loop7: detected capacity change from 0 to 8192
__nla_validate_parse: 1 callbacks suppressed
netlink: 12 bytes leftover after parsing attributes in process `syz.7.2039'.
==================================================================
BUG: KCSAN: data-race in fat12_ent_put / fat_mirror_bhs

write to 0xffff888104f38521 of 1 bytes by task 9523 on cpu 0:
 fat12_ent_put+0x74/0x170 fs/fat/fatent.c:168
 fat_alloc_clusters+0x4ce/0xa80 fs/fat/fatent.c:503
 fat_add_cluster fs/fat/inode.c:107 [inline]
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:189
 __block_write_begin_int+0x419/0xf70 fs/buffer.c:2151
 block_write_begin fs/buffer.c:2262 [inline]
 cont_write_begin+0x5a3/0x8e0 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+0x198/0x8e0 fs/buffer.c:2591
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 generic_cont_expand_simple+0xad/0x150 fs/buffer.c:2492
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
 fat_setattr+0x2a5/0x8a0 fs/fat/file.c:513
 notify_change+0x806/0x890 fs/attr.c:552
 do_truncate fs/open.c:65 [inline]
 do_ftruncate+0x327/0x430 fs/open.c:192
 do_sys_ftruncate fs/open.c:207 [inline]
 __do_sys_ftruncate fs/open.c:212 [inline]
 __se_sys_ftruncate fs/open.c:210 [inline]
 __x64_sys_ftruncate+0x68/0xc0 fs/open.c:210
 x64_sys_call+0xd65/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:78
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd0/0x1a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888104f38400 of 512 bytes by task 9516 on cpu 1:
 fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
 fat_alloc_clusters+0x98b/0xa80 fs/fat/fatent.c:543
 fat_add_cluster fs/fat/inode.c:107 [inline]
 __fat_get_block fs/fat/inode.c:154 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:189
 __block_write_begin_int+0x419/0xf70 fs/buffer.c:2151
 block_write_begin fs/buffer.c:2262 [inline]
 cont_write_begin+0x5a3/0x8e0 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+0x198/0x8e0 fs/buffer.c:2591
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
 generic_cont_expand_simple+0xad/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+0x410/0x450 fs/open.c:338
 ksys_fallocate fs/open.c:362 [inline]
 __do_sys_fallocate fs/open.c:367 [inline]
 __se_sys_fallocate fs/open.c:365 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:365
 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+0xd0/0x1a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 9516 Comm: syz.7.2039 Not tainted 6.15.0-rc5-syzkaller-00032-g0d8d44db295c #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/19/2025
==================================================================
FAT-fs (loop7): error, clusters badly computed (504 != 502)
FAT-fs (loop7): error, clusters badly computed (510 != 503)
FAT-fs (loop7): error, clusters badly computed (512 != 504)
FAT-fs (loop7): error, clusters badly computed (513 != 505)
FAT-fs (loop7): error, clusters badly computed (514 != 506)
FAT-fs (loop7): error, clusters badly computed (515 != 507)
FAT-fs (loop7): error, clusters badly computed (516 != 508)
FAT-fs (loop7): error, clusters badly computed (517 != 509)
FAT-fs (loop7): error, clusters badly computed (518 != 510)
FAT-fs (loop7): error, clusters badly computed (520 != 511)
FAT-fs (loop7): error, clusters badly computed (522 != 512)
FAT-fs (loop7): error, clusters badly computed (523 != 513)
FAT-fs (loop7): error, clusters badly computed (523 != 514)
FAT-fs (loop7): error, clusters badly computed (524 != 515)
FAT-fs (loop7): error, clusters badly computed (525 != 516)
FAT-fs (loop7): error, clusters badly computed (527 != 517)
FAT-fs (loop7): error, clusters badly computed (530 != 518)
FAT-fs (loop7): error, clusters badly computed (532 != 519)
FAT-fs (loop7): error, clusters badly computed (534 != 520)
FAT-fs (loop7): error, clusters badly computed (536 != 521)
FAT-fs (loop7): error, clusters badly computed (538 != 522)
FAT-fs (loop7): error, clusters badly computed (540 != 523)
FAT-fs (loop7): error, clusters badly computed (542 != 524)
FAT-fs (loop7): error, clusters badly computed (544 != 525)
FAT-fs (loop7): error, clusters badly computed (546 != 526)
FAT-fs (loop7): error, clusters badly computed (548 != 527)
FAT-fs (loop7): error, fat_bmap_cluster: request beyond EOF (i_pos 2074)