+ Peter Collingbourne as he added the SA_EXPOSE_TAGBITS flag.
On Thu, Apr 10, 2025 at 08:47:20AM +0100, Yeoreum Yun wrote:
> Use the original FAR_EL1 value when an MTE tag check fault occurs,
> if ARM64_MTE_FAR is supported.
> This allows reports to include not only the logical tag (memory tag)
> but also the address tag information.
>
> Applications that require this information should install a signal handler with
> the SA_EXPOSE_TAGBITS flag.
> While this introduces a minor ABI change,
> most applications do not set this flag and therefore will not be affected.
It is indeed a minor ABI in that a tag check fault resulting in a
signal will report the bits 63:60 as well, not just 59:56 of the address
(if the signal handler was registered with SA_EXPOSE_TAGBITS).
I don't think user-space would notice but asking Peter.
> Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
> ---
> arch/arm64/mm/fault.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index ec0a337891dd..f21d972f99b1 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -837,9 +837,12 @@ static int do_tag_check_fault(unsigned long far, unsigned long esr,
> /*
> * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN
> * for tag check faults. Set them to corresponding bits in the untagged
> - * address.
> + * address if ARM64_MTE_FAR isn't supported.
> + * Otherwise, bits 63:60 of FAR_EL1 are KNOWN.
> */
> - far = (__untagged_addr(far) & ~MTE_TAG_MASK) | (far & MTE_TAG_MASK);
> + if (!cpus_have_cap(ARM64_MTE_FAR))
> + far = (__untagged_addr(far) & ~MTE_TAG_MASK) | (far & MTE_TAG_MASK);
> +
> do_bad_area(far, esr, regs);
> return 0;
> }
> --
> LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}
