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