Re: [PATCH 0/2] kselftest: Make MTE memory access robust

[Muhammad Usama Anjum <usama.anjum@arm.com>]

On 22/09/2026 11:59 am, Vladimir Murzin wrote:
> Currently MTE selftests assume that they can skip (expected) MTE
> faults by advancing the PC by 4 in mte_default_handler(), but this is
> not generally safe, as the faults are triggered from arbitrary library
> functions (e.g. memset() and memcpy()). For instance, memset() could
> be implemented as simple as:
> 
>       mov   x3, x0       // copy pointer
>       add   x4, x0, x2   // calculate end
>     loop:
>       strb  w1, [x3], #1 // faulting access
>       cmp   x3, x4
>       b.ne  loop
>       ret
> 
> which leads to an infinite loop since X3 could not be updated.
> 
> Or, it could be having advanced implementation (FEAT_MOPS):
> 
>       mov   x3, x0         // copy pointer
>       setp  [x3]!, x2!, x1 // prologue
>       setm  [x3]!, x2!, x1 // main
>       sete  [x3]!, x2!, x1 // epilogue
>       ret
> 
> with faulting access at prologue advancing PC to main could lead to
> infinite loop due to main could be triggering unaligned access fault
> which kernel fixing up by advancing PC back to prologue.
> 
> Address this by introducing memory access helpers with predictable
> behavior, allowing faults to be safely handled, and switch the MTE
> selftests over to them.

Tested on Pixel 8:

Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>

> 
> Vladimir Murzin (2):
>   kselftest/arm64/mte: Introduce MTE safe memory accessors
>   kselftest/arm64/mte: Use MTE safe memory accessors
> 
>  .../selftests/arm64/mte/check_buffer_fill.c   | 10 ++---
>  .../selftests/arm64/mte/check_child_memory.c  |  6 +--
>  .../arm64/mte/check_hugetlb_options.c         |  4 +-
>  .../selftests/arm64/mte/check_mmap_options.c  | 10 ++---
>  .../arm64/mte/check_tags_inclusion.c          |  4 +-
>  .../selftests/arm64/mte/mte_common_util.h     |  4 ++
>  .../testing/selftests/arm64/mte/mte_helper.S  | 44 +++++++++++++++++++
>  7 files changed, 65 insertions(+), 17 deletions(-)
> 


-- 
Thanks,
Usama