[Patch 2/2] vfs: allow file truncations when both suid and write permissions set

[Amerigo Wang]

With patch 1/2 applied, now we can just simply add ATTR_FORCE flag, to
remove suid on truncation.

Signed-off-by: WANG Cong <amwang@xxxxxxxxxx>
Cc: Eric Sandeen <esandeen@xxxxxxxxxx>
Cc: Eric Paris <eparis@xxxxxxxxxx>
Cc: Eugene Teo <eteo@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>

---
diff --git a/fs/open.c b/fs/open.c
index dd98e80..40d1fa2 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -199,7 +199,7 @@ out:
 int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
 	struct file *filp)
 {
-	int err;
+	int ret;
 	struct iattr newattrs;
 
 	/* Not pretty: "inode->i_size" shouldn't really be signed. But it is. */
@@ -214,12 +214,14 @@ int do_truncate(struct dentry *dentry, loff_t length, unsigned int time_attrs,
 	}
 
 	/* Remove suid/sgid on truncate too */
-	newattrs.ia_valid |= should_remove_suid(dentry);
+	ret = should_remove_suid(dentry);
+	if (ret)
+		newattrs.ia_valid |= ret | ATTR_FORCE;
 
 	mutex_lock(&dentry->d_inode->i_mutex);
-	err = notify_change(dentry, &newattrs);
+	ret = notify_change(dentry, &newattrs);
 	mutex_unlock(&dentry->d_inode->i_mutex);
-	return err;
+	return ret;
 }
 
 static long do_sys_truncate(const char __user *pathname, loff_t length)
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