ARM: 9321/1: 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")
Reviewed-by: Ard Biesheuvel <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Signed-off-by: Kursad Oney <[e-mail]>
Signed-off-by: Russell King (Oracle) <[e-mail]>

arch/arm/lib/memset.S

···
 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