Re: [PATCH] handle unwind tables that are embedded within unwinding code, [PR111731]

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Mar 15, 2024 at 11:31 AM Thomas Neumann
<thomas.neumann@in.tum.de> wrote:
>
> Original bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111731
> Given that this is a regression, is this okay for gcc 13 and mainline?

It does look straightforward but I hope Jason or Florian can provide the ACK.

Thanks,
Richard.

> The unwinding mechanism registers both the code range and the unwind
> table itself within a b-tree lookup structure. That data structure
> assumes that is consists of non-overlappping intervals. This
> becomes a problem if the unwinding table is embedded within the
> code itself, as now the intervals do overlap.
>
> To fix this problem we now keep the unwind tables in a separate
> b-tree, which prevents the overlap.
>
> libgcc/ChangeLog:
>         PR libgcc/111731
>         * unwind-dw2-fde.c: Split unwind ranges if they contain the
>         unwind table.
> ---
>   libgcc/unwind-dw2-fde.c | 37 +++++++++++++++++++++----------------
>   1 file changed, 21 insertions(+), 16 deletions(-)
>
> diff --git a/libgcc/unwind-dw2-fde.c b/libgcc/unwind-dw2-fde.c
> index 61a578d097e..9d503545677 100644
> --- a/libgcc/unwind-dw2-fde.c
> +++ b/libgcc/unwind-dw2-fde.c
> @@ -48,6 +48,7 @@ typedef __UINTPTR_TYPE__ uintptr_type;
>   #include "unwind-dw2-btree.h"
>
>   static struct btree registered_frames;
> +static struct btree registered_objects;
>   static bool in_shutdown;
>
>   static void
> @@ -58,6 +59,7 @@ release_registered_frames (void)
>     /* Release the b-tree and all frames. Frame releases that happen later are
>      * silently ignored */
>     btree_destroy (&registered_frames);
> +  btree_destroy (&registered_objects);
>     in_shutdown = true;
>   }
>
> @@ -103,6 +105,21 @@ static __gthread_mutex_t object_mutex;
>   #endif
>   #endif
>
> +#ifdef ATOMIC_FDE_FAST_PATH
> +// Register the pc range for a given object in the lookup structure.
> +static void
> +register_pc_range_for_object (uintptr_type begin, struct object *ob)
> +{
> +  // Register the object itself to know the base pointer on deregistration.
> +  btree_insert (&registered_objects, begin, 1, ob);
> +
> +  // Register the frame in the b-tree
> +  uintptr_type range[2];
> +  get_pc_range (ob, range);
> +  btree_insert (&registered_frames, range[0], range[1] - range[0], ob);
> +}
> +#endif
> +
>   /* Called from crtbegin.o to register the unwind info for an object.  */
>
>   void
> @@ -124,13 +141,7 @@ __register_frame_info_bases (const void *begin, struct object *ob,
>   #endif
>
>   #ifdef ATOMIC_FDE_FAST_PATH
> -  // Register the object itself to know the base pointer on deregistration.
> -  btree_insert (&registered_frames, (uintptr_type) begin, 1, ob);
> -
> -  // Register the frame in the b-tree
> -  uintptr_type range[2];
> -  get_pc_range (ob, range);
> -  btree_insert (&registered_frames, range[0], range[1] - range[0], ob);
> +  register_pc_range_for_object ((uintptr_type) begin, ob);
>   #else
>     init_object_mutex_once ();
>     __gthread_mutex_lock (&object_mutex);
> @@ -178,13 +189,7 @@ __register_frame_info_table_bases (void *begin, struct object *ob,
>     ob->s.b.encoding = DW_EH_PE_omit;
>
>   #ifdef ATOMIC_FDE_FAST_PATH
> -  // Register the object itself to know the base pointer on deregistration.
> -  btree_insert (&registered_frames, (uintptr_type) begin, 1, ob);
> -
> -  // Register the frame in the b-tree
> -  uintptr_type range[2];
> -  get_pc_range (ob, range);
> -  btree_insert (&registered_frames, range[0], range[1] - range[0], ob);
> +  register_pc_range_for_object ((uintptr_type) begin, ob);
>   #else
>     init_object_mutex_once ();
>     __gthread_mutex_lock (&object_mutex);
> @@ -232,7 +237,7 @@ __deregister_frame_info_bases (const void *begin)
>
>   #ifdef ATOMIC_FDE_FAST_PATH
>     // Find the originally registered object to get the base pointer.
> -  ob = btree_remove (&registered_frames, (uintptr_type) begin);
> +  ob = btree_remove (&registered_objects, (uintptr_type) begin);
>
>     // Remove the corresponding PC range.
>     if (ob)
> @@ -240,7 +245,7 @@ __deregister_frame_info_bases (const void *begin)
>         uintptr_type range[2];
>         get_pc_range (ob, range);
>         if (range[0] != range[1])
> -    btree_remove (&registered_frames, range[0]);
> +       btree_remove (&registered_frames, range[0]);
>       }
>
>     // Deallocate the sort array if any.
> --
> 2.43.0
>