syzbot

==================================================================
BUG: KCSAN: data-race in fat32_ent_put / fat_mirror_bhs

read-write to 0xffff88813c23f07c of 4 bytes by task 7304 on cpu 1:
 fat32_ent_put+0x4e/0x90 fs/fat/fatent.c:191
 fat_alloc_clusters+0x57b/0xc50 fs/fat/fatent.c:503
 fat_add_new_entries fs/fat/dir.c:1233 [inline]
 fat_add_entries+0xa2b/0x10b0 fs/fat/dir.c:1390
 vfat_add_entry+0x2103/0x2280 fs/fat/namei_vfat.c:675
 vfat_create+0xb2/0x1b0 fs/fat/namei_vfat.c:769
 lookup_open fs/namei.c:4489 [inline]
 open_last_lookups fs/namei.c:4589 [inline]
 path_openat+0xd75/0x2050 fs/namei.c:4833
 do_file_open+0x16c/0x290 fs/namei.c:4865
 do_sys_openat2+0x94/0x130 fs/open.c:1366
 do_sys_open fs/open.c:1372 [inline]
 __do_sys_creat fs/open.c:1450 [inline]
 __se_sys_creat fs/open.c:1444 [inline]
 __x64_sys_creat+0x65/0x90 fs/open.c:1444
 x64_sys_call+0x2ea0/0x3020 arch/x86/include/generated/asm/syscalls_64.h:86
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88813c23f000 of 512 bytes by task 7306 on cpu 0:
 fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
 fat_alloc_clusters+0xb48/0xc50 fs/fat/fatent.c:543
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2142
 block_write_begin fs/buffer.c:2253 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2591
 fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x183/0x490 mm/filemap.c:4319
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4436
 generic_file_write_iter+0x8d/0x310 mm/filemap.c:4462
 __kernel_write_iter+0x319/0x590 fs/read_write.c:621
 __kernel_write+0x99/0xc0 fs/read_write.c:641
 __dump_emit fs/coredump.c:1221 [inline]
 dump_emit+0x5ff/0x720 fs/coredump.c:1259
 elf_core_dump+0x1cc8/0x2330 fs/binfmt_elf.c:2089
 coredump_write+0xacc/0xdd0 fs/coredump.c:1050
 do_coredump fs/coredump.c:1127 [inline]
 vfs_coredump+0x26c7/0x3130 fs/coredump.c:1201
 get_signal+0xd80/0xf60 kernel/signal.c:3019
 arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
 __exit_to_user_mode_loop kernel/entry/common.c:64 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:98 [inline]
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
 irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:270 [inline]
 irqentry_exit_to_user_mode include/linux/irq-entry-common.h:339 [inline]
 irqentry_exit+0xf7/0x520 kernel/entry/common.c:219
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 7306 Comm: syz.6.920 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================