[PATCH 2/3] fs: Extracts pivot_root so it can be made optional

bosrsf04
2017-02-25T00:23:12Z

From: Brian Ashworth <bosrsf04@gmail.com>

add/remove: 0/0 grow/shrink: 1/0 up/down: 4/0 (4)
function old new delta
sys_pivot_root 610 614 +4
Total: Before=1899573, After=1899577, chg +0.00%

Signed-off-by Brian Ashworth <bosrsf04@gmail.com>
---
fs/Makefile | 2 +-
fs/namespace.c | 123 -----------------------------------------------------
fs/pivot_root.c | 129 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 130 insertions(+), 124 deletions(-)
create mode 100644 fs/pivot_root.c

diff --git a/fs/Makefile b/fs/Makefile
index 7bbaca9c67b1..34cb58c4127d 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -11,7 +11,7 @@ obj-y := open.o read_write.o file_table.o super.o \
attr.o bad_inode.o file.o filesystems.o namespace.o \
seq_file.o xattr.o libfs.o fs-writeback.o \
pnode.o splice.o sync.o utimes.o \
- stack.o fs_struct.o statfs.o fs_pin.o nsfs.o
+ stack.o fs_struct.o statfs.o fs_pin.o nsfs.o pivot_root.o

ifeq ($(CONFIG_BLOCK),y)
obj-y += buffer.o block_dev.o direct-io.o mpage.o
diff --git a/fs/namespace.c b/fs/namespace.c
index d49d615e30a1..36e4faf4c6e6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3043,129 +3043,6 @@ bool path_is_under(const struct path *path1, const struct path *path2)
}
EXPORT_SYMBOL(path_is_under);

