Re: [PATCH v10 1/7] lib/stackdepot: Fix first entry having a 0-handle

[Peter Collingbourne]

On Thu, Feb 15, 2024 at 4:25 PM Peter Collingbourne <pcc@xxxxxxxxxx> wrote:
>
> On Thu, Feb 15, 2024 at 3:37 PM Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:
> >
> > On Thu, Feb 15, 2024 at 10:58 PM Oscar Salvador <osalvador@susede> wrote:
> > >
> > > The very first entry of stack_record gets a handle of 0, but this is wrong
> > > because stackdepot treats a 0-handle as a non-valid one.
> > > E.g: See the check in stack_depot_fetch()
> > >
> > > Fix this by adding and offset of 1.
> > >
> > > 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.
> > >
> > > Co-developed-by: Marco Elver <elver@xxxxxxxxxx>
> > > Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> > > Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> > > ---
> > >  lib/stackdepot.c | 16 +++++++++-------
> > >  1 file changed, 9 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> > > index 4a7055a63d9f..c043a4186bc5 100644
> > > --- a/lib/stackdepot.c
> > > +++ b/lib/stackdepot.c
> > > @@ -45,15 +45,16 @@
> > >  #define DEPOT_POOL_INDEX_BITS (DEPOT_HANDLE_BITS - DEPOT_OFFSET_BITS - \
> > >                                STACK_DEPOT_EXTRA_BITS)
> > >  #define DEPOT_POOLS_CAP 8192
> > > +/* The pool_index is offset by 1 so the first record does not have a 0 handle. */
> > >  #define DEPOT_MAX_POOLS \
> > > -       (((1LL << (DEPOT_POOL_INDEX_BITS)) < DEPOT_POOLS_CAP) ? \
> > > -        (1LL << (DEPOT_POOL_INDEX_BITS)) : DEPOT_POOLS_CAP)
> > > +       (((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
> > > +        (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
> > >
> > >  /* Compact structure that stores a reference to a stack. */
> > >  union handle_parts {
> > >         depot_stack_handle_t handle;
> > >         struct {
> > > -               u32 pool_index  : DEPOT_POOL_INDEX_BITS;
> > > +               u32 pool_index  : DEPOT_POOL_INDEX_BITS; /* pool_index is offset by 1 */
>
> Can we rename this, say to pool_index_plus_1? This will make the code
> a bit clearer, as well as make it possible for debugging tools such as
> drgn [1] to be able to tell when the off-by-one was introduced and
> adapt accordingly.
>
> Peter
>
> [1] https://github.com/osandov/drgn/pull/376

Unfortunately this message was not acted upon, and it looks like akpm
picked up the patch and it made its way into Linus's tree. So I sent a
followup to fix this here:
https://lore.kernel.org/all/20240402001500.53533-1-pcc@xxxxxxxxxx/

Peter