syzbot

loop3: detected capacity change from 0 to 128
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BUG: KCSAN: data-race in fat32_ent_put / fat_mirror_bhs

read-write to 0xffff88810ad1a134 of 4 bytes by task 8303 on cpu 0:
 fat32_ent_put+0x4e/0x90 fs/fat/fatent.c:191
 fat_alloc_clusters+0x4d1/0xa80 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+0x400/0xf90 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
 generic_perform_write+0x181/0x490 mm/filemap.c:4112
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4229
 generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4255
 __kernel_write_iter+0x253/0x4c0 fs/read_write.c:619
 dump_emit_page fs/coredump.c:1112 [inline]
 dump_user_range+0x407/0x8c0 fs/coredump.c:1186
 elf_core_dump+0x1dc2/0x1f80 fs/binfmt_elf.c:2075
 do_coredump+0x1dfd/0x27b0 fs/coredump.c:950
 get_signal+0xd85/0xf70 kernel/signal.c:3019
 arch_do_signal_or_restart+0x96/0x480 arch/x86/kernel/signal.c:337
 exit_to_user_mode_loop kernel/entry/common.c:111 [inline]
 exit_to_user_mode_prepare include/linux/entry-common.h:330 [inline]
 irqentry_exit_to_user_mode+0x5e/0xa0 kernel/entry/common.c:184
 irqentry_exit+0x12/0x50 kernel/entry/common.c:287
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:623

read to 0xffff88810ad1a000 of 512 bytes by task 8301 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_new_entries fs/fat/dir.c:1230 [inline]
 fat_add_entries+0x97b/0xfc0 fs/fat/dir.c:1388
 vfat_add_entry+0x19a2/0x1b90 fs/fat/namei_vfat.c:674
 vfat_create+0xa8/0x190 fs/fat/namei_vfat.c:768
 lookup_open fs/namei.c:3717 [inline]
 open_last_lookups fs/namei.c:3816 [inline]
 path_openat+0x1102/0x2170 fs/namei.c:4052
 do_filp_open+0x109/0x230 fs/namei.c:4082
 do_sys_openat2+0xa6/0x110 fs/open.c:1437
 do_sys_open fs/open.c:1452 [inline]
 __do_sys_openat fs/open.c:1468 [inline]
 __se_sys_openat fs/open.c:1463 [inline]
 __x64_sys_openat+0xf2/0x120 fs/open.c:1463
 x64_sys_call+0x1af/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:258
 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: 1 UID: 0 PID: 8301 Comm: syz.3.1491 Not tainted 6.16.0-rc3-syzkaller-00044-g7595b66ae9de #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
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