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[PATCH v1 1/1] arm64: Fix strrchr() matching of null terminator
- To: xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>
- Date: Tue, 19 May 2026 01:43:53 +0200
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- Cc: sstabellini@xxxxxxxxxx, julien@xxxxxxx, bertrand.marquis@xxxxxxx, michal.orzel@xxxxxxx, Volodymyr_Babchuk@xxxxxxxx, edgar.iglesias@xxxxxxx
- Delivery-date: Mon, 18 May 2026 23:44:12 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
The generic Xen strrchr() implementation returns a pointer to the string
terminator when searching for '\0', matching the standard C semantics.
The ARM64 assembly version stopped as soon as it loaded the terminator and
returned the previous match pointer instead. This made strrchr("", '\0')
return NULL.
Compare the loaded byte against the requested character before deciding
whether to stop at the terminator, so the terminator itself can be returned
when it is the requested character.
Fixes: 42c4eb6a83 ("xen: arm64: assembly optimised mem* and str*")
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>
---
xen/arch/arm/arm64/lib/strrchr.S | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/xen/arch/arm/arm64/lib/strrchr.S b/xen/arch/arm/arm64/lib/strrchr.S
index 81033c0822..31f304b183 100644
--- a/xen/arch/arm/arm64/lib/strrchr.S
+++ b/xen/arch/arm/arm64/lib/strrchr.S
@@ -30,11 +30,10 @@ FUNC(strrchr)
mov x3, #0
and w1, w1, #0xff
1: ldrb w2, [x0], #1
- cbz w2, 2f
cmp w2, w1
- b.ne 1b
+ b.ne 2f
sub x3, x0, #1
- b 1b
-2: mov x0, x3
+2: cbnz w2, 1b
+ mov x0, x3
ret
END(strrchr)
--
2.43.0
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