linux-kernel - Re: [syzbot] [bpf?] WARNING in maybe_exit

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Message-ID: <81bb1cf72e9c5f56c92ab43636a0626a1046d748.camel@gmail.com>
Date: Mon, 15 Sep 2025 15:34:28 -0700
From: Eduard Zingerman <eddyz87@...il.com>
To: syzbot <syzbot+3afc814e8df1af64b653@...kaller.appspotmail.com>, 
	andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
 daniel@...earbox.net, 	haoluo@...gle.com, john.fastabend@...il.com,
 jolsa@...nel.org, kpsingh@...nel.org, 	linux-kernel@...r.kernel.org,
 martin.lau@...ux.dev, sdf@...ichev.me, 	song@...nel.org,
 syzkaller-bugs@...glegroups.com, yonghong.song@...ux.dev
Subject: Re: [syzbot] [bpf?] WARNING in maybe_exit_scc

On Mon, 2025-09-15 at 11:28 -0700, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    f83ec76bf285 Linux 6.17-rc6
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=137d0e42580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8f01d8629880e620
> dashboard link: https://syzkaller.appspot.com/bug?extid=3afc814e8df1af64b653
> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=104a947c580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14467b62580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/be9b26c66bc1/disk-f83ec76b.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/53dc5627e608/vmlinux-f83ec76b.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/398506a67fd8/bzImage-f83ec76b.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+3afc814e8df1af64b653@...kaller.appspotmail.com
>
> ------------[ cut here ]------------
> verifier bug: scc exit: no visit info for call chain (1)(1)
> WARNING: CPU: 1 PID: 6013 at kernel/bpf/verifier.c:1949 maybe_exit_scc+0x768/0x8d0 kernel/bpf/verifier.c:1949

Both this and [1] are reported for very similar programs:

<this>                                      <[1]>
--------------------------------------------------------------------------------------------
(b7) r0 = -1023213567                       (b7) r0 = -1023213567				
(bf) r3 = r10				    (bf) r3 = r10					
(07) r3 += -512				    (07) r3 += -504					
(72) *(u8 *)(r10 -16) = -8		    (72) *(u8 *)(r10 -16) = -8			
(71) r4 = *(u8 *)(r10 -16)		    (71) r4 = *(u8 *)(r10 -16)			
(65) if r4 s> 0xff000000 goto pc+2	    (65) if r4 s> 0xff000000 goto pc+2		
(2d) if r0 > r4 goto pc+5		    (2d) if r0 > r4 goto pc+5			
(20) r0 = *(u32 *)skb[60673]		    (20) r0 = *(u32 *)skb[60673]			
(7b) *(u64 *)(r3 +0) = r0		    (7b) *(u64 *)(r3 +0) = r0			
(1d) if r4 == r4 goto pc+0		    (1d) if r4 == r4 goto pc+0			
(7a) *(u64 *)(r10 -512) = -256		    (7a) *(u64 *)(r10 -512) = -256			
(db) lock *(u64 *)(r3 +0) |= r0		    (db) r0 = atomic64_fetch_and((u64 *)(r3 +0), r0)	
(b5) if r0 <= 0x0 goto pc-2		    (b5) if r0 <= 0x0 goto pc-2			
(95) exit				    (95) exit					

So, I assume it's the same issue. Looking into it.

[1] https://lore.kernel.org/bpf/68c85b0d.050a0220.2ff435.03a5.GAE@google.com/T/#u


> Modules linked in:
> CPU: 1 UID: 0 PID: 6013 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
> RIP: 0010:maybe_exit_scc+0x768/0x8d0 kernel/bpf/verifier.c:1949
> Code: ff ff e8 cb 8e e7 ff c6 05 0a b5 bf 0e 01 90 48 89 ee 48 89 df e8 f8 41 fb ff 48 c7 c7 a0 9b b5 8b 48 89 c6 e8 59 33 a6 ff 90 <0f> 0b 90 90 e9 4e ff ff ff e8 0a ee 4d 00 e9 7f f9 ff ff 4c 8b 4c
> RSP: 0018:ffffc900041bf500 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: ffff888079840000 RCX: ffffffff817a4388
> RDX: ffff88807d3f8000 RSI: ffffffff817a4395 RDI: 0000000000000001
> RBP: ffff888079846328 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000001 R11: 0000000000000000 R12: 1ffff92000837ea7
> R13: 0000000000000000 R14: ffff88805cf87400 R15: dffffc0000000000
> FS:  000055557c9b5500(0000) GS:ffff8881247b2000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000055557c9b5808 CR3: 0000000073b4d000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  update_branch_counts kernel/bpf/verifier.c:2040 [inline]
>  do_check kernel/bpf/verifier.c:20135 [inline]
>  do_check_common+0x20cc/0xb410 kernel/bpf/verifier.c:23264
>  do_check_main kernel/bpf/verifier.c:23347 [inline]
>  bpf_check+0x869f/0xc670 kernel/bpf/verifier.c:24707
>  bpf_prog_load+0xe41/0x2490 kernel/bpf/syscall.c:2979
>  __sys_bpf+0x4a3f/0x4de0 kernel/bpf/syscall.c:6029
>  __do_sys_bpf kernel/bpf/syscall.c:6139 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:6137 [inline]
>  __x64_sys_bpf+0x78/0xc0 kernel/bpf/syscall.c:6137
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xcd/0x4e0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fd1d078eba9
> 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:00007ffee0400aa8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
> RAX: ffffffffffffffda RBX: 00007fd1d09d5fa0 RCX: 00007fd1d078eba9
> RDX: 0000000000000048 RSI: 00002000000017c0 RDI: 0000000000000005
> RBP: 00007fd1d0811e19 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007fd1d09d5fa0 R14: 00007fd1d09d5fa0 R15: 0000000000000003
>  </TASK>
>
>
> ---
> 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@...glegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> 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