fuse: truncate pagecache range on hole punch

fuse supports hole punch via the fallocate() FALLOC_FL_PUNCH_HOLE
interface. When a hole punch is passed through, the page cache
is not cleared and thus allows reading stale data from the cache.

This is easily demonstrable (using FOPEN_KEEP_CACHE) by reading a
smallish random data file into cache, punching a hole and creating
a copy of the file. Drop caches or remount and observe that the
original file no longer matches the file copied after the hole
punch. The original file contains a zeroed range and the latter
file contains stale data.

Protect against writepage requests in progress and punch out the
associated page cache range after a successful client fs hole
punch.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -16,6 +16,7 @@
 #include <linux/compat.h>
 #include <linux/swap.h>
 #include <linux/aio.h>
+#include <linux/falloc.h>
 
 static const struct file_operations fuse_direct_io_file_operations;
 
@@ -2453,6 +2454,7 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 				loff_t length)
 {
 	struct fuse_file *ff = file->private_data;
+	struct inode *inode = file->f_inode;
 	struct fuse_conn *fc = ff->fc;
 	struct fuse_req *req;
 	struct fuse_fallocate_in inarg = {
@@ -2466,9 +2468,16 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	if (fc->no_fallocate)
 		return -EOPNOTSUPP;
 
+	if (mode & FALLOC_FL_PUNCH_HOLE) {
+		mutex_lock(&inode->i_mutex);
+		fuse_set_nowrite(inode);
+	}
+
 	req = fuse_get_req_nopages(fc);
-	if (IS_ERR(req))
-		return PTR_ERR(req);
+	if (IS_ERR(req)) {
+		err = PTR_ERR(req);
+		goto out;
+	}
 
 	req->in.h.opcode = FUSE_FALLOCATE;
 	req->in.h.nodeid = ff->nodeid;
@@ -2483,6 +2492,15 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	}
 	fuse_put_request(fc, req);
 
+out:
+	if (mode & FALLOC_FL_PUNCH_HOLE) {
+		if (!err)
+			truncate_pagecache_range(inode, offset,
+						 offset + length - 1);
+		fuse_release_nowrite(inode);
+		mutex_unlock(&inode->i_mutex);
+	}
+
 	return err;
 }