[uclibc-ng-devel] Re: Bug in memset on ARM

From 0a45e40834eeb8fc42f74645be715ca14432a767 Mon Sep 17 00:00:00 2001 From: Tom Bannink <tombannink(a)gmail.com> Date: Tue, 12 Apr 2022 11:15:41 +0200 Subject: [PATCH] Fix bug in ARM memset implementation The ARM implementation of memset has a bug when the fill-value is negative or outside the [0, 255] range. To reproduce: char array[256]; memset(array, -5, 256); This is supposed to fill the array with int8 values -5, -5, -5, ... . On ARM, this does not work because the implementation assumes the high bytes of the fill-value argument are already zero. However in this test case they are filled with 1-bits. The aarch64 and x86_64 implementations do not have this problem: they first convert the fill-value to an unsigned byte following the specification of memset. With GCC one can use `memset(ptr, (-5 & 0xFF), size)` as a workaround, but for clang users that does not work: clang optimizes the `& 0xFF` away because it assumes that memset will do it. --- libc/string/arm/memset.S | 2 ++ 1 file changed, 2 insertions(+) diff --git a/libc/string/arm/memset.S b/libc/string/arm/memset.S index 412270f50..f4b30b3dc 100644 --- a/libc/string/arm/memset.S +++ b/libc/string/arm/memset.S @@ -32,6 +32,7 @@ memset: cmp r2, #8 @ at least 8 bytes to do? bcc 2f + uxtb r1, r1 lsl r3, r1, #8 orr r1, r3 lsl r3, r1, #16 @@ -68,6 +69,7 @@ memset: mov a4, a1 cmp a3, $8 @ at least 8 bytes to do? blo 2f + uxtb a2, a2 orr a2, a2, a2, lsl $8 orr a2, a2, a2, lsl $16 1: -- 2.35.1