Bug 216457

KASAN says:

==================================================================
BUG: KASAN: slab-out-of-bounds in _find_next_bit+0x143/0x160 lib/find_bit.c:109
Read of size 8 at addr ffff8880175766b8 by task kworker/1:1/26
...
The buggy address belongs to the object at ffff888017576600
 which belongs to the cache kmalloc-192 of size 192
The buggy address is located 184 bytes inside of
 192-byte region [ffff888017576600, ffff8880175766c0)
...
Memory state around the buggy address:
>ffff888017576680: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
                                        ^
==================================================================

This "address is located 184 bytes inside of 192-byte region" is confusing b/c it does not look like an out-of-bounds access.

What happens here is that the allocation request was for 184 bytes, so the last 8 bytes in the 192-byte slab are poisoned. But KASAN does not store the requested size in the object header, so it just prints the full slab size everywhere.

User-space ASAN does store 48-bit requested size in the header. But KASAN uses additional bytes in the header for:

struct kasan_alloc_meta {
	depot_stack_handle_t aux_stack[2];

So we don't have space for requested size w/o increasing header size (currently should be 16 bytes).

We could either try to infer requested size from the shadow (count poisoned bytes at the end); or improve wording of the message at least to make it clear that 192 is just full slab size.

For context see:
https://lore.kernel.org/all/20220906173154.6f2664c8fc6b83470c5dfea1@linux-foundation.org/