[syzbot] [nilfs?] general protection fault in nilfs_mdt_save_to_shadow_map

Hello,

syzbot found the following issue on:

HEAD commit:    b84a0ebe421c Add linux-next specific files for 20260313
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=115098ba580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d079c776411aaf59
dashboard link: https://syzkaller.appspot.com/bug?extid=4b4093b1f24ad789bf37
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15e844da580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11775d52580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/ed9cdd5f6ba1/disk-b84a0ebe.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/856b866a24e9/vmlinux-b84a0ebe.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b1d5ecff5545/bzImage-b84a0ebe.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/66467c518306/mount_0.gz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email protected]

loop0: detected capacity change from 0 to 4096
NILFS (loop0): invalid segment: Checksum error in segment payload
NILFS (loop0): trying rollback from an earlier position
NILFS (loop0): recovery complete
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037]
CPU: 0 UID: 0 PID: 6003 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
RIP: 0010:nilfs_mdt_save_to_shadow_map+0x141/0x1c0 fs/nilfs2/mdt.c:559
Code: 3f 4c 8d 63 d8 4c 89 e0 48 c1 e8 03 42 80 3c 28 00 74 08 4c 89 e7 e8 2e 0b 83 fe 4d 8b 24 24 49 83 c4 30 4c 89 e0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 e7 e8 10 0b 83 fe 49 8b 34 24 4c 89 ff
RSP: 0018:ffffc90002767708 EFLAGS: 00010206
RAX: 0000000000000006 RBX: ffff8880605d4560 RCX: 0000000000000000
RDX: ffff88802cde8000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff88802cde8000 R09: 0000000000000003
R10: 0000000000000406 R11: 0000000000000000 R12: 0000000000000030
R13: dffffc0000000000 R14: ffff8880764a6538 R15: ffff8880605d3b18
FS:  000055556403c500(0000) GS:ffff888125436000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b30263fff CR3: 0000000074cae000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 nilfs_clean_segments+0x162/0xa50 fs/nilfs2/segment.c:2521
 nilfs_ioctl_clean_segments fs/nilfs2/ioctl.c:916 [inline]
 nilfs_ioctl+0x261f/0x2780 fs/nilfs2/ioctl.c:1346
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fd2a5f9c799
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd6d77cd18 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fd2a6215fa0 RCX: 00007fd2a5f9c799
RDX: 0000200000000640 RSI: 0000000040786e88 RDI: 0000000000000004
RBP: 00007fd2a6032c99 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fd2a6215fac R14: 00007fd2a6215fa0 R15: 00007fd2a6215fa0
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:nilfs_mdt_save_to_shadow_map+0x141/0x1c0 fs/nilfs2/mdt.c:559
Code: 3f 4c 8d 63 d8 4c 89 e0 48 c1 e8 03 42 80 3c 28 00 74 08 4c 89 e7 e8 2e 0b 83 fe 4d 8b 24 24 49 83 c4 30 4c 89 e0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 e7 e8 10 0b 83 fe 49 8b 34 24 4c 89 ff
RSP: 0018:ffffc90002767708 EFLAGS: 00010206
RAX: 0000000000000006 RBX: ffff8880605d4560 RCX: 0000000000000000
RDX: ffff88802cde8000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffff88802cde8000 R09: 0000000000000003
R10: 0000000000000406 R11: 0000000000000000 R12: 0000000000000030
R13: dffffc0000000000 R14: ffff8880764a6538 R15: ffff8880605d3b18
FS:  000055556403c500(0000) GS:ffff888125536000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f1536619000 CR3: 0000000074cae000 CR4: 00000000003526f0
----------------
Code disassembly (best guess), 1 bytes skipped:
   0:	4c 8d 63 d8          	lea    -0x28(%rbx),%r12
   4:	4c 89 e0             	mov    %r12,%rax
   7:	48 c1 e8 03          	shr    $0x3,%rax
   b:	42 80 3c 28 00       	cmpb   $0x0,(%rax,%r13,1)
  10:	74 08                	je     0x1a
  12:	4c 89 e7             	mov    %r12,%rdi
  15:	e8 2e 0b 83 fe       	call   0xfe830b48
  1a:	4d 8b 24 24          	mov    (%r12),%r12
  1e:	49 83 c4 30          	add    $0x30,%r12
  22:	4c 89 e0             	mov    %r12,%rax
  25:	48 c1 e8 03          	shr    $0x3,%rax
* 29:	42 80 3c 28 00       	cmpb   $0x0,(%rax,%r13,1) <-- trapping instruction
  2e:	74 08                	je     0x38
  30:	4c 89 e7             	mov    %r12,%rdi
  33:	e8 10 0b 83 fe       	call   0xfe830b48
  38:	49 8b 34 24          	mov    (%r12),%rsi
  3c:	4c 89 ff             	mov    %r15,%rdi


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