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

[Puranjay Mohan]

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.


Thanks,
Puranjay