kasan: false use-after-scope warnings with KCOV

[Mark Rutland]

Hi,

As a heads-up, I'm seeing a number of what appear to be false-positive
use-after-scope warnings when I enable both KCOV and KASAN (inline or outline),
when using the Linaro 17.08 GCC7.1.1 for arm64. So far I haven't spotted these
without KCOV selected, and I'm only seeing these for sanitize-use-after-scope.

The reports vary depending on configuration even with the same trigger. I'm not
sure if it's the reporting that's misleading, or whether the detection is going
wrong.

For example, with v4.15-rc1, defconfig + KCOV + KASAN_OUTLINE, I can trigger a
splat:

$ perf record true

[   37.577497] ==================================================================
[   37.584702] BUG: KASAN: use-after-scope in __alloc_pages_nodemask+0x104/0x1608
[   37.591883] Write of size 24 at addr ffff80092d65f160 by task perf/2430
[   37.598452] 
[   37.599944] CPU: 1 PID: 2430 Comm: perf Not tainted 4.15.0-rc1-00001-gaf82bf81ebae #1
[   37.607725] Hardware name: ARM Juno development board (r1) (DT)
[   37.613605] Call trace:
[   37.616051]  dump_backtrace+0x0/0x320
[   37.619700]  show_stack+0x20/0x30
[   37.623005]  dump_stack+0x108/0x174
[   37.626481]  print_address_description+0x60/0x270
[   37.631162]  kasan_report+0x210/0x2f0
[   37.634811]  check_memory_region+0x148/0x198
[   37.639063]  __asan_storeN+0x14/0x20
[   37.642624]  __alloc_pages_nodemask+0x104/0x1608
[   37.647221]  alloc_pages_vma+0xa0/0x2d8
[   37.651042]  wp_page_copy+0x15c/0xee0
[   37.654689]  do_wp_page+0x404/0xa70
[   37.658165]  __handle_mm_fault+0xb28/0x13e0
[   37.662331]  handle_mm_fault+0x290/0x390
[   37.666237]  do_page_fault+0x32c/0x5c0
[   37.669969]  do_mem_abort+0xa8/0x1e0
[   37.673528]  el0_da+0x20/0x24
[   37.676477] 
[   37.677961] The buggy address belongs to the page:
[   37.682730] page:ffff7e0024b597c0 count:0 mapcount:0 mapping:          (null) index:0x0
[   37.690692] flags: 0x1fffc00000000000()
[   37.694518] raw: 1fffc00000000000 0000000000000000 0000000000000000 00000000ffffffff
[   37.702225] raw: 0000000000000000 ffff7e0024b597e0 0000000000000000 0000000000000000
[   37.709922] page dumped because: kasan: bad access detected
[   37.715457] 
[   37.716941] Memory state around the buggy address:
[   37.721709]  ffff80092d65f000: f2 f2 04 f2 f2 f2 f2 f2 f2 f2 00 f2 f2 f2 f2 f2
[   37.728893]  ffff80092d65f080: f2 f2 00 f2 f2 f2 f2 f2 f2 f2 00 f2 f2 f2 f2 f2
[   37.736078] >ffff80092d65f100: f2 f2 00 f2 f2 f2 f2 f2 f2 f2 f8 f8 f8 f8 00 f2
[   37.743257]                                                        ^
[   37.749576]  ffff80092d65f180: f2 f2 f2 f2 f2 f2 00 00 00 00 00 00 00 f2 f3 f3
[   37.756761]  ffff80092d65f200: f3 f3 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[   37.763939] ==================================================================
[   37.771117] Disabling lock debugging due to kernel taint

$ ./scripts/faddr2line vmlinux __alloc_pages_nodemask+0x104/0x1608
__alloc_pages_nodemask+0x104/0x1608:
__alloc_pages_nodemask at mm/page_alloc.c:4215

... which is the declaration+initialisation of a local variable in
__alloc_pages_nodemask:

4208 struct page *
4209 __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
4210                                                         nodemask_t *nodemask)
4211 {
4212         struct page *page;
4213         unsigned int alloc_flags = ALLOC_WMARK_LOW;
4214         gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
4215         struct alloc_context ac = { };

... which is clearly not a use-after-scope bug.

If I separate the declaration and assignment, I get a splat corresponding to the
assignment to ac.

I wondered if we were missing some shadow initialisation, so I hacked a call to
kasan_unpoison_task_stack() into dup_task_struct(), but this had no effect. I
also wondered if this was the result of an overflow caused by instrumentation
bloating the stack, but doubling my stack size (from 32K to 64K) also had no
effect.

Any ideas? I'm a bit confused by this.

Thanks,
Mark.