linux-kernel - Re: [PATCH] kernel:bpf Remove structure passing and assignment to save stack and no coping structures

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Date:   Sat, 13 Jan 2018 16:43:34 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Karim Eshapa <karim.eshapa@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, ecree@...arflare.com,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] kernel:bpf Remove structure passing and assignment to
 save stack and no coping structures

On Sun, Jan 14, 2018 at 12:03:42AM +0200, Karim Eshapa wrote:
> Use pointers to structure as arguments to function instead of coping
> structures and less stack size. Also transfer TNUM(_v, _m) to
> tnum.h file to be used in differnet files for creating anonymous structures
> statically.
> 
> Signed-off-by: Karim Eshapa <karim.eshapa@...il.com>
...
> +/* Statically tnum constant */
> +#define TNUM(_v, _m)	(struct tnum){.value = _v, .mask = _m}
>  /* Represent a known constant as a tnum. */
>  struct tnum tnum_const(u64 value);
>  /* A completely unknown value */
> @@ -26,7 +28,7 @@ struct tnum tnum_lshift(struct tnum a, u8 shift);
>  /* Shift a tnum right (by a fixed shift) */
>  struct tnum tnum_rshift(struct tnum a, u8 shift);
>  /* Add two tnums, return @a + @b */
> -struct tnum tnum_add(struct tnum a, struct tnum b);
> +void tnum_add(struct tnum *res, struct tnum *a, struct tnum *b);
...
> -	reg_off = tnum_add(reg->var_off, tnum_const(ip_align + reg->off + off));
> +	tnum_add(&reg_off, &reg->var_off, &TNUM(ip_align + reg->off + off, 0));
>  	if (!tnum_is_aligned(reg_off, size)) {
>  		char tn_buf[48];
>  
> @@ -1023,8 +1023,7 @@ static int check_generic_ptr_alignment(struct bpf_verifier_env *env,
>  	/* Byte size accesses are always allowed. */
>  	if (!strict || size == 1)
>  		return 0;
> -
> -	reg_off = tnum_add(reg->var_off, tnum_const(reg->off + off));
> +	tnum_add(&reg_off, &reg->var_off, &TNUM(reg->off + off, 0));
...
> -		dst_reg->var_off = tnum_add(ptr_reg->var_off, off_reg->var_off);
> +		tnum_add(&dst_reg->var_off, &ptr_reg->var_off,
> +			&off_reg->var_off);

I think that looks much worse and error prone.
Is it gnu or intel style of argumnets ? where is src or dest ?
Can the same pointer be used as src and as dst ? etc, etc
I don't think it saves stack either.
I'd rather leave things as-is.