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
2014-07-30T12:05:26Z

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