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

Andy Lutomirski
2019-04-14T16:16:42Z

On Sun, Apr 14, 2019 at 9:02 AM Thomas Gleixner <tglx@linutronix.de> 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@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: linux-mm@kvack.org
> ---
> 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