On Tue, 2025-08-05 at 10:45 -0700, Yonghong Song wrote: > > > On 8/5/25 4:55 AM, KaFai Wan wrote: > > OpenWRT users reported regression on ARMv6 devices after updating > > to latest > > HEAD, where tcpdump filter: > > > > tcpdump -i mon1 \ > > "not wlan addr3 3c37121a2b3c and not wlan addr2 184ecbca2a3a \ > > and not wlan addr2 14130b4d3f47 and not wlan addr2 f0f61cf440b7 \ > > and not wlan addr3 a84b4dedf471 and not wlan addr3 d022be17e1d7 \ > > and not wlan addr3 5c497967208b and not wlan addr2 706655784d5b" > > > > fails with warning: "Kernel filter failed: No error information" > > when using config: > > # CONFIG_BPF_JIT_ALWAYS_ON is not set > > CONFIG_BPF_JIT_DEFAULT_ON=y > > > > The issue arises because commits: > > 1. "bpf: Fix array bounds error with may_goto" changed default > > runtime to > > __bpf_prog_ret0_warn when jit_requested = 1 > > 2. "bpf: Avoid __bpf_prog_ret0_warn when jit fails" returns error > > when > > jit_requested = 1 but jit fails > > > > This change restores interpreter fallback capability for BPF > > programs with > > stack size <= 512 bytes when jit fails. > > > > Reported-by: Felix Fietkau <nbd@xxxxxxxx> > > Closes: > > https://lore.kernel.org/bpf/2e267b4b-0540-45d8-9310-e127bf95fc63@xxxxxxxx/ > > Fixes: 6ebc5030e0c5 ("bpf: Fix array bounds error with may_goto") > > Fixes: 86bc9c742426 ("bpf: Avoid __bpf_prog_ret0_warn when jit > > fails") > > Signed-off-by: KaFai Wan <kafai.wan@xxxxxxxxx> > > --- > > kernel/bpf/core.c | 12 +++++++----- > > 1 file changed, 7 insertions(+), 5 deletions(-) > > > > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c > > index 5d1650af899d..2d86bd4b0b97 100644 > > --- a/kernel/bpf/core.c > > +++ b/kernel/bpf/core.c > > @@ -2366,8 +2366,8 @@ static unsigned int > > __bpf_prog_ret0_warn(const void *ctx, > > const struct bpf_insn > > *insn) > > { > > /* If this handler ever gets executed, then > > BPF_JIT_ALWAYS_ON > > - * is not working properly, or interpreter is being used > > when > > - * prog->jit_requested is not 0, so warn about it! > > + * or may_goto may cause stack size > 512 is not working > > properly, > > + * so warn about it! > > */ > > WARN_ON_ONCE(1); > > return 0; > > @@ -2478,10 +2478,10 @@ static void bpf_prog_select_func(struct > > bpf_prog *fp) > > * But for non-JITed programs, we don't need bpf_func, so > > no bounds > > * check needed. > > */ > > - if (!fp->jit_requested && > > - !WARN_ON_ONCE(idx >= ARRAY_SIZE(interpreters))) { > > + if (idx < ARRAY_SIZE(interpreters)) { > > fp->bpf_func = interpreters[idx]; > > } else { > > + WARN_ON_ONCE(!fp->jit_requested); > > fp->bpf_func = __bpf_prog_ret0_warn; > > } > > Your logic here is to do interpreter even if fp->jit_requested is > true. > This is different from the current implementation. > > Also see below code: > > static unsigned int __bpf_prog_ret0_warn(const void *ctx, > const struct bpf_insn > *insn) > { > /* If this handler ever gets executed, then > BPF_JIT_ALWAYS_ON > * is not working properly, or interpreter is being used > when > * prog->jit_requested is not 0, so warn about it! > */ > WARN_ON_ONCE(1); > return 0; > } > > > It mentions to warn if the interpreter is being used when > prog->jit_requested is not 0. > > So if prog->jit_requested is not 0, it is expected not to use > interpreter. > The commit 6ebc5030e0c5 ("bpf: Fix array bounds error with may_goto") [1] this patch fix change the code to that, before this commit it was: static unsigned int __bpf_prog_ret0_warn(const void *ctx, const struct bpf_insn *insn) { /* If this handler ever gets executed, then BPF_JIT_ALWAYS_ON * is not working properly, so warn about it! */ WARN_ON_ONCE(1); return 0; } And static void bpf_prog_select_func(struct bpf_prog *fp) { #ifndef CONFIG_BPF_JIT_ALWAYS_ON u32 stack_depth = max_t(u32, fp->aux->stack_depth, 1); fp->bpf_func = interpreters[(round_up(stack_depth, 32) / 32) - 1]; #else fp->bpf_func = __bpf_prog_ret0_warn; #endif } so it can fall back to the interpreter when jit fails. And this fit the intent of bpf_prog_select_runtime(), see comment: /** * bpf_prog_select_runtime - select exec runtime for BPF program * @fp: bpf_prog populated with BPF program * @err: pointer to error variable * * Try to JIT eBPF program, if JIT is not available, use interpreter. * The BPF program will be executed via bpf_prog_run() function. * * Return: the &fp argument along with &err set to 0 for success or * a negative errno code on failure */ struct bpf_prog *bpf_prog_select_runtime(struct bpf_prog *fp, int *err) And this: bpf_prog_select_func(fp); /* eBPF JITs can rewrite the program in case constant * blinding is active. However, in case of error during * blinding, bpf_int_jit_compile() must always return a * valid program, which in this case would simply not * be JITed, but falls back to the interpreter. */ if (!bpf_prog_is_offloaded(fp->aux)) { The commit [1] mismatch the intent of bpf_prog_select_runtime(), so it should be fixed. [1] https://lore.kernel.org/all/20250214091823.46042-2-mrpre@xxxxxxx/ > > > #else > > @@ -2505,7 +2505,7 @@ struct bpf_prog > > *bpf_prog_select_runtime(struct bpf_prog *fp, int *err) > > /* In case of BPF to BPF calls, verifier did all the prep > > * work with regards to JITing, etc. > > */ > > - bool jit_needed = fp->jit_requested; > > + bool jit_needed = false; > > > > if (fp->bpf_func) > > goto finalize; > > @@ -2515,6 +2515,8 @@ struct bpf_prog > > *bpf_prog_select_runtime(struct bpf_prog *fp, int *err) > > jit_needed = true; > > > > bpf_prog_select_func(fp); > > + if (fp->bpf_func == __bpf_prog_ret0_warn) > > + jit_needed = true; > > > > /* eBPF JITs can rewrite the program in case constant > > * blinding is active. However, in case of error during > -- Thanks, KaFai
