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Message-ID: <c8c870e25c07aee5c84c84aa62cebd655ff53f50.camel@linux.dev>
Date: Wed, 06 Aug 2025 18:57:43 +0800
From: KaFai Wan <kafai.wan@...ux.dev>
To: Yonghong Song <yonghong.song@...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 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/
>
> > #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
>
--
Thanks,
KaFai
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