Re: [PATCH] ARM: memset: cast the constant byte to unsigned char

[Linus Walleij]

On Wed, May 17, 2023 at 8:14 PM Kursad Oney <kursad.oney@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>

Wow you found this old thing!
Reviewed-by: Linus Walleij <linus.walleij@xxxxxxxxxx>

Can you please put this into Russell's patch tracker?
https://www.arm.linux.org.uk/developer/

Yours,
Linus Walleij