Re: [PATCH] [gdb/build] Fix frame_list position in frame.c

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

On 5/3/23 20:47, Simon Marchi wrote:
> On 5/3/23 13:58, Tom de Vries wrote:
>> In commit 995a34b1772 ("Guard against frame.c destructors running before
>> frame-info.c's") the following problem was addressed.
>>
>> The frame_info_ptr destructor:
>> ...
>>    ~frame_info_ptr ()
>>    {
>>      frame_list.erase (frame_list.iterator_to (*this));
>>    }
>> ...
>> uses frame_list, which is a static member of class frame_info_ptr,
>> instantiated in frame-info.c:
>> ...
>> intrusive_list<frame_info_ptr> frame_info_ptr::frame_list;
>> ...
>>
>> Then there's a static frame_info_pointer variable named selected_frame in
>> frame.c:
>> ...
>> static frame_info_ptr selected_frame;
>> ...
>>
>> Because the destructor of selected_frame uses frame_list, its destructor needs
>> to be called before the destructor of frame_list.
>>
>> But because they're in different compilation units, the initialization order and
>> consequently destruction order is not guarantueed.
>>
>> The commit fixed this by handling the case that the destructor of frame_list
>> is called first, adding a check on is_linked ():
>> ...
>>     ~frame_info_ptr ()
>>     {
>> -    frame_list.erase (frame_list.iterator_to (*this));
>> +    /* If this node has static storage, it may be deleted after
>> +       frame_list.  Attempting to erase ourselves would then trigger
>> +       internal errors, so make sure we are still linked first.  */
>> +    if (is_linked ())
>> +      frame_list.erase (frame_list.iterator_to (*this));
>>     }
>> ...
>>
>> However, since then frame_list has been moved into frame.c, and
>> initialization/destruction order is guarantueed inside a compilation unit.
>>
>> Revert aforementioned commit, and fix the destruction order problem by moving
>> frame_list before selected_frame.
>>
>> Reverting the commit is another way of fixing the already fixed
>> Wdangling-pointer warning reported in PR build/30413, in a different way than
>> commit 9b0ccb1ebae ("Pass const frame_info_ptr reference for
>> skip_[language_]trampoline").
>>
>> Tested on x86_64-linux.
>>
>> PR build/30413
>> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30413
>> ---
>>   gdb/frame.c | 11 +++++++----
>>   gdb/frame.h |  9 ++++-----
>>   2 files changed, 11 insertions(+), 9 deletions(-)
>>
>> diff --git a/gdb/frame.c b/gdb/frame.c
>> index 36fb02f3c8e..531eadf3d54 100644
>> --- a/gdb/frame.c
>> +++ b/gdb/frame.c
>> @@ -1733,6 +1733,13 @@ get_current_frame (void)
>>   static frame_id selected_frame_id = null_frame_id;
>>   static int selected_frame_level = -1;
>>   
>> +/* See frame.h.  This definition should come before any definition of a static
>> +   frame_info_ptr, to ensure that frame_list is destroyed after any static
>> +   frame_info_ptr.  This is  necessary because the destructor of frame_info_ptr
> 
> Spurious double space.
> 

Fixed.

>> +   uses frame_list.  */
>> +
>> +intrusive_list<frame_info_ptr> frame_info_ptr::frame_list;
>> +
>>   /* The cached frame_info object pointing to the selected frame.
>>      Looked up on demand by get_selected_frame.  */
>>   static frame_info_ptr selected_frame;
>> @@ -3275,10 +3282,6 @@ maintenance_print_frame_id (const char *args, int from_tty)
>>   
>>   /* See frame-info-ptr.h.  */
>>   
>> -intrusive_list<frame_info_ptr> frame_info_ptr::frame_list;
>> -
>> -/* See frame-info-ptr.h.  */
>> -
>>   frame_info_ptr::frame_info_ptr (struct frame_info *ptr)
>>     : m_ptr (ptr)
>>   {
>> diff --git a/gdb/frame.h b/gdb/frame.h
>> index 6ed8db0af56..ed19dfdc090 100644
>> --- a/gdb/frame.h
>> +++ b/gdb/frame.h
>> @@ -254,11 +254,10 @@ class frame_info_ptr : public intrusive_list_node<frame_info_ptr>
>>   
>>     ~frame_info_ptr ()
>>     {
>> -    /* If this node has static storage, it may be deleted after
>> -       frame_list.  Attempting to erase ourselves would then trigger
>> -       internal errors, so make sure we are still linked first.  */
>> -    if (is_linked ())
>> -      frame_list.erase (frame_list.iterator_to (*this));
>> +    /* If this node has static storage, it should be be deleted before
>> +       frame_list.  Verify this by checking that it is still in the list.  */
>> +    gdb_assert (is_linked ());
>> +    frame_list.erase (frame_list.iterator_to (*this));
> 
> The assert is a bit redundant with the assertions in
> intrusive_list::erase_element:
> 
>      gdb_assert (elem_node->prev != INTRUSIVE_LIST_UNLINKED_VALUE);
>      gdb_assert (elem_node->next != INTRUSIVE_LIST_UNLINKED_VALUE);
> 
> I would maybe remove the assert, but keep the comment (at least the
> first sentence)?
> 

Ack, I checked by doing:
...
+static frame_info_ptr bad_frame;
  intrusive_list<frame_info_ptr> frame_info_ptr::frame_list;
...
and indeed those assertions trigger, so done.

> In any case, this LGTM, thanks for doing this.
> 
> Approved-By: Simon Marchi <simon.marchi@efficios.com>
> 

Committed, thanks for the review.

- Tom