Re: [PATCH] [gdb/tui] Make tui_setup_io more robust

[Tom de Vries <tdevries@suse.de>]

On 4/2/26 11:18 AM, Andrew Burgess wrote:
> Tom de Vries <tdevries@suse.de> writes:
> 
>> The following sequence of events may happen when enabling TUI:
>> - tui_enable () is called,
>> - a SIGFPE happens, before tui_setup_io (1) is called,
>> - the SIGFPE triggers handle_fatal_signal, which calls tui_disable (),
>> - during tui_disable (), tui_setup_io (0) is called, and
>> - tui_setup_io (0) tries to restore things that were saved during a previous
>>    tui_setup_io (1) call, but tui_setup_io (1) was never called so the saving
>>    never happened.
>>
>> This can cause current_ui.m_ui_stderr to be nullptr, which then can cause a
>> crash in sig_write (PR33918).
> 
> If this bug number is worth mentioning, then we should add a 'Bug:...'
> tag to the commit message.  I like them if for nothing else they are
> clickable in my terminal :)
Hi Andrew,

thanks for the review.

I didn't add the tag, because I usually only add the tag if the commit 
claims to fix the PR, and I considered the problem fixed by this patch 
to be different from the one in the PR (I could have filed a another PR, 
but didn't bother).   But that is a bit of a strict approach, so anyway, 
added.

>> Fix this by:
>> - adding a variable tui_io_mode, and
>> - using the variable to bail out of tui_setup_io in case the current mode
>>    doesn't change.
> 
> What's one more global, the tui already has so many!

I tried to explore a scenario where this mattered, and ran into a 
problem, for which I filed a PR ( 
https://sourceware.org/pipermail/gdb-prs/2026q2/047033.html ).
But I might also file a PR for the generic problem.

> This seems a reasonable change to me.
> 
> Approved-By: Andrew Burgess <aburgess@redhat.com>
Thanks, pushed.

- Tom

> Thanks,
> Andrew
> 
> 
>>
>> Tested on x86_64-linux.
>> ---
>>   gdb/tui/tui-io.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>>
>> diff --git a/gdb/tui/tui-io.c b/gdb/tui/tui-io.c
>> index f673fbf36f6..5a3bc604592 100644
>> --- a/gdb/tui/tui-io.c
>> +++ b/gdb/tui/tui-io.c
>> @@ -824,6 +824,10 @@ tui_rl_display_match_list (char **matches, int len, int max)
>>     gdb_display_match_list (matches, len, max, &displayer);
>>   }
>>   
>> +/* Whether the IO is setup for curses (1) or non-curses (0).  */
>> +
>> +static int tui_io_mode;
>> +
>>   /* Setup the IO for curses or non-curses mode.
>>      - In non-curses mode, readline and gdb use the standard input and
>>      standard output/error directly.
>> @@ -837,6 +841,14 @@ tui_setup_io (int mode)
>>   {
>>     extern int _rl_echoing_p;
>>   
>> +  if (tui_io_mode == mode)
>> +    {
>> +      /* Nothing to do.  Also, this makes sure that we don't try to restore
>> +	 things before having saved them. */
>> +      return;
>> +    }
>> +  tui_io_mode = mode;
>> +
>>     if (mode)
>>       {
>>         /* Ensure that readline has been initialized before saving any
>>
>> base-commit: 646982f4295bf0a1e64867606d496c34c9a15a0c
>> -- 
>> 2.51.0
>