Re: [PATCH v5 1/9] target/arm: Fix mte_checkN

[Peter Maydell <peter.maydell@linaro.org>]

On Fri, 16 Apr 2021 at 19:31, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> We were incorrectly assuming that only the first byte of an MTE access
> is checked against the tags.  But per the ARM, unaligned accesses are
> pre-decomposed into single-byte accesses.  So by the time we reach the
> actual MTE check in the ARM pseudocode, all accesses are aligned.
>
> Therefore, the first failure is always either the first byte of the
> access, or the first byte of the granule.
>
> In addition, some of the arithmetic is off for last-first -> count.
> This does not become directly visible until a later patch that passes
> single bytes into this function, so ptr == ptr_last.
>
> Buglink: https://bugs.launchpad.net/bugs/1921948
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  target/arm/mte_helper.c | 38 +++++++++++++++++---------------------
>  1 file changed, 17 insertions(+), 21 deletions(-)
>
> diff --git a/target/arm/mte_helper.c b/target/arm/mte_helper.c
> index 8be17e1b70..c87717127c 100644
> --- a/target/arm/mte_helper.c
> +++ b/target/arm/mte_helper.c
> @@ -757,10 +757,10 @@ uint64_t mte_checkN(CPUARMState *env, uint32_t desc,
>                      uint64_t ptr, uintptr_t ra)
>  {
>      int mmu_idx, ptr_tag, bit55;
> -    uint64_t ptr_last, ptr_end, prev_page, next_page;
> -    uint64_t tag_first, tag_end;
> -    uint64_t tag_byte_first, tag_byte_end;
> -    uint32_t esize, total, tag_count, tag_size, n, c;
> +    uint64_t ptr_last, prev_page, next_page;
> +    uint64_t tag_first, tag_last;
> +    uint64_t tag_byte_first, tag_byte_last;
> +    uint32_t total, tag_count, tag_size, n, c;
>      uint8_t *mem1, *mem2;
>      MMUAccessType type;
>
> @@ -779,29 +779,27 @@ uint64_t mte_checkN(CPUARMState *env, uint32_t desc,
>
>      mmu_idx = FIELD_EX32(desc, MTEDESC, MIDX);
>      type = FIELD_EX32(desc, MTEDESC, WRITE) ? MMU_DATA_STORE : MMU_DATA_LOAD;
> -    esize = FIELD_EX32(desc, MTEDESC, ESIZE);
>      total = FIELD_EX32(desc, MTEDESC, TSIZE);
>
>      /* Find the addr of the end of the access, and of the last element. */
> -    ptr_end = ptr + total;
> -    ptr_last = ptr_end - esize;
> +    ptr_last = ptr + total - 1;

Code change means the comment needs updating, since we're no longer
finding two things. Change to just
 /* Find the addr of the end of the access */

?

>
>      /* Round the bounds to the tag granule, and compute the number of tags. */
>      tag_first = QEMU_ALIGN_DOWN(ptr, TAG_GRANULE);
> -    tag_end = QEMU_ALIGN_UP(ptr_last, TAG_GRANULE);
> -    tag_count = (tag_end - tag_first) / TAG_GRANULE;
> +    tag_last = QEMU_ALIGN_DOWN(ptr_last, TAG_GRANULE);
> +    tag_count = ((tag_last - tag_first) / TAG_GRANULE) + 1;
>
>      /* Round the bounds to twice the tag granule, and compute the bytes. */
>      tag_byte_first = QEMU_ALIGN_DOWN(ptr, 2 * TAG_GRANULE);
> -    tag_byte_end = QEMU_ALIGN_UP(ptr_last, 2 * TAG_GRANULE);
> +    tag_byte_last = QEMU_ALIGN_DOWN(ptr_last, 2 * TAG_GRANULE);
>
>      /* Locate the page boundaries. */
>      prev_page = ptr & TARGET_PAGE_MASK;
>      next_page = prev_page + TARGET_PAGE_SIZE;
>
> -    if (likely(tag_end - prev_page <= TARGET_PAGE_SIZE)) {
> +    if (likely(tag_last - prev_page <= TARGET_PAGE_SIZE)) {
>          /* Memory access stays on one page. */
> -        tag_size = (tag_byte_end - tag_byte_first) / (2 * TAG_GRANULE);
> +        tag_size = ((tag_byte_last - tag_byte_first) / (2 * TAG_GRANULE)) + 1;
>          mem1 = allocation_tag_mem(env, mmu_idx, ptr, type, total,
>                                    MMU_DATA_LOAD, tag_size, ra);
>          if (!mem1) {
> @@ -815,9 +813,9 @@ uint64_t mte_checkN(CPUARMState *env, uint32_t desc,
>          mem1 = allocation_tag_mem(env, mmu_idx, ptr, type, next_page - ptr,
>                                    MMU_DATA_LOAD, tag_size, ra);
>
> -        tag_size = (tag_byte_end - next_page) / (2 * TAG_GRANULE);
> +        tag_size = ((tag_byte_last - next_page) / (2 * TAG_GRANULE)) + 1;
>          mem2 = allocation_tag_mem(env, mmu_idx, next_page, type,
> -                                  ptr_end - next_page,
> +                                  ptr_last - next_page + 1,
>                                    MMU_DATA_LOAD, tag_size, ra);
>
>          /*
> @@ -838,15 +836,13 @@ uint64_t mte_checkN(CPUARMState *env, uint32_t desc,
>      }
>
>      /*
> -     * If we failed, we know which granule.  Compute the element that
> -     * is first in that granule, and signal failure on that element.
> +     * If we failed, we know which granule.  For the first granule, the
> +     * failure address is @ptr, the first byte accessed.  Otherwise the
> +     * failure address is the first byte of the nth granule.
>       */
>      if (unlikely(n < tag_count)) {
> -        uint64_t fail_ofs;
> -
> -        fail_ofs = tag_first + n * TAG_GRANULE - ptr;
> -        fail_ofs = ROUND_UP(fail_ofs, esize);
> -        mte_check_fail(env, desc, ptr + fail_ofs, ra);
> +        uint64_t fault = (n == 0 ? ptr : tag_first + n * TAG_GRANULE);
> +        mte_check_fail(env, desc, fault, ra);
>      }
>

This pointer arithmetic makes my head hurt. But I think it's right now.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM