syzbot

loop6: detected capacity change from 0 to 8192
==================================================================
BUG: KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

write to 0xffff8881773f368c of 1 bytes by task 18314 on cpu 1:
 fat12_ent_put+0x74/0x170 fs/fat/fatent.c:168
 fat_free_clusters+0x2a9/0x790 fs/fat/fatent.c:597
 vfat_mkdir+0xe9/0x1f0 fs/fat/namei_vfat.c:883
 vfs_mkdir+0x210/0x340 fs/namei.c:4375
 do_mkdirat+0x132/0x3f0 fs/namei.c:4408
 __do_sys_mkdir fs/namei.c:4430 [inline]
 __se_sys_mkdir fs/namei.c:4428 [inline]
 __x64_sys_mkdir+0x42/0x50 fs/namei.c:4428
 x64_sys_call+0x2289/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:84
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff8881773f3600 of 512 bytes by task 18312 on cpu 0:
 fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
 fat_ent_write+0xd0/0xe0 fs/fat/fatent.c:423
 fat_chain_add+0x15b/0x3f0 fs/fat/misc.c:136
 fat_add_cluster fs/fat/inode.c:112 [inline]
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x46c/0x5e0 fs/fat/inode.c:189
 __block_write_begin_int+0x3fd/0xf90 fs/buffer.c:2151
 block_write_begin fs/buffer.c:2262 [inline]
 cont_write_begin+0x5fc/0x970 fs/buffer.c:2601
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 cont_expand_zero fs/buffer.c:2528 [inline]
 cont_write_begin+0x1ad/0x970 fs/buffer.c:2591
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2492
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
 fat_fallocate+0x177/0x1c0 fs/fat/file.c:301
 vfs_fallocate+0x413/0x450 fs/open.c:341
 ksys_fallocate fs/open.c:365 [inline]
 __do_sys_fallocate fs/open.c:370 [inline]
 __se_sys_fallocate fs/open.c:368 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:368
 x64_sys_call+0x2b88/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:286
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 18312 Comm: syz.6.4784 Not tainted 6.16.0-rc2-syzkaller-00269-g11313e2f7812 #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
==================================================================
FAT-fs (loop6): error, corrupted directory (invalid entries)
FAT-fs (loop6): Filesystem has been set read-only