[PATCH v2 2/3] arm64/mm/fault: use original FAR_EL1 value when ARM64_MTE_FAR is supported

[Yeoreum Yun]

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.

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;
 }
-- 
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