[DTrace-devel] [PATCH] Switch ALU instruction to fit a signed 32-bit IMM

[Kris Van Hees]

On Thu, Aug 10, 2023 at 01:47:42PM -0400, eugene.loh--- via DTrace-devel wrote:
> From: Eugene Loh <eugene.loh at oracle.com>
> 
> ALU instructions take a signed 32-bit IMM.  Strictly speaking,
> 0xffffffff overflows it.  Switch to -1 to be safe.
> 
> Signed-off-by: Eugene Loh <eugene.loh at oracle.com>

Reviewed-byb: Kris Van Hees <kris.van.hees at oracle.com>

... but I would suggest you change the subject.  You are not really switching
ALU instructions (it is still an AND) - you are just adjusting the immediate
operand because the instruction ex[ects a signed 32-bit IMM value.  So perhaps
make it:

	Fix overflow in immediate operand to 'and'

> ---
>  bpf/index.S | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/bpf/index.S b/bpf/index.S
> index f64a24dd..c8b1d7cd 100644
> --- a/bpf/index.S
> +++ b/bpf/index.S
> @@ -36,7 +36,7 @@
>   *                       //   perfect match    any mismatches
>   *                       //        == 0              != 0
>   *     r0 |= (r0 >> 32); //        == 0              != 0
> - *     r0 &= 0xffffffff; //        == 0              >  0
> + *     r0 &= -1;         //        == 0              >  0
>   *     r0 -= 1;          //        <  0              >= 0
>   *     r0 >>= 63;        //        == 1              == 0
>   *
> @@ -69,7 +69,7 @@ dt_index_match:
>  	mov	%r4, %r0
>  	rsh	%r4, 32
>  	or	%r0, %r4
> -	and	%r0, 0xffffffff
> +	and	%r0, -1
>  	sub	%r0, 1
>  	rsh	%r0, 63
>  
> -- 
> 2.18.4
> 
> 
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