[PATCH] splice: fix bugs in pipe_to_file()

Found by Oleg Nesterov <oleg@tv-sign.ru>, fixed by me.

- Only allow full pages to go to the page cache.
- Check page != buf->page instead of using PIPE_BUF_FLAG_STOLEN.
- Remember to clear 'stolen' if add_to_page_cache() fails.

And as a cleanup on that:

- Make the bottom fall-through logic a little less convoluted. Also make
  the steal path hold an extra reference to the page, so we don't have
  to differentiate between stolen and non-stolen at the end.
Signed-off-by: Jens Axboe <axboe@suse.de>

fs/pipe.c

@@ -99,8 +99,6 @@ static void anon_pipe_buf_release(struct pipe_inode_info *pipe,
 {
 	struct page *page = buf->page;
 
-	buf->flags &= ~PIPE_BUF_FLAG_STOLEN;
-
 	/*
 	 * If nobody else uses this page, and we don't already have a
 	 * temporary page, let's keep track of it as a one-deep
@@ -130,7 +128,6 @@ static int anon_pipe_buf_steal(struct pipe_inode_info *pipe,
 	struct page *page = buf->page;
 
 	if (page_count(page) == 1) {
-		buf->flags |= PIPE_BUF_FLAG_STOLEN;
 		lock_page(page);
 		return 0;
 	}

fs/splice.c

@@ -78,7 +78,7 @@ static int page_cache_pipe_buf_steal(struct pipe_inode_info *info,
 		return 1;
 	}
 
-	buf->flags |= PIPE_BUF_FLAG_STOLEN | PIPE_BUF_FLAG_LRU;
+	buf->flags |= PIPE_BUF_FLAG_LRU;
 	return 0;
 }
 
@@ -87,7 +87,7 @@ static void page_cache_pipe_buf_release(struct pipe_inode_info *info,
 {
 	page_cache_release(buf->page);
 	buf->page = NULL;
-	buf->flags &= ~(PIPE_BUF_FLAG_STOLEN | PIPE_BUF_FLAG_LRU);
+	buf->flags &= ~PIPE_BUF_FLAG_LRU;
 }
 
 static void *page_cache_pipe_buf_map(struct file *file,
@@ -587,9 +587,10 @@ static int pipe_to_file(struct pipe_inode_info *info, struct pipe_buffer *buf,
 		this_len = PAGE_CACHE_SIZE - offset;
 
 	/*
-	 * Reuse buf page, if SPLICE_F_MOVE is set.
+	 * Reuse buf page, if SPLICE_F_MOVE is set and we are doing a full
+	 * page.
 	 */
-	if (sd->flags & SPLICE_F_MOVE) {
+	if ((sd->flags & SPLICE_F_MOVE) && this_len == PAGE_CACHE_SIZE) {
 		/*
 		 * If steal succeeds, buf->page is now pruned from the vm
 		 * side (LRU and page cache) and we can reuse it. The page
@@ -604,6 +605,8 @@ static int pipe_to_file(struct pipe_inode_info *info, struct pipe_buffer *buf,
 			goto find_page;
 		}
 
+		page_cache_get(page);
+
 		if (!(buf->flags & PIPE_BUF_FLAG_LRU))
 			lru_cache_add(page);
 	} else {
@@ -662,7 +665,7 @@ find_page:
 	} else if (ret)
 		goto out;
 
-	if (!(buf->flags & PIPE_BUF_FLAG_STOLEN)) {
+	if (buf->page != page) {
 		char *dst = kmap_atomic(page, KM_USER0);
 
 		memcpy(dst + offset, src + buf->offset, this_len);
@@ -671,22 +674,20 @@ find_page:
 	}
 
 	ret = mapping->a_ops->commit_write(file, page, offset, offset+this_len);
-	if (ret == AOP_TRUNCATED_PAGE) {
+	if (!ret) {
+		/*
+		 * Return the number of bytes written and mark page as
+		 * accessed, we are now done!
+		 */
+		ret = this_len;
+		mark_page_accessed(page);
+		balance_dirty_pages_ratelimited(mapping);
+	} else if (ret == AOP_TRUNCATED_PAGE) {
 		page_cache_release(page);
 		goto find_page;
-	} else if (ret)
-		goto out;
-
-	/*
-	 * Return the number of bytes written.
-	 */
-	ret = this_len;
-	mark_page_accessed(page);
-	balance_dirty_pages_ratelimited(mapping);
+	}
 out:
-	if (!(buf->flags & PIPE_BUF_FLAG_STOLEN))
-		page_cache_release(page);
-
+	page_cache_release(page);
 	unlock_page(page);
 out_nomem:
 	buf->ops->unmap(info, buf);

include/linux/pipe_fs_i.h

@@ -5,8 +5,7 @@
 
 #define PIPE_BUFFERS (16)
 
-#define PIPE_BUF_FLAG_STOLEN	0x01
-#define PIPE_BUF_FLAG_LRU	0x02
+#define PIPE_BUF_FLAG_LRU	0x01
 
 struct pipe_buffer {
 	struct page *page;