Re: [patch V3 01/32] mm/slab: Fix broken stack trace storage

[Andy Lutomirski]

On Sun, Apr 14, 2019 at 9:02 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> kstack_end() is broken on interrupt stacks as they are not guaranteed to be
> sized THREAD_SIZE and THREAD_SIZE aligned.
>
> Use the stack tracer instead. Remove the pointless pointer increment at the
> end of the function while at it.
>
> Fixes: 98eb235b7feb ("[PATCH] page unmapping debug") - History tree
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> ---
>  mm/slab.c |   28 ++++++++++++----------------
>  1 file changed, 12 insertions(+), 16 deletions(-)
>
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1470,33 +1470,29 @@ static bool is_debug_pagealloc_cache(str
>  static void store_stackinfo(struct kmem_cache *cachep, unsigned long *addr,
>                             unsigned long caller)
>  {
> -       int size = cachep->object_size;
> +       int size = cachep->object_size / sizeof(unsigned long);
>
>         addr = (unsigned long *)&((char *)addr)[obj_offset(cachep)];
>
> -       if (size < 5 * sizeof(unsigned long))
> +       if (size < 5)
>                 return;
>
>         *addr++ = 0x12345678;
>         *addr++ = caller;
>         *addr++ = smp_processor_id();
> -       size -= 3 * sizeof(unsigned long);
> +#ifdef CONFIG_STACKTRACE
>         {
> -               unsigned long *sptr = &caller;
> -               unsigned long svalue;
> -
> -               while (!kstack_end(sptr)) {
> -                       svalue = *sptr++;
> -                       if (kernel_text_address(svalue)) {
> -                               *addr++ = svalue;
> -                               size -= sizeof(unsigned long);
> -                               if (size <= sizeof(unsigned long))
> -                                       break;
> -                       }
> -               }
> +               struct stack_trace trace = {
> +                       .max_entries    = size - 4;
> +                       .entries        = addr;
> +                       .skip           = 3;
> +               };

This looks correct, but I think that it would have been clearer if you
left the size -= 3 above.  You're still incrementing addr, but you're
not decrementing size, so they're out of sync and the resulting code
is hard to follow.

--Andy