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.
#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
