Re: [pushed+7.8] Re: [PATCH] Fix "attach" command vs user input race [Re: Regression for attach from stdin [Re: [pushed] Re: [PATCH v6 0/2] enable target-async by default]]

[Pedro Alves <palves at redhat dot com>]

On 07/29/2014 10:48 PM, Doug Evans wrote:
> On Wed, Jul 9, 2014 at 10:09 AM, Pedro Alves <palves@redhat.com> wrote:
>> On 07/09/2014 05:37 PM, Doug Evans wrote:
>>
>>> spurious change
>>
>> Fixed.
>>
>>> I like this a lot better.  Thanks.
>>> The patch is ok with me, modulo removing the spurious change.
>>
>> Here's what I pushed to both master and gdb-7.8-branch.
>>
>> Thanks.
>>
>> -------------
>> From 1fe2833b6dd03602ba86aa334e81466ea9abe66a Mon Sep 17 00:00:00 2001
>> From: Pedro Alves <palves@redhat.com>
>> Date: Wed, 9 Jul 2014 17:52:58 +0100
>> Subject: [PATCH] Fix "attach" command vs user input race
>>
>> On async targets, a synchronous attach is done like this:
>>
>>    #1 - target_attach is called (PTRACE_ATTACH is issued)
>>    #2 - a continuation is installed
>>    #3 - we go back to the event loop
>>    #4 - target reports stop (SIGSTOP), event loop wakes up, and
>>         attach continuation is called
>>    #5 - among other things, the continuation calls
>>         target_terminal_inferior, which removes stdin from the event
>>         loop
>>
>> Note that in #3, GDB is still processing user input.  If the user is
>> fast enough, e.g., with something like:
>>
>>   echo -e "attach PID\nset xxx=1" | gdb
>>
>> ... then the "set" command is processed before the attach completes.
>>
>> We get worse behavior even, if input is a tty and therefore
>> readline/editing is enabled, with e.g.,:
>>
>>  (gdb) attach PID\nset xxx=1
>>
>> we then crash readline/gdb, with:
>>
>>  Attaching to program: attach-wait-input, process 14537
>>  readline: readline_callback_read_char() called with no handler!
>>  Aborted
>>  $
>>
>> Fix this by calling target_terminal_inferior before #3 above.
>>
>> The test covers both scenarios by running with editing/readline forced
>> to both on and off.
>>
>> gdb/
>> 2014-07-09  Pedro Alves  <palves@redhat.com>
>>
>>         * infcmd.c (attach_command_post_wait): Don't call
>>         target_terminal_inferior here.
>>         (attach_command): Call it here instead.
>>
>> gdb/testsuite/
>> 2014-07-09  Pedro Alves  <palves@redhat.com>
>>
>>         * gdb.base/attach-wait-input.exp: New file.
>>         * gdb.base/attach-wait-input.c: New file.
> 
> Hi.
> 
> Is this TODO still needed after this patch?
> 
> infcmd.c:
> 
> /*
>  * TODO:
>  * Should save/restore the tty state since it might be that the
>  * program to be debugged was started on this tty and it wants
>  * the tty in some state other than what we want.  If it's running
>  * on another terminal or without a terminal, then saving and
>  * restoring the tty state is a harmless no-op.
>  * This only needs to be done if we are attaching to a process.
>  */
> 

As usual, git blame/log is your friend...

That's been in place for over 20 years.   In bd5635a1 (1991), we see:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/*
 * TODO:
 * Should save/restore the tty state since it might be that the
 * program to be debugged was started on this tty and it wants
 * the tty in some state other than what we want.  If it's running
 * on another terminal or without a terminal, then saving and
 * restoring the tty state is a harmless no-op.
 * This only needs to be done if we are attaching to a process.
 */

/*
 * attach_command --
 * takes a program started up outside of gdb and ``attaches'' to it.
 * This stops it cold in its tracks and allows us to start tracing it.
 * For this to work, we must be able to send the process a
 * signal and we must have the same effective uid as the program.
 */
void
attach_command (args, from_tty)
     char *args;
     int from_tty;
{
  target_attach (args, from_tty);
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


So clearly the TODO has been stale for a long while.
We've been saving/restoring the tty state way before
my patch.

Thanks,
Pedro Alves