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

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

On Monday, January 29 2018, Pedro Alves wrote:

> 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.

Done.

>> 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/

Fixed.

>> 
>> 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.

Added.

> and it's good to me.

Pushed.

69ab5edb4d601611ba7b4d05e56689d4b60ca3b1

Thanks,

-- 
Sergio
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