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

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Date:   Thu, 21 Sep 2017 17:24:10 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.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 21/09/17 16:52, Alexei Starovoitov wrote:
> 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.
Agreed.
>>  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.
Good point.
> imo
> (u16) r4 endian be
> isn't intuitive.
> Can we come up with some better syntax?
> Like
> bswap16be r4
> bswap32le r4
Hmm, I don't like these, since bswapbe is a swap on *le* and a nop on be.
> or
>
> to_be16 r4
> to_le32 r4
And the problem here is that it's not just to_be, it's also from_be.
 Otherwise we could write `(be16) r4 = endian (u16) r4` and be much more
 explicit about what's happening.
Really the operation is something like `cpu_tofrom_be16 r4`, but that also
 seems a bit clumsy and longwinded.  Also it's inconsistent with the other
 ops that all indicate sizes with these (u16) etc casts.
`endian (be16) r4`, perhaps?