add unrestricted_chown option to vfs (mount)

Unrestricted chown: adds "unrestricted_chown" mount option.

Add a new unrestricted_chown mount option to the VFS. This option is
used to disable the Posix _POSIX_CHOWN_RESTRICTED chown option on a given
filesystem which forbids non-privilegued users to give away files.

This is currently implemented by XFS as a sysctl, but making it per
filesystem and available not just for XFS makes more sense.  This has
also been requested in http://oss.sgi.com/bugzilla/show_bug.cgi?id=768.

See http://www.opengroup.org/onlinepubs/009695399/functions/c...
for details.  A patch to util-linux-ng to add the new mount option
is also beeing submitted.

This patch is on top of 2.6.28-rc5.

Signed-off-by: Niraj Kumar <niraj17@gmail.com>


diff --git a/fs/attr.c b/fs/attr.c
index 7a83819..4547563 100644
--- a/fs/attr.c
+++ b/fs/attr.c
@@ -30,15 +30,27 @@ int inode_change_ok(struct inode *inode, struct iattr *attr)
 	/* Make sure a caller can chown. */
 	if ((ia_valid & ATTR_UID) &&
 	    (current->fsuid != inode->i_uid ||
-	     attr->ia_uid != inode->i_uid) && !capable(CAP_CHOWN))
-		goto error;
+	     attr->ia_uid != inode->i_uid) && !capable(CAP_CHOWN)) {
+		/* if "unrestricted_chown" is set, make sure owner can chown */
+		if (!((inode->i_sb->s_flags & MS_UNRESTRICTED_CHOWN) &&
+			(current->fsuid == inode->i_uid)))
+				goto error;
+	}
 
 	/* Make sure caller can chgrp. */
 	if ((ia_valid & ATTR_GID) &&
 	    (current->fsuid != inode->i_uid ||
 	    (!in_group_p(attr->ia_gid) && attr->ia_gid != inode->i_gid)) &&
-	    !capable(CAP_CHOWN))
-		goto error;
+	    !capable(CAP_CHOWN)) {
+		/*
+		 * if "unrestricted_chown" is set, make sure the owner can
+		 * change the group id only if he is a member of that group.
+		 */
+		if (!((inode->i_sb->s_flags & MS_UNRESTRICTED_CHOWN) &&
+			(current->fsuid == inode->i_uid &&
+			in_group_p(attr->ia_gid))))
+				goto error;
+	}
 
 	/* Make sure a caller can chmod. */
 	if (ia_valid & ATTR_MODE) {
diff --git a/fs/namespace.c b/fs/namespace.c
index 65b3dc8..3da02c3 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -757,6 +757,7 @@ static int show_sb_opts(struct seq_file *m, struct super_block *sb)
 		{ MS_SYNCHRONOUS, ",sync" },
 		{ MS_DIRSYNC, ",dirsync" },
 		{ MS_MANDLOCK, ",mand" },
+		{ MS_UNRESTRICTED_CHOWN, ",unrestricted_chown" },
 		{ 0, NULL }
 	};
 	const struct proc_fs_info *fs_infop;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 0dcdd94..090697b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -119,6 +119,7 @@ extern int dir_notify_enable;
 #define MS_REMOUNT	32	/* Alter flags of a mounted FS */
 #define MS_MANDLOCK	64	/* Allow mandatory locks on an FS */
 #define MS_DIRSYNC	128	/* Directory modifications are synchronous */
+#define MS_UNRESTRICTED_CHOWN	256	/* Unrestricted chown */
 #define MS_NOATIME	1024	/* Do not update access times. */
 #define MS_NODIRATIME	2048	/* Do not update directory access times */
 #define MS_BIND		4096
@@ -141,7 +142,8 @@ extern int dir_notify_enable;
 /*
  * Superblock flags that can be altered by MS_REMOUNT
  */
-#define MS_RMT_MASK	(MS_RDONLY|MS_SYNCHRONOUS|MS_MANDLOCK|MS_I_VERSION)
+#define MS_RMT_MASK	(MS_RDONLY|MS_SYNCHRONOUS|MS_MANDLOCK|MS_I_VERSION|\
+			MS_UNRESTRICTED_CHOWN)
 
 /*
  * Old magic mount flag and mask
--
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