Messages in this thread | | | From | Ard Biesheuvel <> | Date | Thu, 3 Aug 2023 15:59:50 +0200 | Subject | Re: [PATCH] ARM: memset: cast the constant byte to unsigned char |
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On Wed, 17 May 2023 at 20:14, Kursad Oney <kursad.oney@broadcom.com> wrote: > > 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@broadcom.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
Yes, this is clearly a bug. The value in R1 is expanded to 32 bits like this
1: orr r1, r1, r1, lsl #8 orr r1, r1, r1, lsl #16
which assumes that the upper bytes are 0x0, which they are not in this case.
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
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