-/*
- * pivot_root Semantics:
- * Moves the root file system of the current process to the directory put_old,
- * makes new_root as the new root file system of the current process, and sets
- * root/cwd of all processes which had them on the current root to new_root.
- *
- * Restrictions:
- * The new_root and put_old must be directories, and must not be on the
- * same file system as the current process root. The put_old must be
- * underneath new_root, i.e. adding a non-zero number of /.. to the string
- * pointed to by put_old must yield the same directory as new_root. No other
- * file system may be mounted on put_old. After all, new_root is a mountpoint.
- *
- * Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem.
- * See Documentation/filesystems/ramfs-rootfs-initramfs.txt for alternatives
- * in this situation.
- *
- * Notes:
- * - we don't move root/cwd if they are not at the root (reason: if something
- * cared enough to change them, it's probably wrong to force them elsewhere)
- * - it's okay to pick a root that isn't the root of a file system, e.g.
- * /nfs/my_root where /nfs is the mount point. It must be a mountpoint,
- * though, so you may need to say mount --bind /nfs/my_root /nfs/my_root
- * first.
- */
-SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
- const char __user *, put_old)
-{
- struct path new, old, parent_path, root_parent, root;
- struct mount *new_mnt, *root_mnt, *old_mnt;
- struct mountpoint *old_mp, *root_mp;
- int error;
-
- if (!may_mount())
- return -EPERM;
-
- error = user_path_dir(new_root, &new);
- if (error)
- goto out0;
-
- error = user_path_dir(put_old, &old);
- if (error)
- goto out1;
-
- error = security_sb_pivotroot(&old, &new);
- if (error)
- goto out2;
-
- get_fs_root(current->fs, &root);
- old_mp = lock_mount(&old);
- error = PTR_ERR(old_mp);
- if (IS_ERR(old_mp))
- goto out3;
-
- error = -EINVAL;
- new_mnt = real_mount(new.mnt);
- root_mnt = real_mount(root.mnt);
- old_mnt = real_mount(old.mnt);
- if (IS_MNT_SHARED(old_mnt) ||
- IS_MNT_SHARED(new_mnt->mnt_parent) ||
- IS_MNT_SHARED(root_mnt->mnt_parent))
- goto out4;
- if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
- goto out4;
- if (new_mnt->mnt.mnt_flags & MNT_LOCKED)
- goto out4;
- error = -ENOENT;
- if (d_unlinked(new.dentry))
- goto out4;
- error = -EBUSY;
- if (new_mnt == root_mnt || old_mnt == root_mnt)
- goto out4; /* loop, on the same file system */
- error = -EINVAL;
- if (root.mnt->mnt_root != root.dentry)
- goto out4; /* not a mountpoint */
- if (!mnt_has_parent(root_mnt))
- goto out4; /* not attached */
- root_mp = root_mnt->mnt_mp;
- if (new.mnt->mnt_root != new.dentry)
- goto out4; /* not a mountpoint */
- if (!mnt_has_parent(new_mnt))
- goto out4; /* not attached */
- /* make sure we can reach put_old from new_root */
- if (!is_path_reachable(old_mnt, old.dentry, &new))
- goto out4;
- /* make certain new is below the root */
- if (!is_path_reachable(new_mnt, new.dentry, &root))
- goto out4;
- root_mp->m_count++; /* pin it so it won't go away */
- lock_mount_hash();
- detach_mnt(new_mnt, &parent_path);
- detach_mnt(root_mnt, &root_parent);
- if (root_mnt->mnt.mnt_flags & MNT_LOCKED) {
- new_mnt->mnt.mnt_flags |= MNT_LOCKED;
- root_mnt->mnt.mnt_flags &= ~MNT_LOCKED;
- }
- /* mount old root on put_old */
- attach_mnt(root_mnt, old_mnt, old_mp);
- /* mount new_root on / */
- attach_mnt(new_mnt, real_mount(root_parent.mnt), root_mp);
- touch_mnt_namespace(current->nsproxy->mnt_ns);
- /* A moved mount should not expire automatically */
- list_del_init(&new_mnt->mnt_expire);
- put_mountpoint(root_mp);
- unlock_mount_hash();
- chroot_fs_refs(&root, &new);
- error = 0;
-out4:
- unlock_mount(old_mp);
- if (!error) {
- path_put(&root_parent);
- path_put(&parent_path);
- }
-out3:
- path_put(&root);
-out2:
- path_put(&old);
-out1:
- path_put(&new);
-out0:
- return error;
-}
-
static void __init init_mount_tree(void)
{
struct vfsmount *mnt;
diff --git a/fs/pivot_root.c b/fs/pivot_root.c
new file mode 100644
index 000000000000..a609b21a1438
--- /dev/null
+++ b/fs/pivot_root.c
@@ -0,0 +1,129 @@
+#include <linux/syscalls.h>
+#include <linux/security.h>
+#include <linux/namei.h>
+#include <linux/fs_struct.h>
+#include "pnode.h"
+#include "internal.h"
+
+/*
+ * pivot_root Semantics:
+ * Moves the root file system of the current process to the directory put_old,
+ * makes new_root as the new root file system of the current process, and sets
+ * root/cwd of all processes which had them on the current root to new_root.
+ *
+ * Restrictions:
+ * The new_root and put_old must be directories, and must not be on the
+ * same file system as the current process root. The put_old must be
+ * underneath new_root, i.e. adding a non-zero number of /.. to the string
+ * pointed to by put_old must yield the same directory as new_root. No other
+ * file system may be mounted on put_old. After all, new_root is a mountpoint.
+ *
+ * Also, the current root cannot be on the 'rootfs' (initial ramfs) filesystem.
+ * See Documentation/filesystems/ramfs-rootfs-initramfs.txt for alternatives
+ * in this situation.
+ *
+ * Notes:
+ * - we don't move root/cwd if they are not at the root (reason: if something
+ * cared enough to change them, it's probably wrong to force them elsewhere)
+ * - it's okay to pick a root that isn't the root of a file system, e.g.
+ * /nfs/my_root where /nfs is the mount point. It must be a mountpoint,
+ * though, so you may need to say mount --bind /nfs/my_root /nfs/my_root
+ * first.
+ */
+SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
+ const char __user *, put_old)
+{
+ struct path new, old, parent_path, root_parent, root;
+ struct mount *new_mnt, *root_mnt, *old_mnt;
+ struct mountpoint *old_mp, *root_mp;
+ int error;
+
+ if (!may_mount())
+ return -EPERM;
+
+ error = user_path_dir(new_root, &new);
+ if (error)
+ goto out0;
+
+ error = user_path_dir(put_old, &old);
+ if (error)
+ goto out1;
+
+ error = security_sb_pivotroot(&old, &new);
+ if (error)
+ goto out2;
+
+ get_fs_root(current->fs, &root);
+ old_mp = lock_mount(&old);
+ error = PTR_ERR(old_mp);
+ if (IS_ERR(old_mp))
+ goto out3;
+
+ error = -EINVAL;
+ new_mnt = real_mount(new.mnt);
+ root_mnt = real_mount(root.mnt);
+ old_mnt = real_mount(old.mnt);
+ if (IS_MNT_SHARED(old_mnt) ||
+ IS_MNT_SHARED(new_mnt->mnt_parent) ||
+ IS_MNT_SHARED(root_mnt->mnt_parent))
+ goto out4;
+ if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
+ goto out4;
+ if (new_mnt->mnt.mnt_flags & MNT_LOCKED)
+ goto out4;
+ error = -ENOENT;
+ if (d_unlinked(new.dentry))
+ goto out4;
+ error = -EBUSY;
+ if (new_mnt == root_mnt || old_mnt == root_mnt)
+ goto out4; /* loop, on the same file system */
+ error = -EINVAL;
+ if (root.mnt->mnt_root != root.dentry)
+ goto out4; /* not a mountpoint */
+ if (!mnt_has_parent(root_mnt))
+ goto out4; /* not attached */
+ root_mp = root_mnt->mnt_mp;
+ if (new.mnt->mnt_root != new.dentry)
+ goto out4; /* not a mountpoint */
+ if (!mnt_has_parent(new_mnt))
+ goto out4; /* not attached */
+ /* make sure we can reach put_old from new_root */
+ if (!is_path_reachable(old_mnt, old.dentry, &new))
+ goto out4;
+ /* make certain new is below the root */
+ if (!is_path_reachable(new_mnt, new.dentry, &root))
+ goto out4;
+ root_mp->m_count++; /* pin it so it won't go away */
+ lock_mount_hash();
+ detach_mnt(new_mnt, &parent_path);
+ detach_mnt(root_mnt, &root_parent);
+ if (root_mnt->mnt.mnt_flags & MNT_LOCKED) {
+ new_mnt->mnt.mnt_flags |= MNT_LOCKED;
+ root_mnt->mnt.mnt_flags &= ~MNT_LOCKED;
+ }
+ /* mount old root on put_old */
+ attach_mnt(root_mnt, old_mnt, old_mp);
+ /* mount new_root on / */
+ attach_mnt(new_mnt, real_mount(root_parent.mnt), root_mp);
+ touch_mnt_namespace(current->nsproxy->mnt_ns);
+ /* A moved mount should not expire automatically */
+ list_del_init(&new_mnt->mnt_expire);
+ put_mountpoint(root_mp);
+ unlock_mount_hash();
+ chroot_fs_refs(&root, &new);
+ error = 0;
+out4:
+ unlock_mount(old_mp);
+ if (!error) {
+ path_put(&root_parent);
+ path_put(&parent_path);
+ }
+out3:
+ path_put(&root);
+out2:
+ path_put(&old);
+out1:
+ path_put(&new);
+out0:
+ return error;
+}
--
2.11.1