Re: [syzbot] [iomap?] kernel BUG in folio_end_read (2)

[Petr Mladek <pmladek@suse.com>]

Adding John into Cc.

On Tue 2025-11-04 09:45:27, Joanne Koong wrote:
> On Mon, Nov 3, 2025 at 6:43 PM syzbot
> <syzbot+3686758660f980b402dc@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> > WARNING in get_data
> >
> > loop0: detected capacity change from 0 to 16
> > ------------[ cut here ]------------
> > WARNING: kernel/printk/printk_ringbuffer.c:1278 at get_data+0x48a/0x840 kernel/printk/printk_ringbuffer.c:1278, CPU#1: syz.0.585/7652

It seems to trigger an "Illegac block description" warning, see :

   1263         /* Regular data block: @begin less than @next and in same wrap. */
   1264         if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next) &&
   1265             blk_lpos->begin < blk_lpos->next) {
   1266                 db = to_block(data_ring, blk_lpos->begin);
   1267                 *data_size = blk_lpos->next - blk_lpos->begin;
   1268 
   1269         /* Wrapping data block: @begin is one wrap behind @next. */
   1270         } else if (!is_blk_wrapped(data_ring,
   1271                                    blk_lpos->begin + DATA_SIZE(data_ring),
   1272                                    blk_lpos->next)) {
   1273                 db = to_block(data_ring, 0);
   1274                 *data_size = DATA_INDEX(data_ring, blk_lpos->next);
   1275 
   1276         /* Illegal block description. */
   1277         } else {
   1278                 WARN_ON_ONCE(1);		<-----------
   1279                 return NULL;
   1280         }


> > Modules linked in:
> > CPU: 1 UID: 0 PID: 7652 Comm: syz.0.585 Not tainted syzkaller #0 PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
> > RIP: 0010:get_data+0x48a/0x840 kernel/printk/printk_ringbuffer.c:1278
> > Code: 83 c4 f8 48 b8 00 00 00 00 00 fc ff df 41 0f b6 04 07 84 c0 0f 85 ee 01 00 00 44 89 65 00 49 83 c5 08 eb 13 e8 a7 19 1f 00 90 <0f> 0b 90 eb 05 e8 9c 19 1f 00 45 31 ed 4c 89 e8 48 83 c4 28 5b 41
> > RSP: 0018:ffffc900035170e0 EFLAGS: 00010293
> > RAX: ffffffff81a1eee9 RBX: 00003fffffffffff RCX: ffff888033255b80
> > RDX: 0000000000000000 RSI: 00003fffffffffff RDI: 0000000000000000
> > RBP: 0000000000000012 R08: 0000000000000e55 R09: 000000325e213cc7
> > R10: 000000325e213cc7 R11: 00001de4c2000037 R12: 0000000000000012
> > R13: 0000000000000000 R14: ffffc90003517228 R15: 1ffffffff1bca646
> > FS:  00007f44eb8da6c0(0000) GS:ffff888125fda000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007f44ea9722e0 CR3: 0000000066344000 CR4: 00000000003526f0
> > Call Trace:
> >  <TASK>
> >  copy_data kernel/printk/printk_ringbuffer.c:1857 [inline]
> >  prb_read kernel/printk/printk_ringbuffer.c:1966 [inline]
> >  _prb_read_valid+0x672/0xa90 kernel/printk/printk_ringbuffer.c:2143
> >  prb_read_valid+0x3c/0x60 kernel/printk/printk_ringbuffer.c:2215
> >  printk_get_next_message+0x15c/0x7b0 kernel/printk/printk.c:2978
> >  console_emit_next_record kernel/printk/printk.c:3062 [inline]
> >  console_flush_one_record kernel/printk/printk.c:3194 [inline]
> >  console_flush_all+0x4cc/0xb10 kernel/printk/printk.c:3268
> >  __console_flush_and_unlock kernel/printk/printk.c:3298 [inline]
> >  console_unlock+0xbb/0x190 kernel/printk/printk.c:3338
> >  vprintk_emit+0x4c5/0x590 kernel/printk/printk.c:2423
> >  _printk+0xcf/0x120 kernel/printk/printk.c:2448
> >  _erofs_printk+0x349/0x410 fs/erofs/super.c:33
> >  erofs_fc_fill_super+0x1591/0x1b20 fs/erofs/super.c:746
> >  get_tree_bdev_flags+0x40e/0x4d0 fs/super.c:1692
> >  vfs_get_tree+0x92/0x2b0 fs/super.c:1752
> >  fc_mount fs/namespace.c:1198 [inline]
> >  do_new_mount_fc fs/namespace.c:3641 [inline]
> >  do_new_mount+0x302/0xa10 fs/namespace.c:3717
> >  do_mount fs/namespace.c:4040 [inline]
> >  __do_sys_mount fs/namespace.c:4228 [inline]
> >  __se_sys_mount+0x313/0x410 fs/namespace.c:4205
> >  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: 0033:0x7f44ea99076a
> > Code: d8 64 89 02 48 c7 c0 ff ff ff ff eb a6 e8 de 1a 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 49 89 ca b8 a5 00 00 00 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:00007f44eb8d9e68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> > RAX: ffffffffffffffda RBX: 00007f44eb8d9ef0 RCX: 00007f44ea99076a
> > RDX: 0000200000000180 RSI: 00002000000001c0 RDI: 00007f44eb8d9eb0
> > RBP: 0000200000000180 R08: 00007f44eb8d9ef0 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 00002000000001c0
> > R13: 00007f44eb8d9eb0 R14: 00000000000001a1 R15: 0000200000000080
> >  </TASK>
> >
> 
> This looks unrelated to the iomap changes and seems tied to the recent
> printk console flushing changes. Hmm, maybe one of these changes
> [1,2,3]?
>> 
> [1] https://lore.kernel.org/all/20251020-printk_legacy_thread_console_lock-v3-1-00f1f0ac055a@thegoodpenguin.co.uk/
> [2] https://lore.kernel.org/all/20251020-printk_legacy_thread_console_lock-v3-2-00f1f0ac055a@thegoodpenguin.co.uk/
> [3] https://lore.kernel.org/all/20251020-printk_legacy_thread_console_lock-v3-3-00f1f0ac055a@thegoodpenguin.co.uk/

These patches modified the callers of the printk_ringbuffer API.
I doubt that they might cause the problem.

It rather looks like an internal bug in the printk_ringbuffer code.
And there is only one recent patch:

   https://patch.msgid.link/20250905144152.9137-2-d-tatianin@yandex-team.ru

The scenario leading to the WARN() is not obvious to me. But the patch
touched this code path. So it is a likely culprit. I have to think
more about it.

Anyway, I wonder if the WARNING is reproducible and if it happens even after
reverting the commit 67e1b0052f6bb82be84e3 ("printk_ringbuffer: don't
needlessly wrap data blocks around")

Best Regards,
Petr

> Thanks,
> Joanne
> 
> >
> > Tested on:
> >
> > commit:         98231209 Add linux-next specific files for 20251103
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1370a292580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=43cc0e31558cb527
> > dashboard link: https://syzkaller.appspot.com/bug?extid=3686758660f980b402dc
> > compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> >
> > Note: no patches were applied.