fuse: zero the partial EOF page when extending a file

Extending a fuse file past a non-page-aligned EOF does not zero the tail of
the old last page.  When that page is cached and has been mmap-dirtied beyond
the old EOF, the now in-bounds tail is served to later reads as stale data
rather than zeros, which violates POSIX file-extension semantics.

Some file systems get this zeroing automatically at writeback time
(block_write_full_folio() / iomap_writeback_handle_eof() zero the tail of the
folio straddling i_size).  A non-writeback caching fuse file system uses neither
path, so it has to zero the tail itself from the size-extending paths, like
XFS (xfs_file_write_zero_eof()) and ext4 (ext4_block_zero_eof()) do.

Call truncate_pagecache_range() over the newly-exposed range up front from the
three paths that extend a file, before the new size is published:

  - a buffered write whose position is past the old EOF (fuse_perform_write());
  - a size-extending setattr/truncate (fuse_do_setattr());
  - a size-extending fallocate (fuse_file_fallocate()).

This unmaps the stale mappings and zeroes the partial tail of the old EOF
folio, so a later read returns zeros.  Truncating [old EOF, write start) before
a buffered write keeps the dropped range disjoint from the written data, so a
write that lands inside the old EOF folio is preserved.

writeback_cache connections are unaffected, as their writes go through
iomap_file_buffered_write(), which zeroes post-EOF folios.  The bug is
observable on a non-writeback_cache server that returns FOPEN_KEEP_CACHE on
writable files (without FOPEN_DIRECT_IO), and is caught by the new
write_extend_eof fuse selftest.

Signed-off-by: Jimmy Zuber <jamz@amazon.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -2289,6 +2289,9 @@ int fuse_do_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
 	 */
 	if ((is_truncate || !is_wb) &&
 	    S_ISREG(inode->i_mode) && oldsize != outarg.attr.size) {
+		if (outarg.attr.size > oldsize)
+			truncate_pagecache_range(inode, oldsize,
+						 outarg.attr.size - 1);
 		truncate_pagecache(inode, outarg.attr.size);
 		invalidate_inode_pages2(mapping);
 	}

fs/fuse/file.c

@@ -1346,9 +1346,13 @@ static ssize_t fuse_perform_write(struct kiocb *iocb, struct iov_iter *ii)
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	loff_t pos = iocb->ki_pos;
+	loff_t old_size = i_size_read(inode);
 	int err = 0;
 	ssize_t res = 0;
 
+	if (pos > old_size)
+		truncate_pagecache_range(inode, old_size, pos - 1);
+
 	if (inode->i_size < pos + iov_iter_count(ii))
 		set_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
 
@@ -2895,6 +2899,11 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 
 	/* we could have extended the file */
 	if (!(mode & FALLOC_FL_KEEP_SIZE)) {
+		loff_t oldsize = i_size_read(inode);
+
+		if (offset + length > oldsize)
+			truncate_pagecache_range(inode, oldsize,
+						 offset + length - 1);
 		if (fuse_write_update_attr(inode, offset + length, length))
 			file_update_time(file);
 	}