init: add an init_chroot helper

Add a simple helper to chroot with a kernel space file name and switch
the early init code over to it.  Remove the now unused ksys_chroot.
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/base/devtmpfs.c

@@ -413,7 +413,7 @@ static int __init devtmpfs_setup(void *p)
 	if (err)
 		goto out;
 	init_chdir("/.."); /* will traverse into overmounted root */
-	ksys_chroot(".");
+	init_chroot(".");
 out:
 	*(int *)p = err;
 	complete(&setup_done);

fs/init.c

@@ -9,6 +9,7 @@
 #include <linux/fs.h>
 #include <linux/fs_struct.h>
 #include <linux/init_syscalls.h>
+#include <linux/security.h>
 #include "internal.h"
 
 int __init init_mount(const char *dev_name, const char *dir_name,
@@ -54,6 +55,29 @@ int __init init_chdir(const char *filename)
 	return error;
 }
 
+int __init init_chroot(const char *filename)
+{
+	struct path path;
+	int error;
+
+	error = kern_path(filename, LOOKUP_FOLLOW | LOOKUP_DIRECTORY, &path);
+	if (error)
+		return error;
+	error = inode_permission(path.dentry->d_inode, MAY_EXEC | MAY_CHDIR);
+	if (error)
+		goto dput_and_out;
+	error = -EPERM;
+	if (!ns_capable(current_user_ns(), CAP_SYS_CHROOT))
+		goto dput_and_out;
+	error = security_path_chroot(&path);
+	if (error)
+		goto dput_and_out;
+	set_fs_root(current->fs, &path);
+dput_and_out:
+	path_put(&path);
+	return error;
+}
+
 int __init init_unlink(const char *pathname)
 {
 	return do_unlinkat(AT_FDCWD, getname_kernel(pathname));

fs/open.c

@@ -530,7 +530,7 @@ SYSCALL_DEFINE1(fchdir, unsigned int, fd)
 	return error;
 }
 
-int ksys_chroot(const char __user *filename)
+SYSCALL_DEFINE1(chroot, const char __user *, filename)
 {
 	struct path path;
 	int error;
@@ -563,11 +563,6 @@ int ksys_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;

include/linux/init_syscalls.h

@@ -4,5 +4,6 @@ int __init init_mount(const char *dev_name, const char *dir_name,
 		const char *type_page, unsigned long flags, void *data_page);
 int __init init_umount(const char *name, int flags);
 int __init init_chdir(const char *filename);
+int __init init_chroot(const char *filename);
 int __init init_unlink(const char *pathname);
 int __init init_rmdir(const char *pathname);

include/linux/syscalls.h

@@ -1235,8 +1235,6 @@ asmlinkage long sys_ni_syscall(void);
  * Instead, use one of the functions which work equivalently, such as
  * the ksys_xyzyyz() functions prototyped below.
  */
-
-int ksys_chroot(const char __user *filename);
 ssize_t ksys_write(unsigned int fd, const char __user *buf, size_t count);
 int ksys_fchown(unsigned int fd, uid_t user, gid_t group);
 ssize_t ksys_read(unsigned int fd, char __user *buf, size_t count);

init/do_mounts.c

@@ -629,7 +629,7 @@ void __init prepare_namespace(void)
 out:
 	devtmpfs_mount();
 	init_mount(".", "/", NULL, MS_MOVE, NULL);
-	ksys_chroot(".");
+	init_chroot(".");
 }
 
 static bool is_tmpfs;

init/do_mounts_initrd.c

@@ -63,7 +63,7 @@ static int __init init_linuxrc(struct subprocess_info *info, struct cred *new)
 	/* move initrd over / and chdir/chroot in initrd root */
 	init_chdir("/root");
 	init_mount(".", "/", NULL, MS_MOVE, NULL);
-	ksys_chroot(".");
+	init_chroot(".");
 	ksys_setsid();
 	return 0;
 }
@@ -101,7 +101,7 @@ static void __init handle_initrd(void)
 	/* move initrd to rootfs' /old */
 	init_mount("..", ".", NULL, MS_MOVE, NULL);
 	/* switch root and cwd back to / of rootfs */
-	ksys_chroot("..");
+	init_chroot("..");
 
 	if (new_decode_dev(real_root_dev) == Root_RAM0) {
 		init_chdir("/old");