linux-kernel - [PATCH] ARM: memset: cast the constant byte to unsigned char

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Message-Id: <20230517181353.381073-1-kursad.oney@broadcom.com>
Date:   Wed, 17 May 2023 14:13:52 -0400
From:   Kursad Oney <kursad.oney@...adcom.com>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        Kursad Oney <kursad.oney@...adcom.com>,
        Russell King <linux@...linux.org.uk>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] ARM: memset: cast the constant byte to unsigned char

memset() description in ISO/IEC 9899:1999 (and elsewhere) says:

	The memset function copies the value of c (converted to an
	unsigned char) into each of the first n characters of the
	object pointed to by s.

The kernel's arm32 memset does not cast c to unsigned char. This results
in the following code to produce erroneous output:

	char a[128];
	memset(a, -128, sizeof(a));

This is because gcc will generally emit the following code before
it calls memset() :

	mov   r0, r7
	mvn   r1, #127        ; 0x7f
	bl    00000000 <memset>

r1 ends up with 0xffffff80 before being used by memset() and the
'a' array will have -128 once in every four bytes while the other
bytes will be set incorrectly to -1 like this (printing the first
8 bytes) :

	test_module: -128 -1 -1 -1
	test_module: -1 -1 -1 -128

The change here is to 'and' r1 with 255 before it is used.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Kursad Oney <kursad.oney@...adcom.com>

---

 arch/arm/lib/memset.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/lib/memset.S b/arch/arm/lib/memset.S
index d71ab61430b2..de75ae4d5ab4 100644
--- a/arch/arm/lib/memset.S
+++ b/arch/arm/lib/memset.S
@@ -17,6 +17,7 @@ ENTRY(__memset)
 ENTRY(mmioset)
 WEAK(memset)
 UNWIND( .fnstart         )
+	and	r1, r1, #255		@ cast to unsigned char
 	ands	r3, r0, #3		@ 1 unaligned?
 	mov	ip, r0			@ preserve r0 as return value
 	bne	6f			@ 1
-- 
2.37.3


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