[PATCH 13/30] ocfs2: abstract the fast symlink NUL-termination check

[Joseph Qi <joseph.qi@linux.alibaba.com>]

Fold the strnlen() test into a predicate that bounds the read itself
against ocfs2_fast_symlink_chars(), which is exactly the space available
for id2.i_symlink in the inode block.

The open-coded version was only safe because the preceding i_size check
had already bailed: i_size comes from disk, so an unbounded strnlen() on
i_size + 1 bytes can run past the end of the buffer.  Keeping that
ordering dependency implicit in a helper would be fragile, especially
once the filecheck variant starts calling it, so the predicate returns
false for an oversized i_size and leaves that case to
ocfs2_dinode_has_oversized_fast_symlink().

At the existing call site the size check still runs first, so behaviour
is unchanged.

No functional change intended.

Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
---
 fs/ocfs2/inode.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
index 51bfa7643ebd..9cf2541b82ba 100644
--- a/fs/ocfs2/inode.c
+++ b/fs/ocfs2/inode.c
@@ -197,6 +197,25 @@ static bool ocfs2_dinode_has_oversized_fast_symlink(struct super_block *sb,
 	return le64_to_cpu(di->i_size) >= ocfs2_fast_symlink_chars(sb);
 }
 
+static bool ocfs2_dinode_has_unterminated_fast_symlink(struct super_block *sb,
+						       struct ocfs2_dinode *di)
+{
+	u64 i_size = le64_to_cpu(di->i_size);
+
+	/*
+	 * Bound the strnlen() here rather than relying on the caller having
+	 * rejected an oversized i_size first.  i_size comes from disk, and
+	 * ocfs2_fast_symlink_chars() is exactly the space available in the
+	 * inode block, so without this bound the read can run past it.
+	 * An oversized i_size is reported by
+	 * ocfs2_dinode_has_oversized_fast_symlink() instead.
+	 */
+	if (i_size >= ocfs2_fast_symlink_chars(sb))
+		return false;
+
+	return strnlen((char *)di->id2.i_symlink, i_size + 1) != i_size;
+}
+
 void ocfs2_set_inode_flags(struct inode *inode)
 {
 	unsigned int flags = OCFS2_I(inode)->ip_attr;
@@ -1801,7 +1820,7 @@ int ocfs2_validate_inode_block(struct super_block *sb,
 			goto bail;
 		}
 
-		if (strnlen((char *)di->id2.i_symlink, i_size + 1) != i_size) {
+		if (ocfs2_dinode_has_unterminated_fast_symlink(sb, di)) {
 			rc = ocfs2_error(sb,
 					 "Invalid dinode #%llu: fast symlink is not NUL-terminated at i_size %llu\n",
 					 (unsigned long long)bh->b_blocknr,
-- 
2.39.3