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

[Kursad Oney]

Hi Linus, Ard,


On Thu, Aug 10, 2023 at 4:15 AM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
>
> 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/

Thank you for the reviews. I just got back from vacation and will add
this into Russell's tracker.

>
> Yours,
> Linus Walleij

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