OpenWRT users reported regression on ARMv6 devices after updating to latest HEAD, where tcpdump filter: tcpdump "not ether host 3c37121a2b3c and not ether host 184ecbca2a3a \ and not ether host 14130b4d3f47 and not ether host f0f61cf440b7 \ and not ether host a84b4dedf471 and not ether host d022be17e1d7 \ and not ether host 5c497967208b and not ether host 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") Signed-off-by: KaFai Wan <kafai.wan@xxxxxxxxx> --- changes: v3: - Remove the selftest (Puranjay and Eduard) v2: - Addressed comments from Alexei - Add selftest https://lore.kernel.org/all/20250813152958.3107403-1-kafai.wan@xxxxxxxxx/ v1: https://lore.kernel.org/all/20250805115513.4018532-1-kafai.wan@xxxxxxxxx/ --- kernel/bpf/core.c | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-) diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c index f8ac77d08ca7..e4568d44e827 100644 --- a/kernel/bpf/core.c +++ b/kernel/bpf/core.c @@ -2366,8 +2366,7 @@ 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! + * is not working properly, so warn about it! */ WARN_ON_ONCE(1); return 0; @@ -2468,8 +2467,9 @@ static int bpf_check_tail_call(const struct bpf_prog *fp) return ret; } -static void bpf_prog_select_func(struct bpf_prog *fp) +static bool bpf_prog_select_interpreter(struct bpf_prog *fp) { + bool select_interpreter = false; #ifndef CONFIG_BPF_JIT_ALWAYS_ON u32 stack_depth = max_t(u32, fp->aux->stack_depth, 1); u32 idx = (round_up(stack_depth, 32) / 32) - 1; @@ -2478,15 +2478,16 @@ 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]; + select_interpreter = true; } else { fp->bpf_func = __bpf_prog_ret0_warn; } #else fp->bpf_func = __bpf_prog_ret0_warn; #endif + return select_interpreter; } /** @@ -2505,7 +2506,7 @@ 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; @@ -2514,7 +2515,8 @@ struct bpf_prog *bpf_prog_select_runtime(struct bpf_prog *fp, int *err) bpf_prog_has_kfunc_call(fp)) jit_needed = true; - bpf_prog_select_func(fp); + if (!bpf_prog_select_interpreter(fp)) + jit_needed = true; /* eBPF JITs can rewrite the program in case constant * blinding is active. However, in case of error during -- 2.43.0
