For compatibility reason, MAY_CHROOT is always set with MAY_CHDIR.
However, this new flag enable to differentiate a chdir form a chroot.

This is needed for the Landlock LSM to be able to evaluate a new root
directory.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: James Morris <jmorris@namei.org>
Cc: John Johansen <john.johansen@canonical.com>
Cc: Kees Cook <redacted>
Cc: Paul Moore <paul@paul-moore.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Stephen Smalley <redacted>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: linux-fsdevel@vger.kernel.org
---
 fs/open.c          | 3 ++-
 include/linux/fs.h | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/open.c b/fs/open.c
index b5b80469b93d..e8767318fd03 100644
--- a/fs/open.c
+++ b/fs/open.c

@@ -494,7 +494,8 @@ int ksys_chroot(const char __user *filename)
 	if (error)
 		goto out;
 
-	error = inode_permission(path.dentry->d_inode, MAY_EXEC | MAY_CHDIR);
+	error = inode_permission(path.dentry->d_inode, MAY_EXEC | MAY_CHDIR |
+			MAY_CHROOT);
 	if (error)
 		goto dput_and_out;
 

diff --git a/include/linux/fs.h b/include/linux/fs.h
index f7fdfe93e25d..fa6c9d0c152b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -99,6 +99,7 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 #define MAY_CHDIR		0x00000040
 /* called from RCU mode, don't block */
 #define MAY_NOT_BLOCK		0x00000080
+#define MAY_CHROOT		0x00000100
 
 /*
  * flags in file.f_mode.  Note that FMODE_READ and FMODE_WRITE must correspond

-- 
2.20.1