On Sat, Jun 03, 2023 at 11:04:45PM -0400, Theodore Ts'o wrote: > I tried testing to see if this fixed [1], and it appears to be > triggering a lockdep warning[2] at this line in the patch: > > [1] https://syzkaller.appspot.com/bug?extid=f4582777a19ec422b517 > [2] https://syzkaller.appspot.com/x/report.txt?x=17260843280000 Looking at this more closely, the fundamental problem is by the time ext4_file_mmap() is called, the mm layer has already taken current->mm->mmap_lock, and when we try to take the inode_lock, this causes locking ordering problems with how buffered write path works, which take the inode_lock first, and then in some cases, may end up taking the mmap_lock if there is a page fault for the buffer used for the buffered write. If we're going to stick with the approach in this patch, I think what we would need is to add a pre_mmap() function to file_operations struct, which would get called by the mmap path *before* taking current->mm->mmap_lock, so we can do the inline conversion before we take the mmap_lock. I'm not sure how the mm folks would react to such a proposal, though. I could be seen as a bit hacky, and it's not clear that any file system other than ext4 would need something like this. Willy, as someone who does a lot of work in both mm and fs worlds --- I'm curious what you think about this idea? Thanks, - Ted > > diff --git a/fs/ext4/file.c b/fs/ext4/file.c > > index d101b3b0c7da..9df82d72eb90 100644 > > --- a/fs/ext4/file.c > > +++ b/fs/ext4/file.c > > @@ -808,6 +809,27 @@ static int ext4_file_mmap(struct file *file, struct vm_area_struct *vma) > > if (!daxdev_mapping_supported(vma, dax_dev)) > > return -EOPNOTSUPP; > > > > + /* > > + * Writing via mmap has no logic to handle inline data, so we > > + * need to call ext4_convert_inline_data() to convert the inode > > + * to normal format before doing so, otherwise a BUG_ON will be > > + * triggered in ext4_writepages() due to the > > + * EXT4_STATE_MAY_INLINE_DATA flag. Moreover, we need to grab > > + * i_rwsem during conversion, since clearing and setting the > > + * inline data flag may race with ext4_buffered_write_iter() > > + * to trigger a BUG_ON. > > + */ > > + if (ext4_has_feature_inline_data(sb) && > > + vma->vm_flags & VM_SHARED && vma->vm_flags & VM_MAYWRITE) { > > + int err; > > + > > + inode_lock(inode); <=================== LOCKDEP warning > > + err = ext4_convert_inline_data(inode); > > + inode_unlock(inode); > > + if (err) > > + return err; > > + } > > > The details of the lockdep warning from [2], which appears to be a > mmap(2) racing with a buffered write(2) are below. Could you take a > look? > > Thanks! > > - Ted > > ====================================================== > WARNING: possible circular locking dependency detected > 6.4.0-rc4-syzkaller-geb1f822c76be-dirty #0 Not tainted > ------------------------------------------------------ > syz-executor.4/5589 is trying to acquire lock: > ffff888024228168 (&mm->mmap_lock){++++}-{3:3}, at: mmap_read_lock include/linux/mmap_lock.h:142 [inline] > ffff888024228168 (&mm->mmap_lock){++++}-{3:3}, at: do_user_addr_fault+0xb3d/0x1210 arch/x86/mm/fault.c:1391 > > but task is already holding lock: > ffff88806a066800 (&sb->s_type->i_mutex_key#8){++++}-{3:3}, at: inode_lock include/linux/fs.h:775 [inline] > ffff88806a066800 (&sb->s_type->i_mutex_key#8){++++}-{3:3}, at: ext4_buffered_write_iter+0xb0/0x460 fs/ext4/file.c:283 > > which lock already depends on the new lock. > > > the existing dependency chain (in reverse order) is: > > -> #1 (&sb->s_type->i_mutex_key#8){++++}-{3:3}: > down_write+0x92/0x200 kernel/locking/rwsem.c:1573 > inode_lock include/linux/fs.h:775 [inline] > ext4_file_mmap+0x62e/0x800 fs/ext4/file.c:826 > call_mmap include/linux/fs.h:1873 [inline] > mmap_region+0x694/0x28d0 mm/mmap.c:2652 > do_mmap+0x831/0xf60 mm/mmap.c:1394 > vm_mmap_pgoff+0x1a2/0x3b0 mm/util.c:543 > ksys_mmap_pgoff+0x41f/0x5a0 mm/mmap.c:1440 > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80 > entry_SYSCALL_64_after_hwframe+0x63/0xcd > > -> #0 (&mm->mmap_lock){++++}-{3:3}: > check_prev_add kernel/locking/lockdep.c:3113 [inline] > check_prevs_add kernel/locking/lockdep.c:3232 [inline] > validate_chain kernel/locking/lockdep.c:3847 [inline] > __lock_acquire+0x2fcd/0x5f30 kernel/locking/lockdep.c:5088 > lock_acquire kernel/locking/lockdep.c:5705 [inline] > lock_acquire+0x1b1/0x520 kernel/locking/lockdep.c:5670 > down_read+0x9c/0x480 kernel/locking/rwsem.c:1520 > mmap_read_lock include/linux/mmap_lock.h:142 [inline] > do_user_addr_fault+0xb3d/0x1210 arch/x86/mm/fault.c:1391 > handle_page_fault arch/x86/mm/fault.c:1534 [inline] > exc_page_fault+0x98/0x170 arch/x86/mm/fault.c:1590 > asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:570 > fault_in_readable+0x1a5/0x210 mm/gup.c:1856 > fault_in_iov_iter_readable+0x252/0x2c0 lib/iov_iter.c:362 > generic_perform_write+0x1ae/0x570 mm/filemap.c:3913 > ext4_buffered_write_iter+0x15b/0x460 fs/ext4/file.c:289 > ext4_file_write_iter+0xbe0/0x1740 fs/ext4/file.c:710 > call_write_iter include/linux/fs.h:1868 [inline] > new_sync_write fs/read_write.c:491 [inline] > vfs_write+0x945/0xd50 fs/read_write.c:584 > ksys_write+0x12b/0x250 fs/read_write.c:637 > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80 > entry_SYSCALL_64_after_hwframe+0x63/0xcd > > other info that might help us debug this: > > Possible unsafe locking scenario: > > CPU0 CPU1 > ---- ---- > lock(&sb->s_type->i_mutex_key#8); > lock(&mm->mmap_lock); > lock(&sb->s_type->i_mutex_key#8); > rlock(&mm->mmap_lock); > > *** DEADLOCK *** > > 3 locks held by syz-executor.4/5589: > #0: ffff88802a7fe0e8 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0xe7/0x100 fs/file.c:1047 > #1: ffff888021fe0460 (sb_writers#4){.+.+}-{0:0}, at: ksys_write+0x12b/0x250 fs/read_write.c:637 > #2: ffff88806a066800 (&sb->s_type->i_mutex_key#8){++++}-{3:3}, at: inode_lock include/linux/fs.h:775 [inline] > #2: ffff88806a066800 (&sb->s_type->i_mutex_key#8){++++}-{3:3}, at: ext4_buffered_write_iter+0xb0/0x460 fs/ext4/file.c:283 > > stack backtrace: > CPU: 0 PID: 5589 Comm: syz-executor.4 Not tainted 6.4.0-rc4-syzkaller-geb1f822c76be-dirty #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/25/2023 > Call Trace: > <TASK> > __dump_stack lib/dump_stack.c:88 [inline] > dump_stack_lvl+0xd9/0x150 lib/dump_stack.c:106 > check_noncircular+0x25f/0x2e0 kernel/locking/lockdep.c:2188 > check_prev_add