Re: [pushed] gdb/mi: consistently notify user when GDB/MI client uses -thread-select

Andrew Burgess via Gdb-patches
2022-03-23T19:12:42Z

Andrew Burgess <aburgess@redhat.com> writes:

> Jan Vrany via Gdb-patches <gdb-patches@sourceware.org> writes:
>
>> On Wed, 2022-03-23 at 11:16 -0400, Simon Marchi wrote:
>>> > I gave it a go but I'm sorry: everything seem to work as expected
>>> > here. I do not get "[Switching to thread 1 (process 1891548)]" message
>>> > on CLI when -thread-select already selected thread.
>>>
>>> Ok, strange.
>>>
>>> > I must be doing something differently. What commit is failing for you?
>>> > What's your ./configure incantation?
>>>
>>> My configure arguments are:
>>>
>>> 'CC=ccache gcc-11' 'CXX=ccache g++-11' '--disable-binutils' '--disable-gold' '--disable-ld' '--disable-gprof' '--disable-gas' '--with-guile' 'CFLAGS=-g3 -O0
>>> -fdiagnostics-color=always -fmax-errors=1 -fsanitize=address' 'CXXFLAGS=-std=c++11 -g3 -O0 -fdiagnostics-color=always -fmax-errors=1 -fsanitize=address -
>>> D_GLIBCXX_DEBUG=1 -D_GLIBCXX_DEBUG_PEDANTIC=1 -D_GLIBCXX_SANITIZE_VECTOR=1' 'LDFLAGS=-fuse-ld=lld -fsanitize=address' '--prefix=/usr' '--enable-ubsan' '--
>>> with-python=python3' '--with-debuginfod' '--enable-silent-rules' '--enable-werror'
>>>
>>
>> Thanks! Now it is failing for me too!
>> I just recompiled GDB using the above command. I'll have a
>> look tomorrow.
>
> These failures are totally my fault. Sorry!
>
> The patch below should fix the problem. Let me know what you think.

Also, this patch should definitely be back ported to the GDB 12 branch.

Thanks,
Andrew

>
> Thanks,
> Andrew
>
> ---
>
> commit fcac0d17e64a18e0600372c2bed51f9e2d9e9d6a
> Author: Andrew Burgess <aburgess@redhat.com>
> Date: Wed Mar 23 19:00:35 2022 +0000
>
> gdb/mi: fix use after free of frame_info causing spurious notifications
>
> In commit:
>
> commit a2757c4ed693cef4ecc4dcdcb2518353eb6b3c3f
> Date: Wed Mar 16 15:08:22 2022 +0000
>
> gdb/mi: consistently notify user when GDB/MI client uses -thread-select
>
> Changes were made to GDB to address some inconsistencies in when
> notifications are sent from a MI terminal to a CLI terminal (when
> multiple terminals are in use, see new-ui command).
>
> Unfortunately, in order to track when the currently selected frame has
> changed, that commit grabs a frame_info pointer before and after an MI
> command has executed, and compares the pointers to see if the frame
> has changed.
>
> This is not safe.
>
> If the frame cache is deleted for any reason then the frame_info
> pointer captured before the command started, is no longer valid, and
> any comparisons based on that pointer are undefined.
>
> This was leading to random test failures for some folk, see:
>
> https://sourceware.org/pipermail/gdb-patches/2022-March/186867.html
>
> This commit changes GDB so we no longer hold frame_info pointers, but
> instead store the frame_id and frame_level, this is safe even when the
> frame cache is flushed.
>
> diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
> index abd033b22ae..4f32bb58939 100644
> --- a/gdb/mi/mi-main.c
> +++ b/gdb/mi/mi-main.c
> @@ -1974,27 +1974,40 @@ struct user_selected_context
> {
> /* Constructor. */
> user_selected_context ()
> - : m_previous_ptid (inferior_ptid),
> - m_previous_frame (deprecated_safe_get_selected_frame ())
> - { /* Nothing. */ }
> + : m_previous_ptid (inferior_ptid)
> + {
> + save_selected_frame (&m_previous_frame_id, &m_previous_frame_level);
> + }
>
> /* Return true if the user selected context has changed since this object
> was created. */
> bool has_changed () const
> {
> + /* Grab details of the currently selected frame, for comparison. */
> + frame_id current_frame_id;
> + int current_frame_level;
> + save_selected_frame (&current_frame_id, &current_frame_level);
> +
> + /* If we end up trying to compare two invalid frame-id's then these
> + will always report themselves as not equal. However, if we are at
> + the top-most level of the stack then we don't want to consider this
> + as a frame change. */
> return ((m_previous_ptid != null_ptid
> && inferior_ptid != null_ptid
> && m_previous_ptid != inferior_ptid)
> - || m_previous_frame != deprecated_safe_get_selected_frame ());
> + || current_frame_level != m_previous_frame_level
> + || (current_frame_level != -1
> + && !frame_id_eq (current_frame_id, m_previous_frame_id)));
> }
> private:
> /* The previously selected thread. This might be null_ptid if there was
> no previously selected thread. */
> ptid_t m_previous_ptid;
>
> - /* The previously selected frame. This might be nullptr if there was no
> - previously selected frame. */
> - frame_info *m_previous_frame;
> + /* The previously selected frame. The frame_id might be null_frame_id
> + if no frame is currently selected. */
> + frame_id m_previous_frame_id;
> + int m_previous_frame_level;
> };
>
> static void