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SubjectRe: [syzbot] BUG: "hc->tx_t_ipi == NUM" holds (exception!) at net/dccp/ccids/ccid3.c:LINE/ccid3_update_send_interval()
From
Hi,

On 1/10/22 08:16, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 82192cb497f9 Merge branch 'ena-capabilities-field-and-cosm..
> git tree: net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=12ec95c7b00000
> kernel config: https://syzkaller.appspot.com/x/.config?x=322a0a6462d9ff7d
> dashboard link: https://syzkaller.appspot.com/bug?extid=94641ba6c1d768b1e35e
> compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=100ea4e3b00000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13977b73b00000
>
> Bisection is inconclusive: the issue happens on the oldest tested release.

Regarding commit 793734b587a6:
Author: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Date: Mon Feb 27 12:29:44 2012 -0700

dccp ccid-3: replace incorrect BUG_ON


Is this an algorithm (RFC) problem or a Linux implementation problem?

Could we round a calculated 0 (unbounded) t_ipi up to 1?

Or is just something "nasty" that the reproducer program is doing?


I ran the C reproducer for about 15 seconds and got 71 occurrences
of this
DCCP_BUG_ON(hc->tx_t_ipi == 0);

>
> bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=13698c63b00000
> final oops: https://syzkaller.appspot.com/x/report.txt?x=10e98c63b00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=17698c63b00000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+94641ba6c1d768b1e35e@syzkaller.appspotmail.com
>
> BUG: "hc->tx_t_ipi == 0" holds (exception!) at net/dccp/ccids/ccid3.c:90/ccid3_update_send_interval()
> CPU: 0 PID: 29976 Comm: syz-executor890 Not tainted 5.16.0-rc8-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
> <TASK>
> __dump_stack lib/dump_stack.c:88 [inline]
> dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
> ccid3_update_send_interval net/dccp/ccids/ccid3.c:90 [inline]
> ccid3_update_send_interval.cold+0x87/0x93 net/dccp/ccids/ccid3.c:86
> ccid3_hc_tx_update_s net/dccp/ccids/ccid3.c:169 [inline]
> ccid3_hc_tx_packet_sent+0x12e/0x160 net/dccp/ccids/ccid3.c:353
> ccid_hc_tx_packet_sent net/dccp/ccid.h:175 [inline]
> dccp_xmit_packet+0x2f2/0x750 net/dccp/output.c:289
> dccp_write_xmit+0x16d/0x1d0 net/dccp/output.c:366
> dccp_sendmsg+0x922/0xc90 net/dccp/proto.c:783
> inet_sendmsg+0x99/0xe0 net/ipv4/af_inet.c:819
> sock_sendmsg_nosec net/socket.c:705 [inline]
> sock_sendmsg+0xcf/0x120 net/socket.c:725
> ____sys_sendmsg+0x331/0x810 net/socket.c:2413
> ___sys_sendmsg+0xf3/0x170 net/socket.c:2467
> __sys_sendmmsg+0x195/0x470 net/socket.c:2553
> __do_sys_sendmmsg net/socket.c:2582 [inline]
> __se_sys_sendmmsg net/socket.c:2579 [inline]
> __x64_sys_sendmmsg+0x99/0x100 net/socket.c:2579
> do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7f2cf5f36da9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 e1 15 00 00 90 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 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f2cf5ee4308 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
> RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007f2cf5f36da9
> RDX: 0000000000000001 RSI: 000000002000bf40 RDI: 0000000000000004
> RBP: 00007f2cf5fbf4c8 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f2cf5fbf4c0
> R13: 00007f2cf5f8c5dc R14: 00007f2cf5ee4400 R15: 0000000000022000
> </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@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
> syzbot can test patches for this issue, for details see:
> https://goo.gl/tpsmEJ#testing-patches

thanks.

--
~Randy

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