filemap: Fix bounds checking in filemap_read()

If the caller supplies an iocb->ki_pos value that is close to the
filesystem upper limit, and an iterator with a count that causes us to
overflow that limit, then filemap_read() enters an infinite loop.

This behaviour was discovered when testing xfstests generic/525 with the
"localio" optimisation for loopback NFS mounts.

Reported-by: Mike Snitzer <[e-mail]>
Fixes: c2a9737f45e2 ("vfs,mm: fix a dead loop in truncate_inode_pages_range()")
Tested-by: Mike Snitzer <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/filemap.c

···
 	if (unlikely(!iov_iter_count(iter)))
 		return 0;
 
-	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
+	iov_iter_truncate(iter, inode->i_sb->s_maxbytes - iocb->ki_pos);
 	folio_batch_init(&fbatch);
 
 	do {