[DTrace-devel] [PATCH] bpf: work around BPF verifier issues with AND and 2-reg branch conditions

[Eugene Loh]

Reviewed-by: Eugene Loh <eugene.loh at oracle.com>
two nits...

On 4/30/24 10:27, Kris Van Hees wrote:
> Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> ---
>   bpf/strchr.S | 22 ++++++++++++----------
>   1 file changed, 12 insertions(+), 10 deletions(-)
>
> diff --git a/bpf/strchr.S b/bpf/strchr.S
> index 8365d9ce..42c6430f 100644
> --- a/bpf/strchr.S
> +++ b/bpf/strchr.S
> @@ -26,8 +26,10 @@
>    *
>    *     // xor the char with every byte;  a match results in NULL byte
>    *     r4 = roundup(r6, 8);
> - *     for (r3 = 0; r3 < r4; r3 += 8)
> - *         ((uint64_t *)dst)[r3] ^= c;
> + *     do {
> + *         r4 -= 8;
> + *         ((uint64_t *)dst)[r4] ^= c;
> + *     } while (r4 > 8);

I think that is wrong, but in any case out of step with the actual 
code.  How about either s/8/7/ or (imho better but then needs change in 
corresponding code) s/>/>=/.

>    *
>    *     // put a safeguard in place, then look for that NULL byte
>    *     dst[r6] = '\0';
> @@ -72,24 +74,24 @@ dt_strchr :
>   	call	BPF_FUNC_probe_read_str	/* r6 = bpf_probe_read_str(dst, STRSZ, src) */
>   	mov	%r6, %r0
>   
> -	jsle	%r6, 0, .Lerror
> +	jsle	%r6, 1, .Lerror
>   
>   	sub	%r6, 1			/* r6-- */
>   
>   	mov	%r4, %r6		/* r4 = roundup(r6, 8) */
>   	add	%r4, 7
> -	and	%r4, -8
> +	rsh	%r4, 3
> +	lsh	%r4, 3

How about a comment on the << >> 3 thing so that some future maintainer 
more easily understands why this looks so funny.

>   	ldxdw	%r1, [%fp+-16]
> -	mov	%r3, 0
> -.Lloop:					/* for (r3 = 0; r3 < r4; r3 += 8) */
> +.Lloop:					/* do { */
> +	sub	%r4, 8			/*	r4 -= 8; */
>   	ldxdw	%r5, [%fp+-24]
> -	add	%r5, %r3
> +	add	%r5, %r4
>   	ldxdw	%r0, [%r5+0]
> -	xor	%r0, %r1		/* 	((uint64_t *)dst)[r3] ^= c; */
> +	xor	%r0, %r1		/* 	((uint64_t *)dst)[r4] ^= c; */
>   	stxdw	[%r5+0], %r0
> -	add	%r3, 8
> -	jlt	%r3, %r4, .Lloop
> +	jgt	%r4, 7, .Lloop		/* } while (r4 > 7); */
>   
>   	ldxdw	%r2, [%fp+-24]
>   	add	%r2, %r6