[syzbot] [bcachefs?] UBSAN: array-index-out-of-bounds in bch2_sb_downgrade_update

[syzbot <syzbot+14c52d86ddbd89bea13e@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    e72e9e693307 Merge tag 'net-6.15-rc4' of git://git.kernel...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11ef21b3980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4f9c44a22d09fd53
dashboard link: https://syzkaller.appspot.com/bug?extid=14c52d86ddbd89bea13e
compiler:       Debian clang version 20.1.2 (++20250402124445+58df0ef89dd6-1~exp1~20250402004600.97), Debian LLD 20.1.2
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11777d9b980000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15ef21b3980000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-e72e9e69.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/2bee8b8591c3/vmlinux-e72e9e69.xz
kernel image: https://storage.googleapis.com/syzbot-assets/97a6564905c3/bzImage-e72e9e69.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/067449ccbbaf/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+14c52d86ddbd89bea13e@syzkaller.appspotmail.com

bcachefs (loop0): bucket 0:26 gen 0 data type btree sector count overflow: 0 + -256 > U32_MAX
  while marking u64s 11 type btree_ptr_v2 SPOS_MAX len 0 ver 0: seq ac62141f8dc7e261 written 24 min_key POS_MIN durability: 1 ptr: 0:26:0 gen 0
  
------------[ cut here ]------------
UBSAN: array-index-out-of-bounds in fs/bcachefs/sb-downgrade.c:268:4
index 0 is out of range for type '__le16[] __counted_by(nr_errors)' (aka 'unsigned short[]')
CPU: 0 UID: 0 PID: 1037 Comm: kworker/u4:6 Not tainted 6.15.0-rc3-syzkaller-00076-ge72e9e693307 #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Workqueue: btree_update btree_interior_update_work
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 ubsan_epilogue+0xa/0x40 lib/ubsan.c:231
 __ubsan_handle_out_of_bounds+0xe9/0xf0 lib/ubsan.c:453
 downgrade_table_extra fs/bcachefs/sb-downgrade.c:268 [inline]
 bch2_sb_downgrade_update+0xb10/0xcc0 fs/bcachefs/sb-downgrade.c:388
 bch2_write_super+0xbf4/0x2cc0 fs/bcachefs/super-io.c:1071
 btree_update_new_nodes_mark_sb fs/bcachefs/btree_update_interior.c:608 [inline]
 btree_update_nodes_written fs/bcachefs/btree_update_interior.c:678 [inline]
 btree_interior_update_work+0x441/0x25e0 fs/bcachefs/btree_update_interior.c:843
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0xadb/0x17a0 kernel/workqueue.c:3319
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3400
 kthread+0x70e/0x8a0 kernel/kthread.c:464
 ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:153
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
---[ end trace ]---
Kernel panic - not syncing: UBSAN: panic_on_warn set ...
CPU: 0 UID: 0 PID: 1037 Comm: kworker/u4:6 Not tainted 6.15.0-rc3-syzkaller-00076-ge72e9e693307 #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Workqueue: btree_update btree_interior_update_work
Call Trace:
 <TASK>
 dump_stack_lvl+0x99/0x250 lib/dump_stack.c:120
 panic+0x2db/0x790 kernel/panic.c:354
 check_panic_on_warn+0x89/0xb0 kernel/panic.c:243
 __ubsan_handle_out_of_bounds+0xe9/0xf0 lib/ubsan.c:453
 downgrade_table_extra fs/bcachefs/sb-downgrade.c:268 [inline]
 bch2_sb_downgrade_update+0xb10/0xcc0 fs/bcachefs/sb-downgrade.c:388
 bch2_write_super+0xbf4/0x2cc0 fs/bcachefs/super-io.c:1071
 btree_update_new_nodes_mark_sb fs/bcachefs/btree_update_interior.c:608 [inline]
 btree_update_nodes_written fs/bcachefs/btree_update_interior.c:678 [inline]
 btree_interior_update_work+0x441/0x25e0 fs/bcachefs/btree_update_interior.c:843
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0xadb/0x17a0 kernel/workqueue.c:3319
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3400
 kthread+0x70e/0x8a0 kernel/kthread.c:464
 ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:153
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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