Re: KCSAN: data-race in __fat_write_inode / fat12_ent_get

[OGAWA Hirofumi]

syzbot <syzbot+6f1624f937d9d6911e2d@xxxxxxxxxxxxxxxxxxxxxxxxx> writes:

> syzbot found the following crash on:
>
> HEAD commit:    40959e34 kcsan: Avoid blocking producers in prepare_report()
> git tree:       https://github.com/google/ktsan.git kcsan
> console output: https://syzkaller.appspot.com/x/log.txt?x=1201d5a3e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=1ab2c758651b11f6
> dashboard link: https://syzkaller.appspot.com/bug?extid=6f1624f937d9d6911e2d
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>
> Unfortunately, I don't have any reproducer for this crash yet.
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+6f1624f937d9d6911e2d@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> FAT-fs (loop1): error, clusters badly computed (876 != 875)
> FAT-fs (loop1): error, clusters badly computed (877 != 876)
> FAT-fs (loop1): error, clusters badly computed (878 != 877)

Hm, looks like the race between a directory entry vs a FAT entry.  This
bug was happened with the corrupted image? Or the image passes the check
of dosfsck?

If the corrupted image, it may be hard to prevent the all races. Well,
anyway, the corrupted image of the report will help to detect this
corruption.

Thanks.

> ==================================================================
> BUG: KCSAN: data-race in __fat_write_inode / fat12_ent_get
>
> write to 0xffff8881015f423c of 4 bytes by task 9966 on cpu 1:
>  __fat_write_inode+0x246/0x510 fs/fat/inode.c:877
>  fat_write_inode+0x67/0xe0 fs/fat/inode.c:909
>  write_inode fs/fs-writeback.c:1312 [inline]
>  __writeback_single_inode+0x722/0x910 fs/fs-writeback.c:1511
>  writeback_single_inode+0x219/0x2f0 fs/fs-writeback.c:1565
>  sync_inode fs/fs-writeback.c:2602 [inline]
>  sync_inode_metadata+0x75/0xa0 fs/fs-writeback.c:2622
>  __generic_file_fsync+0x117/0x180 fs/libfs.c:1081
>  fat_file_fsync+0x54/0x120 fs/fat/file.c:190
>  vfs_fsync_range+0x7c/0x150 fs/sync.c:197
>  generic_write_sync include/linux/fs.h:2867 [inline]
>  generic_file_write_iter+0x31c/0x38e mm/filemap.c:3452
>  call_write_iter include/linux/fs.h:1901 [inline]
>  do_iter_readv_writev+0x4a7/0x5d0 fs/read_write.c:693
>  do_iter_write fs/read_write.c:998 [inline]
>  do_iter_write+0x137/0x3a0 fs/read_write.c:979
>  vfs_iter_write+0x56/0x80 fs/read_write.c:1039
>  iter_file_splice_write+0x530/0x830 fs/splice.c:760
>  do_splice_from fs/splice.c:863 [inline]
>  direct_splice_actor+0x97/0xb0 fs/splice.c:1037
>  splice_direct_to_actor+0x22f/0x540 fs/splice.c:992
>  do_splice_direct+0x152/0x1d0 fs/splice.c:1080
>  do_sendfile+0x396/0x810 fs/read_write.c:1520
>  __do_sys_sendfile64 fs/read_write.c:1575 [inline]
>  __se_sys_sendfile64 fs/read_write.c:1567 [inline]
>  __x64_sys_sendfile64+0xb8/0x140 fs/read_write.c:1567
>  do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> read to 0xffff8881015f423d of 1 bytes by task 9960 on cpu 0:
>  fat12_ent_get+0x5e/0x120 fs/fat/fatent.c:125
>  fat_ent_read+0x3de/0x560 fs/fat/fatent.c:370
>  fat_get_cluster+0x52b/0x920 fs/fat/cache.c:266
>  fat_bmap_cluster fs/fat/cache.c:299 [inline]
>  fat_get_mapped_cluster+0x105/0x230 fs/fat/cache.c:320
>  fat_bmap+0x146/0x28e fs/fat/cache.c:384
>  __fat_get_block fs/fat/inode.c:165 [inline]
>  fat_get_block+0x244/0x4f0 fs/fat/inode.c:190
>  __block_write_begin_int+0x306/0xf80 fs/buffer.c:2008
>  __block_write_begin fs/buffer.c:2058 [inline]
>  block_write_begin+0x76/0x160 fs/buffer.c:2117
>  cont_write_begin+0x3bd/0x660 fs/buffer.c:2466
>  fat_write_begin+0x69/0xc0 fs/fat/inode.c:236
>  pagecache_write_begin+0x67/0x90 mm/filemap.c:3106
>  cont_expand_zero fs/buffer.c:2393 [inline]
>  cont_write_begin+0x176/0x660 fs/buffer.c:2456
>  fat_write_begin+0x69/0xc0 fs/fat/inode.c:236
>  generic_perform_write+0x13a/0x320 mm/filemap.c:3287
>  __generic_file_write_iter+0x240/0x370 mm/filemap.c:3416
>  generic_file_write_iter+0x294/0x38e mm/filemap.c:3448
>  call_write_iter include/linux/fs.h:1901 [inline]
>  new_sync_write+0x303/0x400 fs/read_write.c:483
>  __vfs_write+0x9e/0xb0 fs/read_write.c:496
>  vfs_write fs/read_write.c:558 [inline]
>  vfs_write+0x189/0x380 fs/read_write.c:542
>  ksys_write+0xc5/0x1a0 fs/read_write.c:611
>  __do_sys_write fs/read_write.c:623 [inline]
>  __se_sys_write fs/read_write.c:620 [inline]
>  __x64_sys_write+0x49/0x60 fs/read_write.c:620
>  do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 PID: 9960 Comm: syz-executor.1 Not tainted 5.6.0-rc1-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> ==================================================================
>
>
> ---
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>
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-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>