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

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Date:   Thu, 21 Sep 2017 09:40:20 -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 05:24:10PM +0100, Edward Cree wrote:
> 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?

How about:
r4 = (be16) (u16) r4 
r4 = (le64) (u64) r4 
most C like and most explicit ?
it should be easy to grasp that (be16) cast on big-endian arch is a nop and
swap on little.