On 4/8/24 2:57 PM, Greg Kroah-Hartman wrote:
> 6.8-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Peter Collingbourne <pcc@xxxxxxxxxx>
>
> [ Upstream commit a6c1d9cb9a68bfa4512248419c4f4d880d19fe90 ]
>
> Commit 3ee34eabac2a ("lib/stackdepot: fix first entry having a 0-handle")
> changed the meaning of the pool_index field to mean "the pool index plus
> 1". This made the code accessing this field less self-documenting, as
> well as causing debuggers such as drgn to not be able to easily remain
> compatible with both old and new kernels, because they typically do that
> by testing for presence of the new field. Because stackdepot is a
> debugging tool, we should make sure that it is debugger friendly.
> Therefore, give the field a different name to improve readability as well
> as enabling debugger backwards compatibility.
>
> This is needed in 6.9, which would otherwise become an odd release with
> the new semantics and old name so debuggers wouldn't recognize the new
> semantics there.
This got me curious so I did check what's going on, so mentioning the result
here others don't need to repeat that.
> Fixes: 3ee34eabac2a ("lib/stackdepot: fix first entry having a 0-handle")
It's because this was backported to 6.8.2 despite:
> This bug has been lurking since the very beginning of stackdepot, but no
> one really cared as it seems. Because of that I am not adding a Fixes
> tag.
Then indeed this commit would be needed too in 6.8.y in order to not confuse
drgn and co.
I forgot that the stable MM extemption is based on source code paths, not
commits going through the mm tree, so it didn't cover stackdepot itself.
> Link: https://lkml.kernel.org/r/20240402001500.53533-1-pcc@xxxxxxxxxx
> Link: https://linux-review.googlesource.com/id/Ib3e70c36c1d230dd0a118dc22649b33e768b9f88
> Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Reviewed-by: Alexander Potapenko <glider@xxxxxxxxxx>
> Acked-by: Marco Elver <elver@xxxxxxxxxx>
> Acked-by: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Omar Sandoval <osandov@xxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
> include/linux/stackdepot.h | 7 +++----
> lib/stackdepot.c | 4 ++--
> 2 files changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/stackdepot.h b/include/linux/stackdepot.h
> index c4b5ad57c0660..bf0136891a0f2 100644
> --- a/include/linux/stackdepot.h
> +++ b/include/linux/stackdepot.h
> @@ -44,10 +44,9 @@ typedef u32 depot_stack_handle_t;
> union handle_parts {
> depot_stack_handle_t handle;
> struct {
> - /* pool_index is offset by 1 */
> - u32 pool_index : DEPOT_POOL_INDEX_BITS;
> - u32 offset : DEPOT_OFFSET_BITS;
> - u32 extra : STACK_DEPOT_EXTRA_BITS;
> + u32 pool_index_plus_1 : DEPOT_POOL_INDEX_BITS;
> + u32 offset : DEPOT_OFFSET_BITS;
> + u32 extra : STACK_DEPOT_EXTRA_BITS;
> };
> };
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index ee4bbe6513aa4..ee830f14afb78 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -330,7 +330,7 @@ static struct stack_record *depot_pop_free_pool(void **prealloc, size_t size)
> stack = current_pool + pool_offset;
>
> /* Pre-initialize handle once. */
> - stack->handle.pool_index = pool_index + 1;
> + stack->handle.pool_index_plus_1 = pool_index + 1;
> stack->handle.offset = pool_offset >> DEPOT_STACK_ALIGN;
> stack->handle.extra = 0;
> INIT_LIST_HEAD(&stack->hash_list);
> @@ -441,7 +441,7 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
> const int pools_num_cached = READ_ONCE(pools_num);
> union handle_parts parts = { .handle = handle };
> void *pool;
> - u32 pool_index = parts.pool_index - 1;
> + u32 pool_index = parts.pool_index_plus_1 - 1;
> size_t offset = parts.offset << DEPOT_STACK_ALIGN;
> struct stack_record *stack;
>
