fs: add ksys_chroot() helper; remove-in kernel calls to sys_chroot()

Using this helper allows us to avoid the in-kernel calls to the
sys_chroot() syscall. The ksys_ prefix denotes that this function is
meant as a drop-in replacement for the syscall. In particular, it uses the
same calling convention as sys_chroot().

In the near future, the fs-external callers of ksys_chroot() should be
converted to use kern_path()/set_fs_root() directly. Then ksys_chroot()
can be moved within sys_chroot() again.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

5 files changed, 11 insertions, 5 deletions

diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index 4afb04686c8e..5743f04014ca 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -387,7 +387,7 @@ static int devtmpfsd(void *p)
         if (*err)
                 goto out;
         sys_chdir("/.."); /* will traverse into overmounted root */
-        sys_chroot(".");
+        ksys_chroot(".");
         complete(&setup_done);
         while (1) {
                 spin_lock(&req_lock);
diff --git a/fs/open.c b/fs/open.c
index 7ea118471dce..7a475e8a2e41 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -479,7 +479,7 @@ out:
         return error;
 }
 
-SYSCALL_DEFINE1(chroot, const char __user *, filename)
+int ksys_chroot(const char __user *filename)
 {
         struct path path;
         int error;
@@ -512,6 +512,11 @@ out:
         return error;
 }
 
+SYSCALL_DEFINE1(chroot, const char __user *, filename)
+{
+        return ksys_chroot(filename);
+}
+
 static int chmod_common(const struct path *path, umode_t mode)
 {
         struct inode *inode = path->dentry->d_inode;
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 50876ae1d17b..920a0db1871d 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -950,5 +950,6 @@ int ksys_mount(char __user *dev_name, char __user *dir_name, char __user *type,
                unsigned long flags, void __user *data);
 int ksys_umount(char __user *name, int flags);
 int ksys_dup(unsigned int fildes);
+int ksys_chroot(const char __user *filename);
 
 #endif
diff --git a/init/do_mounts.c b/init/do_mounts.c
index eb768de43d84..2f06f7827b0c 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -600,7 +600,7 @@ void __init prepare_namespace(void)
 out:
         devtmpfs_mount("dev");
         ksys_mount(".", "/", NULL, MS_MOVE, NULL);
-        sys_chroot(".");
+        ksys_chroot(".");
 }
 
 static bool is_tmpfs;
diff --git a/init/do_mounts_initrd.c b/init/do_mounts_initrd.c
index e8573e1776f6..71293265ac4b 100644
--- a/init/do_mounts_initrd.c
+++ b/init/do_mounts_initrd.c
@@ -44,7 +44,7 @@ static int init_linuxrc(struct subprocess_info *info, struct cred *new)
         /* move initrd over / and chdir/chroot in initrd root */
         sys_chdir("/root");
         ksys_mount(".", "/", NULL, MS_MOVE, NULL);
-        sys_chroot(".");
+        ksys_chroot(".");
         sys_setsid();
         return 0;
 }
@@ -83,7 +83,7 @@ static void __init handle_initrd(void)
         /* move initrd to rootfs' /old */
         ksys_mount("..", ".", NULL, MS_MOVE, NULL);
         /* switch root and cwd back to / of rootfs */
-        sys_chroot("..");
+        ksys_chroot("..");
 
         if (new_decode_dev(real_root_dev) == Root_RAM0) {
                 sys_chdir("/old");