On Fri, 16 Apr 2021 at 19:31, Richard Henderson
<richard.hender...@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.hender...@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.mayd...@linaro.org>
thanks
-- PMM
