[syzbot] [bcachefs?] divide error in bch2_sb_members_v2_to_text

[syzbot <syzbot+7c8101d4d0ba2eb511d7@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    b27cc623e01b Add linux-next specific files for 20250610
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=169ac60c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=846e731334efc0f8
dashboard link: https://syzkaller.appspot.com/bug?extid=7c8101d4d0ba2eb511d7
compiler:       Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1c0c417339c8/disk-b27cc623.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/fa29c0f3a1fa/vmlinux-b27cc623.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b902a80b6e7e/bzImage-b27cc623.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7c8101d4d0ba2eb511d7@syzkaller.appspotmail.com

loop2: detected capacity change from 0 to 32768
Oops: divide error: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 6339 Comm: syz.2.181 Not tainted 6.16.0-rc1-next-20250610-syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
RIP: 0010:bch2_sb_members_v2_to_text+0x10a/0x3c0 fs/bcachefs/sb-members.c:347
Code: 49 89 f4 49 c1 ec 03 43 0f b6 0c 34 84 c9 48 89 b4 24 a0 00 00 00 0f 85 67 02 00 00 0f b7 0e 48 89 c2 48 c1 ea 20 74 07 48 99 <48> f7 f9 eb 04 31 d2 f7 f1 48 89 84 24 b0 00 00 00 48 8b 84 24 c8
RSP: 0000:ffffc90003e5ed40 EFLAGS: 00010a02
RAX: ffff888029b58368 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffffffffffffffff RSI: ffff888029b58338 RDI: 0000000000001de6
RBP: ffffc90003e5ef70 R08: 000000000000003a R09: 000000000000003a
R10: dffffc0000000000 R11: ffffffff844fcfb0 R12: 1ffff1100536b067
R13: 000000000000000b R14: dffffc0000000000 R15: 1ffff920007cbdc4
FS:  00007f61376166c0(0000) GS:ffff888125c4b000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f5581bb5f78 CR3: 000000007e792000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 bch2_sb_field_validate+0x1c6/0x280 fs/bcachefs/super-io.c:1380
 bch2_sb_validate+0x14bd/0x1980 fs/bcachefs/super-io.c:552
 __bch2_read_super+0xba4/0x1040 fs/bcachefs/super-io.c:925
 bch2_fs_open+0x1fe/0x2570 fs/bcachefs/super.c:2382
 bch2_fs_get_tree+0x437/0x14f0 fs/bcachefs/fs.c:2473
 vfs_get_tree+0x8f/0x2b0 fs/super.c:1802
 do_new_mount+0x24a/0xa40 fs/namespace.c:3885
 do_mount fs/namespace.c:4222 [inline]
 __do_sys_mount fs/namespace.c:4433 [inline]
 __se_sys_mount+0x317/0x410 fs/namespace.c:4410
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f61367900ca
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:00007f6137615e68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f6137615ef0 RCX: 00007f61367900ca
RDX: 0000200000000000 RSI: 0000200000011a40 RDI: 00007f6137615eb0
RBP: 0000200000000000 R08: 00007f6137615ef0 R09: 00000000028080c9
R10: 00000000028080c9 R11: 0000000000000246 R12: 0000200000011a40
R13: 00007f6137615eb0 R14: 00000000000119f9 R15: 0000200000013d00
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:bch2_sb_members_v2_to_text+0x10a/0x3c0 fs/bcachefs/sb-members.c:347
Code: 49 89 f4 49 c1 ec 03 43 0f b6 0c 34 84 c9 48 89 b4 24 a0 00 00 00 0f 85 67 02 00 00 0f b7 0e 48 89 c2 48 c1 ea 20 74 07 48 99 <48> f7 f9 eb 04 31 d2 f7 f1 48 89 84 24 b0 00 00 00 48 8b 84 24 c8
RSP: 0000:ffffc90003e5ed40 EFLAGS: 00010a02
RAX: ffff888029b58368 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffffffffffffffff RSI: ffff888029b58338 RDI: 0000000000001de6
RBP: ffffc90003e5ef70 R08: 000000000000003a R09: 000000000000003a
R10: dffffc0000000000 R11: ffffffff844fcfb0 R12: 1ffff1100536b067
R13: 000000000000000b R14: dffffc0000000000 R15: 1ffff920007cbdc4
FS:  00007f61376166c0(0000) GS:ffff888125c4b000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f961d79c000 CR3: 000000007e792000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
   0:	49 89 f4             	mov    %rsi,%r12
   3:	49 c1 ec 03          	shr    $0x3,%r12
   7:	43 0f b6 0c 34       	movzbl (%r12,%r14,1),%ecx
   c:	84 c9                	test   %cl,%cl
   e:	48 89 b4 24 a0 00 00 	mov    %rsi,0xa0(%rsp)
  15:	00
  16:	0f 85 67 02 00 00    	jne    0x283
  1c:	0f b7 0e             	movzwl (%rsi),%ecx
  1f:	48 89 c2             	mov    %rax,%rdx
  22:	48 c1 ea 20          	shr    $0x20,%rdx
  26:	74 07                	je     0x2f
  28:	48 99                	cqto
* 2a:	48 f7 f9             	idiv   %rcx <-- trapping instruction
  2d:	eb 04                	jmp    0x33
  2f:	31 d2                	xor    %edx,%edx
  31:	f7 f1                	div    %ecx
  33:	48 89 84 24 b0 00 00 	mov    %rax,0xb0(%rsp)
  3a:	00
  3b:	48                   	rex.W
  3c:	8b                   	.byte 0x8b
  3d:	84 24 c8             	test   %ah,(%rax,%rcx,8)


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