From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-178.mta1.migadu.com (out-178.mta1.migadu.com [95.215.58.178]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4F85322333D for ; Wed, 22 Oct 2025 16:57:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.178 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761152265; cv=none; b=Bbg8qkLYrPTOPT7a16CSPIcwGJIseA4jCwYowmEwLqgZ5Yu0mYuVT15foXHzgN1i4p+q0mb6yK7UPRCbYkiiDmos8DQQ9wb7hOg4ilUrojk8tASTr1Ek/fYO7UTCpl7sdFuwmbPFhC+7SyY9O0OeYp9oR4Mj0fWJ/MyShSuAI50= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761152265; c=relaxed/simple; bh=ytm0ejnHzvO2wfwThkZuPsjyf68Bo1+nSesP3d8YWjM=; h=Message-ID:Date:MIME-Version:Subject:To:References:From: In-Reply-To:Content-Type; b=msFxGvucDATI/byUBgmQ46TZHpmmANaUTeVapyjlP194OiHFM8erJGxXEhuXVXD2aODfZowjla4pOM1uXP/GIQ8DDmXww13RhU7d4elkrO8akxx7cZK+4AUc2d5fY02ye5Oz4pPfhN77V3cq6I128dT0Ho8L0rlZvEpSr081Mts= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=ric7Gw9I; arc=none smtp.client-ip=95.215.58.178 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="ric7Gw9I" Message-ID: <14371cf8-e49a-4c68-b763-fa7563a9c764@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761152250; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=NFRxEnfnc4VPVFTxGFAyLIkhm0TFgIiDXqEVdaKjeWU=; b=ric7Gw9IqSWQA5EEA9T60kJ2JkX47fhNif7Nk8x8lWfHx6a8ccyGLoFuCef9ytW6lmB/Ce dHKi0ZDFO0O0tRIut8KK/qTu9qklJ/Fd459a4oOOra66uvj64C2JgGmVS6+2DqgxUs/9V+ PMZ4MrNeeXtjGgkHlQrOfWwWY58vD50= Date: Wed, 22 Oct 2025 09:57:22 -0700 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [syzbot] [bpf?] WARNING in bpf_bprintf_prepare (3) Content-Language: en-GB To: syzbot , andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org, chandna.sahil@gmail.com, daniel@iogearbox.net, eddyz87@gmail.com, haoluo@google.com, john.fastabend@gmail.com, jolsa@kernel.org, kpsingh@kernel.org, linux-kernel@vger.kernel.org, listout@listout.xyz, martin.lau@linux.dev, netdev@vger.kernel.org, sdf@fomichev.me, song@kernel.org, syzkaller-bugs@googlegroups.com References: <68f6a4c8.050a0220.1be48.0011.GAE@google.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <68f6a4c8.050a0220.1be48.0011.GAE@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 10/20/25 2:08 PM, syzbot wrote: > Hello, > > syzbot found the following issue on: > > HEAD commit: a1e83d4c0361 selftests/bpf: Fix redefinition of 'off' as d.. > git tree: bpf > console output: https://syzkaller.appspot.com/x/log.txt?x=12d21de2580000 > kernel config: https://syzkaller.appspot.com/x/.config?x=9ad7b090a18654a7 > dashboard link: https://syzkaller.appspot.com/bug?extid=b0cff308140f79a9c4cb > compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8 > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=160cf542580000 > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=128d5c58580000 > > Downloadable assets: > disk image: https://storage.googleapis.com/syzbot-assets/2f6a7a0cd1b7/disk-a1e83d4c.raw.xz > vmlinux: https://storage.googleapis.com/syzbot-assets/873984cfc71e/vmlinux-a1e83d4c.xz > kernel image: https://storage.googleapis.com/syzbot-assets/16711d84070c/bzImage-a1e83d4c.xz > > The issue was bisected to: > > commit 7c33e97a6ef5d84e98b892c3e00c6d1678d20395 > Author: Sahil Chandna > Date: Tue Oct 14 18:56:35 2025 +0000 > > bpf: Do not disable preemption in bpf_test_run(). > > bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=172fe492580000 > final oops: https://syzkaller.appspot.com/x/report.txt?x=14afe492580000 > console output: https://syzkaller.appspot.com/x/log.txt?x=10afe492580000 > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > Reported-by: syzbot+b0cff308140f79a9c4cb@syzkaller.appspotmail.com > Fixes: 7c33e97a6ef5 ("bpf: Do not disable preemption in bpf_test_run().") > > ------------[ cut here ]------------ > WARNING: CPU: 1 PID: 6145 at kernel/bpf/helpers.c:781 bpf_try_get_buffers kernel/bpf/helpers.c:781 [inline] > WARNING: CPU: 1 PID: 6145 at kernel/bpf/helpers.c:781 bpf_bprintf_prepare+0x12cf/0x13a0 kernel/bpf/helpers.c:834 Okay, the warning is due to the following WARN_ON_ONCE: static DEFINE_PER_CPU(struct bpf_bprintf_buffers[MAX_BPRINTF_NEST_LEVEL], bpf_bprintf_bufs); static DEFINE_PER_CPU(int, bpf_bprintf_nest_level); int bpf_try_get_buffers(struct bpf_bprintf_buffers **bufs) { int nest_level; nest_level = this_cpu_inc_return(bpf_bprintf_nest_level); if (WARN_ON_ONCE(nest_level > MAX_BPRINTF_NEST_LEVEL)) { this_cpu_dec(bpf_bprintf_nest_level); return -EBUSY; } *bufs = this_cpu_ptr(&bpf_bprintf_bufs[nest_level - 1]); return 0; } Basically without preempt disable, at process level, it is possible more than one process may trying to take bpf_bprintf_buffers. Adding softirq and nmi, it is totally likely to have more than 3 level for buffers. Also, more than one process with bpf_bprintf_buffers will cause problem in releasing buffers, so we need to have preempt_disable surrounding bpf_try_get_buffers() and bpf_put_buffers(). There are some kfuncs/helpers need such preempt_disable protection, e.g. bpf_stream_printk, bpf_snprintf, bpf_trace_printk, bpf_trace_vprintk, bpf_seq_printf. But please double check. > Modules linked in: > CPU: 1 UID: 0 PID: 6145 Comm: syz.4.53 Not tainted syzkaller #0 PREEMPT(full) > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025 > RIP: 0010:bpf_try_get_buffers kernel/bpf/helpers.c:781 [inline] > RIP: 0010:bpf_bprintf_prepare+0x12cf/0x13a0 kernel/bpf/helpers.c:834 > Code: ff e9 ce fe ff ff e8 10 ec e0 ff e9 be fe ff ff e8 06 ec e0 ff e9 b4 fe ff ff e8 fc eb e0 ff e9 aa fe ff ff e8 f2 eb e0 ff 90 <0f> 0b 90 65 ff 0d 27 fd b2 10 b8 f0 ff ff ff e9 17 ff ff ff e8 d8 > RSP: 0018:ffffc90003797840 EFLAGS: 00010293 > RAX: ffffffff81df57fe RBX: ffffc90003797a10 RCX: ffff888026493c80 > RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000003 > RBP: ffffc90003797970 R08: 0000000000585870 R09: 0000000000000005 > R10: dffffc0000000000 R11: fffff520006f2f20 R12: dffffc0000000000 > R13: 0000000000000004 R14: 0000000000000003 R15: 1ffff920006f2f42 > FS: 00005555805f5500(0000) GS:ffff888125e0c000(0000) knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: 0000000000000000 CR3: 000000007c04e000 CR4: 00000000003526f0 > Call Trace: > > ____bpf_trace_printk kernel/trace/bpf_trace.c:372 [inline] > bpf_trace_printk+0xdb/0x190 kernel/trace/bpf_trace.c:362 > bpf_prog_bfbd7bf4bf171090+0x41/0x5a > bpf_dispatcher_nop_func include/linux/bpf.h:1350 [inline] > __bpf_prog_run include/linux/filter.h:721 [inline] > bpf_prog_run include/linux/filter.h:728 [inline] > bpf_prog_run_pin_on_cpu include/linux/filter.h:745 [inline] > bpf_flow_dissect+0x225/0x720 net/core/flow_dissector.c:1024 > bpf_prog_test_run_flow_dissector+0x37c/0x5c0 net/bpf/test_run.c:1414 > bpf_prog_test_run+0x2c7/0x340 kernel/bpf/syscall.c:4688 > __sys_bpf+0x562/0x860 kernel/bpf/syscall.c:6167 > __do_sys_bpf kernel/bpf/syscall.c:6259 [inline] > __se_sys_bpf kernel/bpf/syscall.c:6257 [inline] > __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:6257 > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] > do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94 > entry_SYSCALL_64_after_hwframe+0x77/0x7f > RIP: 0033:0x7f25b0f8efc9 > Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48 > RSP: 002b:00007ffe036cd5e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141 > RAX: ffffffffffffffda RBX: 00007f25b11e5fa0 RCX: 00007f25b0f8efc9 > RDX: 0000000000000050 RSI: 0000200000000180 RDI: 000000000000000a > RBP: 00007f25b1011f91 R08: 0000000000000000 R09: 0000000000000000 > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 > R13: 00007f25b11e5fa0 R14: 00007f25b11e5fa0 R15: 0000000000000003 > > > > --- > This report is generated by a bot. It may contain errors. > See https://goo.gl/tpsmEJ for more information about syzbot. > syzbot engineers can be reached at syzkaller@googlegroups.com. > > syzbot will keep track of this issue. See: > https://goo.gl/tpsmEJ#status for how to communicate with syzbot. > For information about bisection process see: https://goo.gl/tpsmEJ#bisection > > If the report is already addressed, let syzbot know by replying with: > #syz fix: exact-commit-title > > If you want syzbot to run the reproducer, reply with: > #syz test: git://repo/address.git branch-or-commit-hash > If you attach or paste a git patch, syzbot will apply it before testing. > > If you want to overwrite report's subsystems, reply with: > #syz set subsystems: new-subsystem > (See the list of subsystem names on the web dashboard) > > If the report is a duplicate of another one, reply with: > #syz dup: exact-subject-of-another-report > > If you want to undo deduplication, reply with: > #syz undup