Re: [RFC] kasan: include the hashed pointer for an object's location

[Dmitry Vyukov]

On Tue, Oct 22, 2019 at 4:19 AM Lyude Paul <lyude@xxxxxxxxxx> wrote:
>
> The vast majority of the kernel that needs to print out pointers as a
> way to keep track of a specific object in the kernel for debugging
> purposes does so using hashed pointers, since these are "good enough".
> Ironically, the one place we don't do this is within kasan. While
> simply printing a hashed version of where an out of bounds memory access
> occurred isn't too useful, printing out the hashed address of the object
> in question usually is since that's the format most of the kernel is
> likely to be using in debugging output.
>
> Of course this isn't perfect though-having the object's originating
> address doesn't help users at all that need to do things like printing
> the address of a struct which is embedded within another struct, but
> it's certainly better then not printing any hashed addresses. And users
> which need to handle less trivial cases like that can simply fall back
> to careful usage of %px.
>
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Cc: Sean Paul <sean@xxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: kasan-dev@xxxxxxxxxxxxxxxx
> ---
>  mm/kasan/report.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 621782100eaa..0a5663fee1f7 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -128,8 +128,9 @@ static void describe_object_addr(struct kmem_cache *cache, void *object,
>         int rel_bytes;
>
>         pr_err("The buggy address belongs to the object at %px\n"
> -              " which belongs to the cache %s of size %d\n",
> -               object, cache->name, cache->object_size);
> +              " (aka %p) which belongs to the cache\n"
> +              " %s of size %d\n",
> +              object, object, cache->name, cache->object_size);

Hi Lyude,

This only prints hashed address for heap objects, but
print_address_description() has 4 different code paths for different
types of addresses (heap, global, stack, page). Plus there is a case
for address without shadow.
Should we print the hashed address at least for all cases in
print_address_description()?


>         if (!addr)
>                 return;
> --
> 2.21.0
>