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>

diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index 4afb046..5743f04 100644 (file)
--- 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 7ea1184..7a475e8 100644 (file)
--- 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 50876ae..920a0db 100644 (file)
--- 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 eb768de..2f06f78 100644 (file)
--- 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 e8573e1..7129326 100644 (file)
--- 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");