kernel/locking/lockdep.c:3113 [inline] > check_prevs_add kernel/locking/lockdep.c:3232 [inline] > validate_chain kernel/locking/lockdep.c:3847 [inline] > __lock_acquire+0x2fcd/0x5f30 kernel/locking/lockdep.c:5088 > lock_acquire kernel/locking/lockdep.c:5705 [inline] > lock_acquire+0x1b1/0x520 kernel/locking/lockdep.c:5670 > down_read+0x9c/0x480 kernel/locking/rwsem.c:1520 > mmap_read_lock include/linux/mmap_lock.h:142 [inline] > do_user_addr_fault+0xb3d/0x1210 arch/x86/mm/fault.c:1391 > handle_page_fault arch/x86/mm/fault.c:1534 [inline] > exc_page_fault+0x98/0x170 arch/x86/mm/fault.c:1590 > asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:570 > RIP: 0010:fault_in_readable+0x1a5/0x210 mm/gup.c:1856 > Code: fc ff df 48 c7 04 02 00 00 00 00 48 83 c4 48 4c 89 e0 5b 5d 41 5c 41 5d 41 5e 41 5f c3 45 31 e4 eb ce e8 ae 51 c4 ff 45 31 f6 <41> 8a 45 00 31 ff 44 89 f6 88 44 24 28 e8 b9 4d c4 ff 45 85 f6 75 > RSP: 0018:ffffc90006187a38 EFLAGS: 00050246 > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000 > RDX: ffff888026905940 RSI: ffffffff81bff672 RDI: 0000000000000007 > RBP: 00000000200002cc R08: 0000000000000007 R09: 0000000000000000 > R10: 00000000000002c0 R11: 1ffffffff219cbe3 R12: 000000000000000c > R13: 00000000200002c0 R14: 0000000000000000 R15: 1ffff92000c30f48 > fault_in_iov_iter_readable+0x252/0x2c0 lib/iov_iter.c:362 > generic_perform_write+0x1ae/0x570 mm/filemap.c:3913 > ext4_buffered_write_iter+0x15b/0x460 fs/ext4/file.c:289 > ext4_file_write_iter+0xbe0/0x1740 fs/ext4/file.c:710 > call_write_iter include/linux/fs.h:1868 [inline] > new_sync_write fs/read_write.c:491 [inline] > vfs_write+0x945/0xd50 fs/read_write.c:584 > ksys_write+0x12b/0x250 fs/read_write.c:637 > do_syscall_x64 arch/x86/entry/common.c:50 [inline] > do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80 > entry_SYSCALL_64_after_hwframe+0x63/0xcd > RIP: 0033:0x7f2359a8c169 > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 19 00 00 90 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 b8 ff ff ff f7 d8 64 89 01 48 > RSP: 002b:00007f235a721168 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 > RAX: ffffffffffffffda RBX: 00007f2359bac050 RCX: 00007f2359a8c169 > RDX: 000000000000000c RSI: 00000000200002c0 RDI: 0000000000000004 > RBP: 00007f2359ae7ca1 R08: 0000000000000000 R09: 0000000000000000 > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 > R13: 00007ffea89659df R14: 00007f235a721300 R15: 0000000000022000 > </TASK> > ---------------- > Code disassembly (best guess), 2 bytes skipped: > 0: df 48 c7 fisttps -0x39(%rax) > 3: 04 02 add $0x2,%al > 5: 00 00 add %al,(%rax) > 7: 00 00 add %al,(%rax) > 9: 48 83 c4 48 add $0x48,%rsp > d: 4c 89 e0 mov %r12,%rax > 10: 5b pop %rbx > 11: 5d pop %rbp > 12: 41 5c pop %r12 > 14: 41 5d pop %r13 > 16: 41 5e pop %r14 > 18: 41 5f pop %r15 > 1a: c3 retq > 1b: 45 31 e4 xor %r12d,%r12d > 1e: eb ce jmp 0xffffffee > 20: e8 ae 51 c4 ff callq 0xffc451d3 > 25: 45 31 f6 xor %r14d,%r14d > * 28: 41 8a 45 00 mov 0x0(%r13),%al <-- trapping instruction > 2c: 31 ff xor %edi,%edi > 2e: 44 89 f6 mov %r14d,%esi > 31: 88 44 24 28 mov %al,0x28(%rsp) > 35: e8 b9 4d c4 ff callq 0xffc44df3 > 3a: 45 85 f6 test %r14d,%r14d > 3d: 75 .byte 0x75 >
