[PATCH] arm64: Fix bit-shifting UB in MIDR_AMPERE1

[Geert Uytterhoeven]

Hi Scott,

On Mon, Oct 17, 2022 at 11:01 PM D Scott Phillips
<scott at os.amperecomputing.com> wrote:
> CONFIG_UBSAN_SHIFT with gcc-5 complains that shifting ARM_CPU_IMP_AMPERE
> (0xC0) into bits [31:24] is undefined behavior. Well, sort of, it actually
> spells the error as:
>
>  /kisskb/src/arch/arm64/kernel/proton-pack.c: In function 'spectre_bhb_loop_affected':
>  /kisskb/src/arch/arm64/include/asm/cputype.h:44:2: error: initializer element is not constant
>    (((imp)   << MIDR_IMPLEMENTOR_SHIFT) | \
>    ^
>
> This isn't an issue for other Implementor codes, as all the other codes
> have zero in the top bit.
>
> Fixes: 0e5d5ae837c8ce04 ("arm64: Add AMPERE1 to the Spectre-BHB affected list")
> Reported-by: Geert Uytterhoeven <geert at linux-m68k.org>
> Signed-off-by: D Scott Phillips <scott at os.amperecomputing.com>

Thanks for your patch!

> --- a/arch/arm64/include/asm/cputype.h
> +++ b/arch/arm64/include/asm/cputype.h
> @@ -60,7 +60,7 @@
>  #define ARM_CPU_IMP_FUJITSU            0x46
>  #define ARM_CPU_IMP_HISI               0x48
>  #define ARM_CPU_IMP_APPLE              0x61
> -#define ARM_CPU_IMP_AMPERE             0xC0
> +#define ARM_CPU_IMP_AMPERE             0xC0u

This makes the entry inconsistent with all others.
So we can either add the "u" suffix everywhere, or add a cast to
"u32" to the definitions[1] of MIDR_CPU_MODEL().

[1] There are two: in arch/arm64/include/asm/cputype.h and in
    tools/arch/arm64/include/asm/cputype.h.
    Looks like the second file is a copy of the first, but still lacking the
    AMPERE definitions.
    I guess it should be replaced by a symlink, to keep them in sync.

>  #define ARM_CPU_PART_AEM_V8            0xD0F
>  #define ARM_CPU_PART_FOUNDATION                0xD00

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds