linux-kernel - Re: [PATCH bpf-next 1/1] bpf: fix WARNING in __bpf_prog_ret0

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Message-ID: <2e267b4b-0540-45d8-9310-e127bf95fc63@nbd.name>
Date: Tue, 22 Jul 2025 15:28:13 +0200
From: Felix Fietkau <nbd@....name>
To: KaFai Wan <mannkafai@...il.com>, ast@...nel.org, daniel@...earbox.net,
 john.fastabend@...il.com, andrii@...nel.org, martin.lau@...ux.dev,
 eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
 kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org,
 bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/1] bpf: fix WARNING in __bpf_prog_ret0_warn
 when jit failed

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@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/bpf/6816e34e.a70a0220.254cdc.002c.GAE@google.com
> Fixes: fa9dd599b4da ("bpf: get rid of pure_initcall dependency to enable jits")
> Signed-off-by: KaFai Wan <mannkafai@...il.com>

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.

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

- Felix