fs: add ksys_mount() helper; remove in-kernel calls to sys_mount()

Using this helper allows us to avoid the in-kernel calls to the sys_mount()
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_mount().

In the near future, all callers of ksys_mount() should be converted to call
do_mount() directly.

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, 19 insertions, 9 deletions

diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
index 50025d7959cb..4afb04686c8e 100644
--- a/drivers/base/devtmpfs.c
+++ b/drivers/base/devtmpfs.c
@@ -356,7 +356,8 @@ int devtmpfs_mount(const char *mntdir)
         if (!thread)
                 return 0;
 
-        err = sys_mount("devtmpfs", (char *)mntdir, "devtmpfs", MS_SILENT, NULL);
+        err = ksys_mount("devtmpfs", (char *)mntdir, "devtmpfs", MS_SILENT,
+                         NULL);
         if (err)
                 printk(KERN_INFO "devtmpfs: error mounting %i\n", err);
         else
@@ -382,7 +383,7 @@ static int devtmpfsd(void *p)
         *err = sys_unshare(CLONE_NEWNS);
         if (*err)
                 goto out;
-        *err = sys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, options);
+        *err = ksys_mount("devtmpfs", "/", "devtmpfs", MS_SILENT, options);
         if (*err)
                 goto out;
         sys_chdir("/.."); /* will traverse into overmounted root */
diff --git a/fs/namespace.c b/fs/namespace.c
index 9d1374ab6e06..642b8b229944 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3032,8 +3032,8 @@ struct dentry *mount_subtree(struct vfsmount *mnt, const char *name)
 }
 EXPORT_SYMBOL(mount_subtree);
 
-SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
-                char __user *, type, unsigned long, flags, void __user *, data)
+int ksys_mount(char __user *dev_name, char __user *dir_name, char __user *type,
+               unsigned long flags, void __user *data)
 {
         int ret;
         char *kernel_type;
@@ -3066,6 +3066,12 @@ out_type:
         return ret;
 }
 
+SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
+                char __user *, type, unsigned long, flags, void __user *, data)
+{
+        return ksys_mount(dev_name, dir_name, type, flags, data);
+}
+
 /*
  * Return true if path is reachable from root
  *
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 6ab7ed71a8b6..3a9f9c534624 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -946,4 +946,7 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
  * the ksys_xyzyyz() functions prototyped below.
  */
 
+int ksys_mount(char __user *dev_name, char __user *dir_name, char __user *type,
+               unsigned long flags, void __user *data);
+
 #endif
diff --git a/init/do_mounts.c b/init/do_mounts.c
index 7cf4f6dafd5f..eb768de43d84 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -363,7 +363,7 @@ static void __init get_fs_names(char *page)
 static int __init do_mount_root(char *name, char *fs, int flags, void *data)
 {
         struct super_block *s;
-        int err = sys_mount(name, "/root", fs, flags, data);
+        int err = ksys_mount(name, "/root", fs, flags, data);
         if (err)
                 return err;
 
@@ -599,7 +599,7 @@ void __init prepare_namespace(void)
         mount_root();
 out:
         devtmpfs_mount("dev");
-        sys_mount(".", "/", NULL, MS_MOVE, NULL);
+        ksys_mount(".", "/", NULL, MS_MOVE, NULL);
         sys_chroot(".");
 }
 
diff --git a/init/do_mounts_initrd.c b/init/do_mounts_initrd.c
index 53d4f0f326e7..7868a6039fb4 100644
--- a/init/do_mounts_initrd.c
+++ b/init/do_mounts_initrd.c
@@ -43,7 +43,7 @@ static int init_linuxrc(struct subprocess_info *info, struct cred *new)
         sys_dup(0);
         /* move initrd over / and chdir/chroot in initrd root */
         sys_chdir("/root");
-        sys_mount(".", "/", NULL, MS_MOVE, NULL);
+        ksys_mount(".", "/", NULL, MS_MOVE, NULL);
         sys_chroot(".");
         sys_setsid();
         return 0;
@@ -81,7 +81,7 @@ static void __init handle_initrd(void)
         current->flags &= ~PF_FREEZER_SKIP;
 
         /* move initrd to rootfs' /old */
-        sys_mount("..", ".", NULL, MS_MOVE, NULL);
+        ksys_mount("..", ".", NULL, MS_MOVE, NULL);
         /* switch root and cwd back to / of rootfs */
         sys_chroot("..");
 
@@ -95,7 +95,7 @@ static void __init handle_initrd(void)
         mount_root();
 
         printk(KERN_NOTICE "Trying to move old root to /initrd ... ");
-        error = sys_mount("/old", "/root/initrd", NULL, MS_MOVE, NULL);
+        error = ksys_mount("/old", "/root/initrd", NULL, MS_MOVE, NULL);
         if (!error)
                 printk("okay\n");
         else {