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

Petr Mladek
2025-11-05T16:49:12Z

On Wed 2025-11-05 16:00:28, John Ogness wrote:
> On 2025-11-04, Petr Mladek <pmladek@suse.com> wrote:
> > Adding John into Cc.
>
> Thanks.
>
> > 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.
>
> I have been digging into this all day and I can find no explanation.
>
> The patch you refer to brings a minor semantic change: is_blk_wrapped()
> returns false if begin_lpos and next_lpos are the same, whereas before
> we would have true. However, these values are not allowed to be the same
> (except for the data-less special case values).
>
> > 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")
>
> Note that a quick search on lore shows another similar report:
>
> https://lore.kernel.org/all/69078fb6.050a0220.29fc44.0029.GAE@google.com/

Great catch!

There is a common pattern. There is always one dropped message before
the WARNING() triggers.

This is from
https://syzkaller.appspot.com/x/log.txt?x=1653a342580000

[ 179.188108][ T7136] ntfs3(loop0): Different NTFS sector size (4096) and media sector size (512).
** 1 printk messages dropped **
[ 179.211874][ T7136] ------------[ cut here ]------------
[ 179.211911][ T7136] WARNING: kernel/printk/printk_ringbuffer.c:1278 at get_data+0x48a/0x840, CPU#1: syz.0.359/7136

And this is from
https://syzkaller.appspot.com/x/log.txt?x=1370a292580000

[ 216.317316][ T7652] loop0: detected capacity change from 0 to 16
** 1 printk messages dropped **
[ 216.327750][ T7652] ------------[ cut here ]------------
[ 216.327789][ T7652] WARNING: kernel/printk/printk_ringbuffer.c:1278 at get_data+0x48a/0x840, CPU#1: syz.0.585/7652

I wonder whether it is related to blk_lpos->begin or blk_lpos->next
overflow. They are supposed to overflow at the end of the 1st wrap,
see kernel/printk/printk_ringbuffer.h:

<paste>
* BLK0_LPOS
* The initial @head_lpos and @tail_lpos for data rings. It is at index
* 0 and the lpos value is such that it will overflow on the first wrap.
[...]
*/
#define BLK0_LPOS(sz_bits) (-(_DATA_SIZE(sz_bits)))
</paste>

Now, the question is why the following check ends by the WARN():

static const char *get_data(struct prb_data_ring *data_ring,
struct prb_data_blk_lpos *blk_lpos,
unsigned int *data_size)
{
[...]
/* Regular data block: @begin less than @next and in same wrap. */
if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next) &&
blk_lpos->begin < blk_lpos->next) {
db = to_block(data_ring, blk_lpos->begin);
*data_size = blk_lpos->next - blk_lpos->begin;

/* Wrapping data block: @begin is one wrap behind @next. */
} else if (!is_blk_wrapped(data_ring,
blk_lpos->begin + DATA_SIZE(data_ring),
blk_lpos->next)) {
db = to_block(data_ring, 0);
*data_size = DATA_INDEX(data_ring, blk_lpos->next);

/* Illegal block description. */
} else {
WARN_ON_ONCE(1);
return NULL;
}
[...]

The new is_blk_wrapped() check makes sense on its own.

But what happens when blk_lpos->next overflows to "0"?
is_blk_wrapped() returns false because it checks (blk_lpos->next - 1).
But the extra check "blk_lpos->begin < blk_lpos->next" fails because
it checks the overflown "blk_lpos->next".

I guess that we should do:

From f9cae42b4a910127fb7694aebe2e46247dbb0fcb Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Wed, 5 Nov 2025 17:14:57 +0100
Subject: [PATCH] printk_ringbuffer: Fix check of valid data size when blk_lpos
overflows

The commit 67e1b0052f6bb8 ("printk_ringbuffer: don't needlessly wrap
data blocks around") allows to use the last 4 bytes of the ring buffer.

But the check for the data_size was not properly updated. It fails
when blk_lpos->next overflows to "0". In this case:

+ is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next)
returns false because it checks "blk_lpos->next - 1"

+ but "blk_lpos->begin < blk_lpos->next" fails because
blk_lpos->next is already 0.

