior <sergiodj@redhat.com>
Date: Mon, 29 Jan 2018 12:29:21 -0500
Subject: [PATCH] Don't call "detach_inferior" on "remote_follow_fork"

This patch fixes a regression that has been introduced by:

commit bc09b0c14fb713a9aec25e09b78499f3bc2441b5
Date:   Fri Jan 19 11:48:11 2018 -0500

    Make linux_nat_detach/thread_db_detach use the inferior parameter

Consider the following example program:

  #include <unistd.h>

  int
  main (int argc, char *argv[])
  {
    fork ();

    return 0;
  }

When running it under gdbserver:

  # ./gdb/gdbserver/gdbserver --multi --once :2345

And debugging it under GDB, we see a segmentation fault:

  # ./gdb/gdb -q -batch -ex 'set remote exec-file ./a.out' -ex 'tar extended-remote :2345' -ex r ./a.out
  Starting program:
  ...
  [Detaching after fork from child process 16102.]
  Segmentation fault (core dumped)

The problem happens on inferior.c:detach_inferior:

  void
  detach_inferior (inferior *inf)
  {
    /* Save the pid, since exit_inferior_1 will reset it.  */
    int pid = inf->pid;
              ^^^^^^^^^

    exit_inferior_1 (inf, 0);

    if (print_inferior_events)
      printf_unfiltered (_("[Inferior %d detached]\n"), pid);
  }

When this code is called from remote.c:remote_follow_fork, the PID is
valid but there is not 'inferior' associated with it, which means that
'inf == NULL'.

The proper fix here is to not call "detach_inferior" when doing remote
follow-fork, because we don't have an inferior to detach on the host
side.

This has been regtested using BuildBot and no regressions were found.

gdb/ChangeLog:
2018-01-29  Sergio Durigan Junior  <sergiodj@redhat.com>

	* remote.c (remote_follow_fork): Don't call "detach_inferior".
---
 gdb/remote.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/gdb/remote.c b/gdb/remote.c
index 5ac84df0a0..74d18f7b17 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -5206,7 +5206,6 @@ remote_follow_fork (struct target_ops *ops, int follow_child,
 	  child_pid = ptid_get_pid (child_ptid);
 
 	  remote_detach_pid (child_pid);
-	  detach_inferior (child_pid);
 	}
     }
   return 0;
-- 
2.14.3