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