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Message-ID: <9b96d710-8e21-4d32-9229-30bc99dfb4f4@linux.dev>
Date: Wed, 6 Aug 2025 10:37:47 -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, 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 8/6/25 3:57 AM, KaFai Wan wrote:
> 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@....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.
>>
> 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@163.com/
Okay, indeed the above [1] changed the behavior. Maybe Alexei can
comment whether we should restore to the behavior before [1].
>
>>> #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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