fuse: fix max i/o size calculation

Fix a bug that Werner Baumann reported: fuse can send a bigger write request
than the maximum specified.  This only affected direct_io operation.

In addition set a sane minimum for the max_read and max_write tunables, so I/O
always makes some progress.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fuse/file.c

@@ -966,14 +966,15 @@ static ssize_t fuse_direct_io(struct file *file, const char __user *buf,
 
 	while (count) {
 		size_t nres;
-		size_t nbytes = min(count, nmax);
-		int err = fuse_get_user_pages(req, buf, nbytes, !write);
+		size_t nbytes_limit = min(count, nmax);
+		size_t nbytes;
+		int err = fuse_get_user_pages(req, buf, nbytes_limit, !write);
 		if (err) {
 			res = err;
 			break;
 		}
 		nbytes = (req->num_pages << PAGE_SHIFT) - req->page_offset;
-		nbytes = min(count, nbytes);
+		nbytes = min(nbytes_limit, nbytes);
 		if (write)
 			nres = fuse_send_write(req, file, inode, pos, nbytes,
 					       current->files);

fs/fuse/inode.c

@@ -584,6 +584,7 @@ static void process_init_reply(struct fuse_conn *fc, struct fuse_req *req)
 		fc->bdi.ra_pages = min(fc->bdi.ra_pages, ra_pages);
 		fc->minor = arg->minor;
 		fc->max_write = arg->minor < 5 ? 4096 : arg->max_write;
+		fc->max_write = min_t(unsigned, 4096, fc->max_write);
 		fc->conn_init = 1;
 	}
 	fuse_put_request(fc, req);
@@ -658,7 +659,7 @@ static int fuse_fill_super(struct super_block *sb, void *data, int silent)
 	fc->flags = d.flags;
 	fc->user_id = d.user_id;
 	fc->group_id = d.group_id;
-	fc->max_read = d.max_read;
+	fc->max_read = min_t(unsigned, 4096, d.max_read);
 
 	/* Used by get_root_inode() */
 	sb->s_fs_info = fc;