eCryptfs: delay writing 0's after llseek until write

Delay writing 0's out in eCryptfs after a seek past the end of the file
until data is actually written.

http://www.opengroup.org/onlinepubs/009695399/functions/lseek.html

``The lseek() function shall not, by itself, extend the size of a
file.''

Without this fix, applications that lseek() past the end of the file without
writing will experience unexpected behavior.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 35 insertions, 3 deletions

diff --git a/fs/ecryptfs/mmap.c b/fs/ecryptfs/mmap.c
index 88ea669..55cec98 100644
--- a/fs/ecryptfs/mmap.c
+++ b/fs/ecryptfs/mmap.c
@@ -376,9 +376,31 @@ out:
 	return 0;
 }
 
+/**
+ * eCryptfs does not currently support holes. When writing after a
+ * seek past the end of the file, eCryptfs fills in 0's through to the
+ * current location. The code to fill in the 0's to all the
+ * intermediate pages calls ecryptfs_prepare_write_no_truncate().
+ */
+static int
+ecryptfs_prepare_write_no_truncate(struct file *file, struct page *page,
+				   unsigned from, unsigned to)
+{
+	int rc = 0;
+
+	if (from == 0 && to == PAGE_CACHE_SIZE)
+		goto out;	/* If we are writing a full page, it will be
+				   up to date. */
+	if (!PageUptodate(page))
+		rc = ecryptfs_do_readpage(file, page, page->index);
+out:
+	return rc;
+}
+
 static int ecryptfs_prepare_write(struct file *file, struct page *page,
 				  unsigned from, unsigned to)
 {
+	loff_t pos;
 	int rc = 0;
 
 	if (from == 0 && to == PAGE_CACHE_SIZE)
@@ -386,6 +408,16 @@ static int ecryptfs_prepare_write(struct file *file, struct page *page,
 				   up to date. */
 	if (!PageUptodate(page))
 		rc = ecryptfs_do_readpage(file, page, page->index);
+	pos = ((loff_t)page->index << PAGE_CACHE_SHIFT) + to;
+	if (pos > i_size_read(page->mapping->host)) {
+		rc = ecryptfs_truncate(file->f_path.dentry, pos);
+		if (rc) {
+			printk(KERN_ERR "Error on attempt to "
+			       "truncate to (higher) offset [%lld];"
+			       " rc = [%d]\n", pos, rc);
+			goto out;
+		}
+	}
 out:
 	return rc;
 }
@@ -744,10 +776,10 @@ int write_zeros(struct file *file, pgoff_t index, int start, int num_zeros)
 		rc = PTR_ERR(tmp_page);
 		goto out;
 	}
-	rc = ecryptfs_prepare_write(file, tmp_page, start, start + num_zeros);
-	if (rc) {
+	if ((rc = ecryptfs_prepare_write_no_truncate(file, tmp_page, start,
+						     (start + num_zeros)))) {
 		ecryptfs_printk(KERN_ERR, "Error preparing to write zero's "
-				"to remainder of page at index [0x%.16x]\n",
+				"to page at index [0x%.16x]\n",
 				index);
 		page_cache_release(tmp_page);
 		goto out;