Re: [PATCH bpf-next 1/1] bpf: fix WARNING in __bpf_prog_ret0_warn when jit failed

[Yonghong Song]

On 7/25/25 10:30 AM, Yonghong Song wrote:
> On 7/22/25 6:28 AM, Felix Fietkau wrote:
> > Hi,
> >
> > On 26.05.25 15:33, KaFai Wan wrote:
> > > syzkaller reported an issue:
> > >
> > > WARNING: CPU: 3 PID: 217 at kernel/bpf/core.c:2357 
> > > __bpf_prog_ret0_warn+0xa/0x20 kernel/bpf/core.c:2357
> > > Modules linked in:
> > > CPU: 3 UID: 0 PID: 217 Comm: kworker/u32:6 Not tainted 
> > > 6.15.0-rc4-syzkaller-00040-g8bac8898fe39 #0 PREEMPT(full)
> > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 
> > > 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> > > Workqueue: ipv6_addrconf addrconf_dad_work
> > > RIP: 0010:__bpf_prog_ret0_warn+0xa/0x20 kernel/bpf/core.c:2357
> > > RSP: 0018:ffffc900031f6c18 EFLAGS: 00010293
> > > RAX: 0000000000000000 RBX: ffffc9000006e000 RCX: 1ffff9200000dc06
> > > RDX: ffff8880234ba440 RSI: ffffffff81ca6979 RDI: ffff888031e93040
> > > RBP: ffffc900031f6cb8 R08: 0000000000000001 R09: 0000000000000000
> > > R10: 0000000000000000 R11: 0000000000000000 R12: ffff88802b61e010
> > > R13: ffff888031e93040 R14: 00000000000000a0 R15: ffff88802c3d4800
> > > FS:  0000000000000000(0000) GS:ffff8880d6ce2000(0000) 
> > > knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 000055557b6d2ca8 CR3: 000000002473e000 CR4: 0000000000352ef0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > Call Trace:
> > >   <TASK>
> > >   bpf_dispatcher_nop_func include/linux/bpf.h:1316 [inline]
> > >   __bpf_prog_run include/linux/filter.h:718 [inline]
> > >   bpf_prog_run include/linux/filter.h:725 [inline]
> > >   cls_bpf_classify+0x74a/0x1110 net/sched/cls_bpf.c:105
> > >   ...
> > >
> > > When creating bpf program, 'fp->jit_requested' depends on 
> > > bpf_jit_enable.
> > > Currently the value of bpf_jit_enable is available from 0 to 2, 0 
> > > means use
> > > interpreter and not jit, 1 and 2 means need to jit. When
> > > CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set
> > > to 1, when it's not set or disabled, we can set bpf_jit_enable via 
> > > proc.
> > >
> > > This issue is triggered because of CONFIG_BPF_JIT_ALWAYS_ON is not set
> > > and bpf_jit_enable is set to 1, causing the arch to attempt JIT the 
> > > prog,
> > > but jit failed due to FAULT_INJECTION. As a result, incorrectly
> > > treats the program as valid, when the program runs it calls
> > > `__bpf_prog_ret0_warn` and triggers the WARN_ON_ONCE(1).
> > >
> > > Reported-by: syzbot+0903f6d7f285e41cdf10@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Closes: 
> > > https://lore.kernel.org/bpf/6816e34e.a70a0220.254cdc.002c.GAE@xxxxxxxxxx 
> > >
> > > Fixes: fa9dd599b4da ("bpf: get rid of pure_initcall dependency to 
> > > enable jits")
> > > Signed-off-by: KaFai Wan <mannkafai@xxxxxxxxx>
> >
> > I think this patch may have caused a regression in configurations 
> > with CONFIG_BPF_JIT_DEFAULT_ON=y when programs can't be JITed. 
> > Attaching the program fails with error -ENOTSUPP.
>
> Could you explain why there is an issue here?
> CONFIG_BPF_JIT_DEFAULT_ON=y but prog cannot be jit'ed. So the end 
> result is to return -ENOTSUPP.
> It looks okay to me since the jit is required but jit failed, the only 
> choice for the kernel
> is to return an error.

BTW, you mentioned programs cannot be jited. Could you explain why programs cannot be jitted?
It would be strange that a program cannot be jitted but can be interpreted.

> > Please see https://github.com/openwrt/openwrt/issues/19405 for more 
> > information.
> >
> > - Felix