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

[Pedro Alves <palves at redhat dot com>]

On 01/29/2018 05:36 PM, Sergio Durigan Junior wrote:
> 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;
>   }
> 

Please also mention gdb.base/foll-fork.exp.

> 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'.

s/there is not/there is no/

> 
> 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.

Add something like this here:

 Before bc09b0c1, that call was already a nop (exit_inferior_1 bails
 out early if you pass it a NULL inferior), except that it printed
 "Inferior PID detached" when "set print inferior-events" is on.
 Since native debugging doesn't call detach_inferior in this case,
 removing the call from remote aligns remote debugging output
 with native debugging output further.

and it's good to me.

Thanks,
Pedro Alves