Re: [syzbot] [dri?] WARNING in drm_fbdev_shmem_helper_fb_dirty

[syzbot <syzbot+369ee6a6e9d0bbf14b37@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    af32da41b032 Merge tag 'net-7.3-rc7' of git://git.kernel.o..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17b540a5580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7d1b274ca9837366
dashboard link: https://syzkaller.appspot.com/bug?extid=369ee6a6e9d0bbf14b37
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10c2fa8e580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1268e0a5580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-af32da41.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6a43965520a9/vmlinux-af32da41.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c79ed41c72d4/bzImage-af32da41.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+369ee6a6e9d0bbf14b37@syzkaller.appspotmail.com

------------[ cut here ]------------
virtio-pci 0000:00:01.0: [drm] Dirty helper failed: ret=-12
WARNING: drivers/gpu/drm/drm_fbdev_shmem.c:118 at drm_fbdev_shmem_helper_fb_dirty+0x1e1/0x310 drivers/gpu/drm/drm_fbdev_shmem.c:118, CPU#0: kworker/0:0/9
Modules linked in:
CPU: 0 UID: 0 PID: 9 Comm: kworker/0:0 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Workqueue: events drm_fb_helper_damage_work
RIP: 0010:drm_fbdev_shmem_helper_fb_dirty+0x288/0x310 drivers/gpu/drm/drm_fbdev_shmem.c:118
Code: 27 fc 4c 89 f8 48 c1 e8 03 42 80 3c 28 00 74 08 4c 89 ff e8 3a 1a 9a fc 4d 8b 27 48 89 df 4c 89 f6 4c 89 e2 8b 5c 24 04 89 d9 <67> 48 0f b9 3a 89 d8 e9 f7 fe ff ff 89 d9 80 e1 07 fe c1 38 c1 0f
RSP: 0018:ffffc9000022f898 EFLAGS: 00010246
RAX: 1ffff1100004481a RBX: 00000000fffffff4 RCX: 00000000fffffff4
RDX: ffff888000b33d00 RSI: ffffffff8c79f2a0 RDI: ffffffff91867840
RBP: ffff8880322ca008 R08: ffffc9000022f5e7 R09: 1ffff92000045ebc
R10: dffffc0000000000 R11: fffff52000045ebd R12: ffff888000b33d00
R13: dffffc0000000000 R14: ffffffff8c79f2a0 R15: ffff8880002240d0
FS:  0000000000000000(0000) GS:ffff88808b0b4000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f76a6223b50 CR3: 0000000038789000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 drm_fb_helper_fb_dirty drivers/gpu/drm/drm_fb_helper.c:339 [inline]
 drm_fb_helper_damage_work+0x84c/0xf20 drivers/gpu/drm/drm_fb_helper.c:365
 process_one_work kernel/workqueue.c:3396 [inline]
 process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3479
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3560
 kthread+0x38b/0x480 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
----------------
Code disassembly (best guess), 1 bytes skipped:
   0:	fc                   	cld
   1:	4c 89 f8             	mov    %r15,%rax
   4:	48 c1 e8 03          	shr    $0x3,%rax
   8:	42 80 3c 28 00       	cmpb   $0x0,(%rax,%r13,1)
   d:	74 08                	je     0x17
   f:	4c 89 ff             	mov    %r15,%rdi
  12:	e8 3a 1a 9a fc       	call   0xfc9a1a51
  17:	4d 8b 27             	mov    (%r15),%r12
  1a:	48 89 df             	mov    %rbx,%rdi
  1d:	4c 89 f6             	mov    %r14,%rsi
  20:	4c 89 e2             	mov    %r12,%rdx
  23:	8b 5c 24 04          	mov    0x4(%rsp),%ebx
  27:	89 d9                	mov    %ebx,%ecx
* 29:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2e:	89 d8                	mov    %ebx,%eax
  30:	e9 f7 fe ff ff       	jmp    0xffffff2c
  35:	89 d9                	mov    %ebx,%ecx
  37:	80 e1 07             	and    $0x7,%cl
  3a:	fe c1                	inc    %cl
  3c:	38 c1                	cmp    %al,%cl
  3e:	0f                   	.byte 0xf


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