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Message-ID: <401418b7-248c-42a3-ba74-9b2b2959e36c@linux.dev>
Date: Tue, 5 Aug 2025 10:45:51 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: KaFai Wan <kafai.wan@...ux.dev>, ast@...nel.org, daniel@...earbox.net,
john.fastabend@...il.com, andrii@...nel.org, martin.lau@...ux.dev,
eddyz87@...il.com, song@...nel.org, kpsingh@...nel.org, sdf@...ichev.me,
haoluo@...gle.com, jolsa@...nel.org, mrpre@....com, mannkafai@...il.com,
bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Felix Fietkau <nbd@....name>
Subject: Re: [PATCH bpf-next 1/1] bpf: Allow fall back to interpreter for
programs with stack size <= 512
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@....name>
> Closes: https://lore.kernel.org/bpf/2e267b4b-0540-45d8-9310-e127bf95fc63@nbd.name/
> 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@...ux.dev>
> ---
> 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
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