+ is_blk_wrapped(data_ring, blk_lpos->begin + DATA_SIZE(data_ring),
blk_lpos->next) returns false because "begin_lpos" is from
next wrap but "next_lpos - 1" is from the previous one

As a result, get_data() triggers the WARN_ON_ONCE() for "Illegal
block description", for example:

[ 216.317316][ T7652] loop0: detected capacity change from 0 to 16
** 1 printk messages dropped **
[ 216.327750][ T7652] ------------[ cut here ]------------
[ 216.327789][ T7652] WARNING: kernel/printk/printk_ringbuffer.c:1278 at get_data+0x48a/0x840, CPU#1: syz.0.585/7652
[ 216.327848][ T7652] Modules linked in:
[ 216.327907][ T7652] CPU: 1 UID: 0 PID: 7652 Comm: syz.0.585 Not tainted syzkaller #0 PREEMPT(full)
[ 216.327933][ T7652] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
[ 216.327953][ T7652] RIP: 0010:get_data+0x48a/0x840
[ 216.327986][ T7652] 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
[ 216.328007][ T7652] RSP: 0018:ffffc900035170e0 EFLAGS: 00010293
[ 216.328029][ T7652] RAX: ffffffff81a1eee9 RBX: 00003fffffffffff RCX: ffff888033255b80
[ 216.328048][ T7652] RDX: 0000000000000000 RSI: 00003fffffffffff RDI: 0000000000000000
[ 216.328063][ T7652] RBP: 0000000000000012 R08: 0000000000000e55 R09: 000000325e213cc7
[ 216.328079][ T7652] R10: 000000325e213cc7 R11: 00001de4c2000037 R12: 0000000000000012
[ 216.328095][ T7652] R13: 0000000000000000 R14: ffffc90003517228 R15: 1ffffffff1bca646
[ 216.328111][ T7652] FS: 00007f44eb8da6c0(0000) GS:ffff888125fda000(0000) knlGS:0000000000000000
[ 216.328131][ T7652] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 216.328147][ T7652] CR2: 00007f44ea9722e0 CR3: 0000000066344000 CR4: 00000000003526f0
[ 216.328168][ T7652] Call Trace:
[ 216.328178][ T7652] <TASK>
[ 216.328199][ T7652] _prb_read_valid+0x672/0xa90
[ 216.328328][ T7652] ? desc_read+0x1b8/0x3f0
[ 216.328381][ T7652] ? __pfx__prb_read_valid+0x10/0x10
[ 216.328422][ T7652] ? panic_on_this_cpu+0x32/0x40
[ 216.328450][ T7652] prb_read_valid+0x3c/0x60
[ 216.328482][ T7652] printk_get_next_message+0x15c/0x7b0
[ 216.328526][ T7652] ? __pfx_printk_get_next_message+0x10/0x10
[ 216.328561][ T7652] ? __lock_acquire+0xab9/0xd20
[ 216.328595][ T7652] ? console_flush_all+0x131/0xb10
[ 216.328621][ T7652] ? console_flush_all+0x478/0xb10
[ 216.328648][ T7652] console_flush_all+0x4cc/0xb10
[ 216.328673][ T7652] ? console_flush_all+0x131/0xb10
[ 216.328704][ T7652] ? __pfx_console_flush_all+0x10/0x10
[ 216.328748][ T7652] ? is_printk_cpu_sync_owner+0x32/0x40
[ 216.328781][ T7652] console_unlock+0xbb/0x190
[ 216.328815][ T7652] ? __pfx___down_trylock_console_sem+0x10/0x10
[ 216.328853][ T7652] ? __pfx_console_unlock+0x10/0x10
[ 216.328899][ T7652] vprintk_emit+0x4c5/0x590
[ 216.328935][ T7652] ? __pfx_vprintk_emit+0x10/0x10
[ 216.328993][ T7652] _printk+0xcf/0x120
[ 216.329028][ T7652] ? __pfx__printk+0x10/0x10
[ 216.329051][ T7652] ? kernfs_get+0x5a/0x90
[ 216.329090][ T7652] _erofs_printk+0x349/0x410
[ 216.329130][ T7652] ? __pfx__erofs_printk+0x10/0x10
[ 216.329161][ T7652] ? __raw_spin_lock_init+0x45/0x100
[ 216.329186][ T7652] ? __init_swait_queue_head+0xa9/0x150
[ 216.329231][ T7652] erofs_fc_fill_super+0x1591/0x1b20
