From 282f4a8aa95c5a01b96e55eb5da297560e5f2986 Mon Sep 17 00:00:00 2001 From: Shao Mingyin Date: Mon, 5 Oct 2026 19:02:38 -0600 Subject: [PATCH] riscv: lib: Fix ZBB strnlen wrap-around regression on huge counts The aligned scan boundary is derived from the last valid byte, (s + count - 1). When count is huge (e.g. SIZE_MAX, which FORTIFY strcat/strlcat pass when the destination size is not known at compile time), s + count wraps around and the boundary lands before s, so the ZBB path returns a bogus length. The original implementation (5ba15d419fab) had the same wrap-around in its (s + count) & ~7 boundary computation; after 5d588c684833 the wrapped boundary is caught by the pre-loop guard "bgeu t0, t4, 2f", which then always exits for aligned strings of 8 or more characters and strnlen() returns 8 instead of the real length. This silently truncates strings built by fortified strcat: the dm sysfs name attribute shows "live-bas" instead of "live-base", the truncated name pollutes the udev database, and blivet/anaconda (as well as LVM/dm-crypt/multipath userspace) break on RISC-V systems. Detect the wrap-around and saturate the boundary to the top of the address space, making the scan equivalent to strlen(). The saturation clamps the increment to ~s, so it stays branchless and wrap-free: s + min(count - 1, ~s) == saturating_add(s, count - 1) Normal counts are unaffected. Fixes: 5ba15d419fab ("riscv: lib: add strnlen() implementation") Cc: stable@vger.kernel.org Suggested-by: David Laight Suggested-by: Qingfang Deng Signed-off-by: Shao Mingyin Acked-by: Michael Neuling Tested-by: Troy Mitchell Reviewed-by: Aurelien Jarno Link: https://patch.msgid.link/20260915152656708z04s4oSYY2BGj34F36RZa@zte.com.cn Signed-off-by: Paul Walmsley --- arch/riscv/lib/strnlen.S | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-) diff --git a/arch/riscv/lib/strnlen.S b/arch/riscv/lib/strnlen.S index a8911605c248..5b3bbf0a3098 100644 --- a/arch/riscv/lib/strnlen.S +++ b/arch/riscv/lib/strnlen.S @@ -87,9 +87,25 @@ strnlen_zbb: * Aligned boundary. Use the address of the last valid byte * (s + count - 1) to avoid loading a word past the count * boundary in the loop below. count == 0 is handled above. + * + * Saturate the boundary when s + count would wrap around (very + * large counts, e.g. SIZE_MAX passed by FORTIFY strcat/strlcat + * with a destination whose size is unknown at compile time). + * Without this, the wrapped boundary lands before s and the + * pre-loop guard below always exits, returning a truncated + * length. + * + * Clamping the increment to ~s (== SIZE_MAX - s) keeps the + * computation branchless and wrap-free: + * + * s + min(count - 1, ~s) == saturating_add(s, count - 1) + * + * Saturating makes the scan equivalent to strlen(). */ - add t4, a0, a1 - addi t4, t4, -1 + addi t4, a1, -1 /* count - 1 */ + not t1, a0 /* SIZE_MAX - s */ + minu t4, t4, t1 /* clamp, so s + t4 can never wrap */ + add t4, a0, t4 /* saturated s + count - 1 */ andi t4, t4, -SZREG /* Get the first word. */