On 12/3/20 10:12 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.
>
> Since this kind of programs are usually handcrafted rather than
> generated by LLVM, those programs tend to be small. To avoid increasing
> the complexity of BPF JIT, this commit just bumps the number of passes
> to 64 as suggested by Daniel to make it less likely to fail on such cases.
>
Another idea would be to stop trying to reduce size of generated
code after a given number of passes have been attempted.
Because even a limit of 64 wont ensure all 'valid' programs can be JITed.
> Signed-off-by: Gary Lin <glin@xxxxxxxx>
> ---
> arch/x86/net/bpf_jit_comp.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 796506dcfc42..43cc80387548 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -2042,7 +2042,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> * may converge on the last pass. In such case do one more
> * pass to emit the final image.
> */
> - for (pass = 0; pass < 20 || image; pass++) {
> + for (pass = 0; pass < 64 || image; pass++) {
> proglen = do_jit(prog, addrs, image, oldproglen, &ctx);
> if (proglen <= 0) {
> out_image:
>
