git.ipfire.org Git - thirdparty/openwrt.git/blob

   1 From: KaFai Wan <kafai.wan@linux.dev>
   2 Date: Tue, 9 Sep 2025 22:46:14 +0800
   3 Subject: [PATCH] bpf: Allow fall back to interpreter for programs with stack
   4  size <= 512
   5
   6 OpenWRT users reported regression on ARMv6 devices after updating to latest
   7 HEAD, where tcpdump filter:
   8
   9 tcpdump "not ether host 3c37121a2b3c and not ether host 184ecbca2a3a \
  10 and not ether host 14130b4d3f47 and not ether host f0f61cf440b7 \
  11 and not ether host a84b4dedf471 and not ether host d022be17e1d7 \
  12 and not ether host 5c497967208b and not ether host 706655784d5b"
  13
  14 fails with warning: "Kernel filter failed: No error information"
  15 when using config:
  16  # CONFIG_BPF_JIT_ALWAYS_ON is not set
  17  CONFIG_BPF_JIT_DEFAULT_ON=y
  18
  19 The issue arises because commits:
  20 1. "bpf: Fix array bounds error with may_goto" changed default runtime to
  21    __bpf_prog_ret0_warn when jit_requested = 1
  22 2. "bpf: Avoid __bpf_prog_ret0_warn when jit fails" returns error when
  23    jit_requested = 1 but jit fails
  24
  25 This change restores interpreter fallback capability for BPF programs with
  26 stack size <= 512 bytes when jit fails.
  27
  28 Reported-by: Felix Fietkau <nbd@nbd.name>
  29 Closes: https://lore.kernel.org/bpf/2e267b4b-0540-45d8-9310-e127bf95fc63@nbd.name/
  30 Fixes: 6ebc5030e0c5 ("bpf: Fix array bounds error with may_goto")
  31 Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
  32 Acked-by: Eduard Zingerman <eddyz87@gmail.com>
  33 Link: https://lore.kernel.org/r/20250909144614.2991253-1-kafai.wan@linux.dev
  34 Signed-off-by: Alexei Starovoitov <ast@kernel.org>
  35 ---
  36
  37 --- a/kernel/bpf/core.c
  38 +++ b/kernel/bpf/core.c
  39 @@ -2299,8 +2299,7 @@ static unsigned int __bpf_prog_ret0_warn
  40                                          const struct bpf_insn *insn)
  41  {
  42         /* If this handler ever gets executed, then BPF_JIT_ALWAYS_ON
  43 -        * is not working properly, or interpreter is being used when
  44 -        * prog->jit_requested is not 0, so warn about it!
  45 +        * is not working properly, so warn about it!
  46          */
  47         WARN_ON_ONCE(1);
  48         return 0;
  49 @@ -2385,8 +2384,9 @@ out:
  50         return ret;
  51  }
  52
  53 -static void bpf_prog_select_func(struct bpf_prog *fp)
  54 +static bool bpf_prog_select_interpreter(struct bpf_prog *fp)
  55  {
  56 +       bool select_interpreter = false;
  57  #ifndef CONFIG_BPF_JIT_ALWAYS_ON
  58         u32 stack_depth = max_t(u32, fp->aux->stack_depth, 1);
  59         u32 idx = (round_up(stack_depth, 32) / 32) - 1;
  60 @@ -2395,15 +2395,16 @@ static void bpf_prog_select_func(struct
  61          * But for non-JITed programs, we don't need bpf_func, so no bounds
  62          * check needed.
  63          */
  64 -       if (!fp->jit_requested &&
  65 -           !WARN_ON_ONCE(idx >= ARRAY_SIZE(interpreters))) {
  66 +       if (idx < ARRAY_SIZE(interpreters)) {
  67                 fp->bpf_func = interpreters[idx];
  68 +               select_interpreter = true;
  69         } else {
  70                 fp->bpf_func = __bpf_prog_ret0_warn;
  71         }
  72  #else
  73         fp->bpf_func = __bpf_prog_ret0_warn;
  74  #endif
  75 +       return select_interpreter;
  76  }
  77
  78  /**
  79 @@ -2422,7 +2423,7 @@ struct bpf_prog *bpf_prog_select_runtime
  80         /* In case of BPF to BPF calls, verifier did all the prep
  81          * work with regards to JITing, etc.
  82          */
  83 -       bool jit_needed = fp->jit_requested;
  84 +       bool jit_needed = false;
  85
  86         if (fp->bpf_func)
  87                 goto finalize;
  88 @@ -2431,7 +2432,8 @@ struct bpf_prog *bpf_prog_select_runtime
  89             bpf_prog_has_kfunc_call(fp))
  90                 jit_needed = true;
  91
  92 -       bpf_prog_select_func(fp);
  93 +       if (!bpf_prog_select_interpreter(fp))
  94 +               jit_needed = true;
  95
  96         /* eBPF JITs can rewrite the program in case constant
  97          * blinding is active. However, in case of error during