[PATCH v2] erofs: fix incorrect symlink detection in fast symlink

[Gao Xiang]

Fast symlink can be used if the on-disk symlink data is stored
in the same block as the on-disk inode, so we don’t need to trigger
another I/O for symlink data.  However, correctly fs correction could be
reported _incorrectly_ if inode xattrs are too large.

In fact, these should be valid images although they cannot be handled as
fast symlinks.

Many thanks to Colin for reporting this!

Reported-by: Colin Walters <walters at verbum.org>
Fixes: 431339ba9042 ("staging: erofs: add inode operations")
Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
---
v2:
 - sent out a wrong version (`m_pofs += vi->xattr_isize` was missed).

 fs/erofs/inode.c | 20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/fs/erofs/inode.c b/fs/erofs/inode.c
index 5f6439a63af7..f2cab9e4f3bc 100644
--- a/fs/erofs/inode.c
+++ b/fs/erofs/inode.c
@@ -178,12 +178,14 @@ static int erofs_fill_symlink(struct inode *inode, void *kaddr,
 			      unsigned int m_pofs)
 {
 	struct erofs_inode *vi = EROFS_I(inode);
-	unsigned int bsz = i_blocksize(inode);
+	loff_t off;
 	char *lnk;
 
-	/* if it cannot be handled with fast symlink scheme */
-	if (vi->datalayout != EROFS_INODE_FLAT_INLINE ||
-	    inode->i_size >= bsz || inode->i_size < 0) {
+	m_pofs += vi->xattr_isize;
+	/* check if it cannot be handled with fast symlink scheme */
+	if (vi->datalayout != EROFS_INODE_FLAT_INLINE || inode->i_size < 0 ||
+	    check_add_overflow(m_pofs, inode->i_size, &off) ||
+	    off > i_blocksize(inode)) {
 		inode->i_op = &erofs_symlink_iops;
 		return 0;
 	}
@@ -192,16 +194,6 @@ static int erofs_fill_symlink(struct inode *inode, void *kaddr,
 	if (!lnk)
 		return -ENOMEM;
 
-	m_pofs += vi->xattr_isize;
-	/* inline symlink data shouldn't cross block boundary */
-	if (m_pofs + inode->i_size > bsz) {
-		kfree(lnk);
-		erofs_err(inode->i_sb,
-			  "inline data cross block boundary @ nid %llu",
-			  vi->nid);
-		DBG_BUGON(1);
-		return -EFSCORRUPTED;
-	}
 	memcpy(lnk, kaddr + m_pofs, inode->i_size);
 	lnk[inode->i_size] = '\0';
 
-- 
2.43.5