Re: [PATCHv2] gdb/mi: fix use after free of frame_info causing spurious notifications

Andrew Burgess via Gdb-patches
2022-03-29T10:10:47Z

Simon Marchi <simark@simark.ca> writes:

> On 2022-03-24 14:52, Andrew Burgess via Gdb-patches wrote:
>> Simon,
>>
>> Thanks for the feedback. I believe this addresses all your points.
>> What do you think?
>>
>> Thanks,
>> Andrew
>>
>> ---
>>
>> 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 pointer captured before the command started, is no longer valid, and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.
>> ---
>> gdb/mi/mi-main.c | 43 +++++++++++++++++++++++++++++++++----------
>> 1 file changed, 33 insertions(+), 10 deletions(-)
>>
>> diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
>> index abd033b22ae..28d4afc28c0 100644
>> --- a/gdb/mi/mi-main.c
>> +++ b/gdb/mi/mi-main.c
>> @@ -1974,27 +1974,50 @@ 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
>> {
>> - return ((m_previous_ptid != null_ptid
>> - && inferior_ptid != null_ptid
>> - && m_previous_ptid != inferior_ptid)
>> - || m_previous_frame != deprecated_safe_get_selected_frame ());
>> + /* Did the selected thread change? */
>> + if (m_previous_ptid != null_ptid && inferior_ptid != null_ptid
>> + && m_previous_ptid != inferior_ptid)
>> + return true;
>> +
>> + /* 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);
>> +
>> + /* Did the selected frame level change? */
>> + if (current_frame_level != m_previous_frame_level)
>> + return true;
>> +
>> + /* Did the selected frame id change? If the innermost frame is
>> + selected then the level will be -1, and the frame-id will be
>> + null_frame_id. As comparing null_frame_id with itself always
>> + reports not-equal, we only do the equality test if we have something
>> + other than the innermost frame selected. */
>> + if (current_frame_level != -1
>> + && !frame_id_eq (current_frame_id, m_previous_frame_id))
>> + return true;
>> +
>> + /* Nothing changed! */
>> + return false;
>> }
>> 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. If the innermost frame is selected
>> + then the frame_id will be null_frame_id, and the level will be -1. */
>
> I noticed you changed this from "if no frame is currently selected" to
> "if the innermost frame is selected". I think that both can happen,
> actually:
>
> - if you execute an MI command while the target is running, there is no
> selected frame
> - if you execute an MI command while the innermost frame is selected
>
> So my understanding is that you could mention both (although it might
> become a bit redundant with the big comment on top of selected_frame_id,
> in frame.c).
>
> LGTM in any case.

I updated the comment as you suggested and pushed this patch to mater
and gdb-12-branch.

Thanks,
Andrew