Re: [PATCH] bpf,x64: pad NOPs to make images converge more easily

On Mon, Dec 14, 2020 at 11:56:22AM +0800, Gary Lin wrote:

On Fri, Dec 11, 2020 at 09:05:05PM +0100, Daniel Borkmann wrote:

quoted

On 12/11/20 9:19 AM, Gary Lin wrote:

quoted

The x64 bpf jit expects bpf images converge within the given passes, but
it could fail to do so with some corner cases. For example:

       l0:     ldh [4]
       l1:     jeq #0x537d, l2, l40
       l2:     ld [0]
       l3:     jeq #0xfa163e0d, l4, l40
       l4:     ldh [12]
       l5:     ldx #0xe
       l6:     jeq #0x86dd, l41, l7
       l8:     ld [x+16]
       l9:     ja 41

         [... repeated ja 41 ]

       l40:    ja 41
       l41:    ret #0
       l42:    ld #len
       l43:    ret a

This bpf program contains 32 "ja 41" instructions which are effectively
NOPs and designed to be replaced with valid code dynamically. Ideally,
bpf jit should optimize those "ja 41" instructions out when translating
the bpf instructions into x86_64 machine code. However, do_jit() can
only remove one "ja 41" for offset==0 on each pass, so it requires at
least 32 runs to eliminate those JMPs and exceeds the current limit of
passes (20). In the end, the program got rejected when BPF_JIT_ALWAYS_ON
is set even though it's legit as a classic socket filter.

To make the image more likely converge within 20 passes, this commit
pads some instructions with NOPs in the last 5 passes:

1. conditional jumps
   A possible size variance comes from the adoption of imm8 JMP. If the
   offset is imm8, we calculate the size difference of this BPF instruction
   between the previous pass and the current pass and fill the gap with NOPs.
   To avoid the recalculation of jump offset, those NOPs are inserted before
   the JMP code, so we have to subtract the 2 bytes of imm8 JMP when
   calculating the NOP number.

2. BPF_JA
   There are two conditions for BPF_JA.
   a.) nop jumps
     If this instruction is not optimized out in the previous pass,
     instead of removing it, we insert the equivalent size of NOPs.
   b.) label jumps
     Similar to condition jumps, we prepend NOPs right before the JMP
     code.

To make the code concise, emit_nops() is modified to use the signed len and
return the number of inserted NOPs.

To support bpf-to-bpf, a new flag, padded, is introduced to 'struct bpf_prog'
so that bpf_int_jit_compile() could know if the program is padded or not.

Please also add multiple hand-crafted test cases e.g. for bpf-to-bpf calls into
test_verifier (which is part of bpf kselftests) that would exercise this corner
case in x86 jit where we would start to nop pad so that there is proper coverage,
too.

The corner case I had in the commit description is likely being rejected by
the verifier because most of those "ja 41" are unreachable instructions.
Is there any known test case that needs more than 15 passes in x86 jit?

Just an idea. Besides the mentioned corner case, how about making
PADDING_PASSES dynamically configurable (sysfs?) and reusing the existing
test cases? So that we can have a script to set PADDING_PASSES from 1 to 20
and run the bpf selftests separately. This guarantees that the padding
strategy will be applied at least in a certain PADDING_PASSES settings.

Gary Lin