netdev - Re: [PATCH net-next] bpf/verifier: improve disassembly of BPF

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Message-ID: <20170921155215.jta52sesbiq54vri@ast-mbp>
Date:   Thu, 21 Sep 2017 08:52:17 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Edward Cree <ecree@...arflare.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH net-next] bpf/verifier: improve disassembly of BPF_END
 instructions

On Thu, Sep 21, 2017 at 04:09:34PM +0100, Edward Cree wrote:
> print_bpf_insn() was treating all BPF_ALU[64] the same, but BPF_END has a
>  different structure: it has a size in insn->imm (even if it's BPF_X) and
>  uses the BPF_SRC (X or K) to indicate which endianness to use.  So it
>  needs different code to print it.
> 
> Signed-off-by: Edward Cree <ecree@...arflare.com>
> ---
> It's not the same format as the new LLVM asm uses, does that matter?
> AFAIK the LLVM format doesn't comprehend BPF_TO_LE, just assumes that all
>  endian ops are necessarily swaps (rather than sometimes nops).

that is being fixed and we will fix asm format too.
Let's pick good format first.

>  kernel/bpf/verifier.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 799b245..e7657a4 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -331,20 +331,29 @@ static void print_bpf_insn(const struct bpf_verifier_env *env,
>  	u8 class = BPF_CLASS(insn->code);
>  
>  	if (class == BPF_ALU || class == BPF_ALU64) {
> -		if (BPF_SRC(insn->code) == BPF_X)
> +		if (BPF_OP(insn->code) == BPF_END) {
> +			if (class == BPF_ALU64)
> +				verbose("BUG_alu64_%02x\n", insn->code);
> +			else
> +				verbose("(%02x) (u%d) r%d %s %s\n",
> +					insn->code, insn->imm, insn->dst_reg,
> +					bpf_alu_string[BPF_END >> 4],
> +					BPF_SRC(insn->code) == BPF_X ? "be" : "le");

yes the bit the same, but please use BPF_SRC(insn->code) == BPF_TO_BE.
imo
(u16) r4 endian be
isn't intuitive.
Can we come up with some better syntax?
Like
bswap16be r4
bswap32le r4

or

to_be16 r4
to_le32 r4

It will be more obvious what's happening.