fat: hold truncate_lock in fat_write_failed()

The current fat_write_failed() implementation calls into
fat_truncate_blocks() without holding truncate_lock.

A failed or short extending write racing with FIBMAP can spuriously make a
FAT filesystem read-only.  FIBMAP bounds its lookup by i_blocks, so it can
walk allocated clusters beyond i_size while fat_write_failed() is removing
them.  If the chain is shortened during the walk, the mapper can report a
bogus "request beyond EOF" error, triggering the default read-only error
policy.  Hold truncate_lock for write around the block cleanup to
serialize it with _fat_bmap().

For comparison, fat_setattr() correctly holds truncate_lock during
truncation:

	down_write(&MSDOS_I(inode)->truncate_lock);
	truncate_setsize(inode, attr->ia_size);
	fat_truncate_blocks(inode, attr->ia_size);
	up_write(&MSDOS_I(inode)->truncate_lock);

Make it so ->truncate_lock is held exclusive while truncating.

Link: https://lore.kernel.org/20261021162105.62525-1-matheus.aalmeida@inf.ufrgs.br
Fixes: 16fab2015099 ("fat: permit to return phy block number by fibmap in fallocated region")
Fixes: 459f6ed3b8c7 ("fat: convert to use the new truncate convention.")
Signed-off-by: Matheus Alves de Almeida <matheus.aalmeida@inf.ufrgs.br>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions