linux-kernel - Re: [PATCH -mm] -funsigned-char, x86: make struct p4_event

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Date:   Thu, 20 Oct 2022 10:28:06 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        mm-commits@...r.kernel.org, torvalds@...ux-foundation.org,
        masahiroy@...nel.org, keescook@...omium.org,
        gregkh@...uxfoundation.org, andriy.shevchenko@...ux.intel.com
Subject: Re: [PATCH -mm] -funsigned-char, x86: make struct p4_event_bind::cntr
 signed array

On Thu, Oct 20, 2022 at 3:49 AM Alexey Dobriyan <adobriyan@...il.com> wrote:
>
> struct p4_event_bind::cntr[][] should be signed because of
> the following code:
>
>         int i, j;
>         for (i = 0; i < P4_CNTR_LIMIT; i++) {
>           ===>  j = bind->cntr[thread][i];
>                 if (j != -1 && !test_bit(j, used_mask))
>                         return j;
>         }
>
> Making this member unsigned will make "j" 255 and fail "j != -1"
> comparison.
>
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>

Nice catch.

Reviewed-by: Jason A. Donenfeld <Jason@...c4.com>

>  arch/x86/events/intel/p4.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- a/arch/x86/events/intel/p4.c
> +++ b/arch/x86/events/intel/p4.c
> @@ -24,7 +24,7 @@ struct p4_event_bind {
>         unsigned int escr_msr[2];               /* ESCR MSR for this event */
>         unsigned int escr_emask;                /* valid ESCR EventMask bits */
>         unsigned int shared;                    /* event is shared across threads */
> -       char cntr[2][P4_CNTR_LIMIT];            /* counter index (offset), -1 on absence */
> +       signed char cntr[2][P4_CNTR_LIMIT];     /* counter index (offset), -1 on absence */
>  };

This is fine, but I wonder if this is a good occasion to use `s8` instead?

Jason