Re: [PATCH bpf-next v2 4/8] arm32, bpf: add support for unconditional bswap instruction

[Russell King (Oracle)]

On Thu, Sep 07, 2023 at 09:08:46AM +0000, Puranjay Mohan wrote:
> On Thu, Sep 07 2023, Russell King (Oracle) wrote:
> 
> > On Wed, Sep 06, 2023 at 06:33:16PM +0000, Puranjay Mohan wrote:
> >> @@ -1633,8 +1633,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
> >>  	/* dst = htobe(dst) */
> >>  	case BPF_ALU | BPF_END | BPF_FROM_LE:
> >>  	case BPF_ALU | BPF_END | BPF_FROM_BE:
> >> +	/* dst = bswap(dst) */
> >> +	case BPF_ALU64 | BPF_END | BPF_TO_LE:
> >>  		rd = arm_bpf_get_reg64(dst, tmp, ctx);
> >> -		if (BPF_SRC(code) == BPF_FROM_LE)
> >> +		if (BPF_SRC(code) == BPF_FROM_LE && BPF_CLASS(code) != BPF_ALU64)
> >
> > With the addition of the BPF_ALU64 case, I'm wondering why this if() is
> > affected. If you were adding:
> >
> > 	case BPF_ALU64 | BPF_END | BPF_FROM_LE:
> >
> > then maybe there would be a reason, but the BPF_ALU64 | BPF_END |
> > BPF_TO_LE case will never match even the original if() statement.
> 
> The reason is that these mean the same thing.
> from: include/uapi/linux/bpf.h
> 
> #define BPF_TO_LE	0x00	/* convert to little-endian */
> #define BPF_TO_BE	0x08	/* convert to big-endian */
> #define BPF_FROM_LE	BPF_TO_LE
> #define BPF_FROM_BE	BPF_TO_BE
> 
> So, to not cause confusion and follow the earlier cases I can add:
> 
> case BPF_ALU64 | BPF_END | BPF_FROM_LE:
> 
> in the next version.

It might be worth adding a comment after each stating one of:

	/* also BPF_TO_LE */
	/* also BPF_TO_BE */

as appropriate to make this more readable.

Thanks.

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