[ 216.329285][ T7652] ? __pfx_erofs_fc_fill_super+0x10/0x10
[ 216.329324][ T7652] ? sb_set_blocksize+0x104/0x180
[ 216.329356][ T7652] ? setup_bdev_super+0x4c1/0x5b0
[ 216.329385][ T7652] get_tree_bdev_flags+0x40e/0x4d0
[ 216.329410][ T7652] ? __pfx_erofs_fc_fill_super+0x10/0x10
[ 216.329444][ T7652] ? __pfx_get_tree_bdev_flags+0x10/0x10
[ 216.329483][ T7652] vfs_get_tree+0x92/0x2b0
[ 216.329512][ T7652] do_new_mount+0x302/0xa10
[ 216.329537][ T7652] ? apparmor_capable+0x137/0x1b0
[ 216.329576][ T7652] ? __pfx_do_new_mount+0x10/0x10
[ 216.329605][ T7652] ? ns_capable+0x8a/0xf0
[ 216.329637][ T7652] ? kmem_cache_free+0x19b/0x690
[ 216.329682][ T7652] __se_sys_mount+0x313/0x410
[ 216.329717][ T7652] ? __pfx___se_sys_mount+0x10/0x10
[ 216.329836][ T7652] ? do_syscall_64+0xbe/0xfa0
[ 216.329869][ T7652] ? __x64_sys_mount+0x20/0xc0
[ 216.329901][ T7652] do_syscall_64+0xfa/0xfa0
[ 216.329932][ T7652] ? lockdep_hardirqs_on+0x9c/0x150
[ 216.329964][ T7652] ? entry_SYSCALL_64_after_hwframe+0x77/0x7f
[ 216.329988][ T7652] ? clear_bhb_loop+0x60/0xb0
[ 216.330017][ T7652] entry_SYSCALL_64_after_hwframe+0x77/0x7f
[ 216.330040][ T7652] RIP: 0033:0x7f44ea99076a
[ 216.330080][ T7652] 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
[ 216.330100][ T7652] RSP: 002b:00007f44eb8d9e68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
[ 216.330128][ T7652] RAX: ffffffffffffffda RBX: 00007f44eb8d9ef0 RCX: 00007f44ea99076a
[ 216.330146][ T7652] RDX: 0000200000000180 RSI: 00002000000001c0 RDI: 00007f44eb8d9eb0
[ 216.330164][ T7652] RBP: 0000200000000180 R08: 00007f44eb8d9ef0 R09: 0000000000000000
[ 216.330181][ T7652] R10: 0000000000000000 R11: 0000000000000246 R12: 00002000000001c0
[ 216.330196][ T7652] R13: 00007f44eb8d9eb0 R14: 00000000000001a1 R15: 0000200000000080
[ 216.330233][ T7652] </TASK>

The check comparing "blk_lpos->next" must decrement 1 as well.

Alternative:

The check can be removed. Instead we might add a check for invalid
*data_size, something like:

if (WARN_ON_ONCE(!data_check_size(data_ring, *data_size))
return NULL;

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
kernel/printk/printk_ringbuffer.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 839f504db6d3..1272c220c8b4 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -1262,7 +1262,7 @@ static const char *get_data(struct prb_data_ring *data_ring,

/* Regular data block: @begin less than @next and in same wrap. */
if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next) &&
- blk_lpos->begin < blk_lpos->next) {
+ blk_lpos->begin < blk_lpos->next - 1) {
db = to_block(data_ring, blk_lpos->begin);
*data_size = blk_lpos->next - blk_lpos->begin;

--
2.51.1

Another question is whether this is the only problem caused the patch.

> We may want to revert the commit until we can take a closer look at
> this.
>
> I will divert my energies from code-reading to trying to reproduce this.

It might help to fill messages with a fixed size which might trigger
blk_lpos->next == 0 in the 1st wrap.

I could try this tomorrow. It is getting late here. But I wanted
to send my thoughts ASAP.

Best Regards,
Petr