[PATCH 069/109] fs: add ksys_chroot() helper; remove-in kernel calls to sys_chroot()

[Dominik Brodowski]

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@xxxxxxxxxxxxxxxxxxxxxxxxxx

Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
---
 drivers/base/devtmpfs.c  | 2 +-
 fs/open.c                | 7 ++++++-
 include/linux/syscalls.h | 1 +
 init/do_mounts.c         | 2 +-
 init/do_mounts_initrd.c  | 4 ++--
 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 @@ SYSCALL_DEFINE1(fchdir, unsigned int, fd)
 	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 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
 	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");
-- 
2.16.3