Re: [PATCH v3 2/3] arm64/mm/fault: use original FAR_EL1 value when ARM64_MTE_FAR is supported

[Catalin Marinas]

+ 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}