vfs: syscall: Add open_tree(2) to reference or clone a mount

open_tree(dfd, pathname, flags)

Returns an O_PATH-opened file descriptor or an error.
dfd and pathname specify the location to open, in usual
fashion (see e.g. fstatat(2)).  flags should be an OR of
some of the following:
	* AT_PATH_EMPTY, AT_NO_AUTOMOUNT, AT_SYMLINK_NOFOLLOW -
same meanings as usual
	* OPEN_TREE_CLOEXEC - make the resulting descriptor
close-on-exec
	* OPEN_TREE_CLONE or OPEN_TREE_CLONE | AT_RECURSIVE -
instead of opening the location in question, create a detached
mount tree matching the subtree rooted at location specified by
dfd/pathname.  With AT_RECURSIVE the entire subtree is cloned,
without it - only the part within in the mount containing the
location in question.  In other words, the same as mount --rbind
or mount --bind would've taken.  The detached tree will be
dissolved on the final close of obtained file.  Creation of such
detached trees requires the same capabilities as doing mount --bind.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-api@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 1f9607e..ae2294d 100644 (file)
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -398,7 +398,8 @@
 384    i386    arch_prctl              sys_arch_prctl                  __ia32_compat_sys_arch_prctl
 385    i386    io_pgetevents           sys_io_pgetevents_time32        __ia32_compat_sys_io_pgetevents
 386    i386    rseq                    sys_rseq                        __ia32_sys_rseq
-# don't use numbers 387 through 392, add new calls at the end
+387    i386    open_tree               sys_open_tree                   __ia32_sys_open_tree
+# don't use numbers 388 through 392, add new calls at the end
 393    i386    semget                  sys_semget                      __ia32_sys_semget
 394    i386    semctl                  sys_semctl                      __ia32_compat_sys_semctl
 395    i386    shmget                  sys_shmget                      __ia32_sys_shmget

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 92ee0b4..a6e06c3 100644 (file)
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -343,6 +343,7 @@
 332    common  statx                   __x64_sys_statx
 333    common  io_pgetevents           __x64_sys_io_pgetevents
 334    common  rseq                    __x64_sys_rseq
+335    common  open_tree               __x64_sys_open_tree
 # don't use numbers 387 through 423, add new calls after the last
 # 'common' entry
 424    common  pidfd_send_signal       __x64_sys_pidfd_send_signal

diff --git a/fs/file_table.c b/fs/file_table.c
index 155d751..3f9c1b4 100644 (file)
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -255,6 +255,7 @@ static void __fput(struct file *file)
        struct dentry *dentry = file->f_path.dentry;
        struct vfsmount *mnt = file->f_path.mnt;
        struct inode *inode = file->f_inode;
+       fmode_t mode = file->f_mode;
 
        if (unlikely(!(file->f_mode & FMODE_OPENED)))
                goto out;
@@ -277,18 +278,20 @@ static void __fput(struct file *file)
        if (file->f_op->release)
                file->f_op->release(inode, file);
        if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
-                    !(file->f_mode & FMODE_PATH))) {
+                    !(mode & FMODE_PATH))) {
                cdev_put(inode->i_cdev);
        }
        fops_put(file->f_op);
        put_pid(file->f_owner.pid);
-       if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
+       if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
                i_readcount_dec(inode);
-       if (file->f_mode & FMODE_WRITER) {
+       if (mode & FMODE_WRITER) {
                put_write_access(inode);
                __mnt_drop_write(mnt);
        }
        dput(dentry);
+       if (unlikely(mode & FMODE_NEED_UNMOUNT))
+               dissolve_on_fput(mnt);
        mntput(mnt);
 out:
        file_free(file);

diff --git a/fs/internal.h b/fs/internal.h
index 6a8b716..f3a027c 100644 (file)
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -94,6 +94,7 @@ extern int __mnt_want_write_file(struct file *);
 extern void __mnt_drop_write(struct vfsmount *);
 extern void __mnt_drop_write_file(struct file *);
 
+extern void dissolve_on_fput(struct vfsmount *);
 /*
  * fs_struct.c
  */

diff --git a/fs/namespace.c b/fs/namespace.c
index c9cab30..b804a1a 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -20,6 +20,7 @@
 #include <linux/init.h>                /* init_rootfs */
 #include <linux/fs_struct.h>   /* get_fs_root et.al. */
 #include <linux/fsnotify.h>    /* fsnotify_vfsmount_delete */
+#include <linux/file.h>
 #include <linux/uaccess.h>
 #include <linux/proc_ns.h>
 #include <linux/magic.h>
@@ -1832,6 +1833,21 @@ struct vfsmount *collect_mounts(const struct path *path)
        return &tree->mnt;
 }
 
+static void free_mnt_ns(struct mnt_namespace *);
+static struct mnt_namespace *alloc_mnt_ns(struct user_namespace *, bool);
+
+void dissolve_on_fput(struct vfsmount *mnt)
+{
+       struct mnt_namespace *ns;
+       namespace_lock();
+       lock_mount_hash();
+       ns = real_mount(mnt)->mnt_ns;
+       umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
+       unlock_mount_hash();
+       namespace_unlock();
+       free_mnt_ns(ns);
+}
+
 void drop_collected_mounts(struct vfsmount *mnt)
 {
        namespace_lock();
@@ -2222,6 +2238,30 @@ static bool has_locked_children(struct mount *mnt, struct dentry *dentry)
        return false;
 }
 
+static struct mount *__do_loopback(struct path *old_path, int recurse)
+{
+       struct mount *mnt = ERR_PTR(-EINVAL), *old = real_mount(old_path->mnt);
+
+       if (IS_MNT_UNBINDABLE(old))
+               return mnt;
+
+       if (!check_mnt(old) && old_path->dentry->d_op != &ns_dentry_operations)
+               return mnt;
+
+       if (!recurse && has_locked_children(old, old_path->dentry))
+               return mnt;
+
+       if (recurse)
+               mnt = copy_tree(old, old_path->dentry, CL_COPY_MNT_NS_FILE);
+       else
+               mnt = clone_mnt(old, old_path->dentry, 0);
+
+       if (!IS_ERR(mnt))
+               mnt->mnt.mnt_flags &= ~MNT_LOCKED;
+
+       return mnt;
+}
+
 /*
  * do loopback mount.
  */
@@ -2229,7 +2269,7 @@ static int do_loopback(struct path *path, const char *old_name,
                                int recurse)
 {
        struct path old_path;
-       struct mount *mnt = NULL, *old, *parent;
+       struct mount *mnt = NULL, *parent;
        struct mountpoint *mp;
        int err;
        if (!old_name || !*old_name)
@@ -2243,38 +2283,21 @@ static int do_loopback(struct path *path, const char *old_name,
                goto out;
 
        mp = lock_mount(path);
-       err = PTR_ERR(mp);
-       if (IS_ERR(mp))
+       if (IS_ERR(mp)) {
+               err = PTR_ERR(mp);
                goto out;
+       }
 
-       old = real_mount(old_path.mnt);
        parent = real_mount(path->mnt);
-
-       err = -EINVAL;
-       if (IS_MNT_UNBINDABLE(old))
-               goto out2;
-
        if (!check_mnt(parent))
                goto out2;
 
-       if (!check_mnt(old) && old_path.dentry->d_op != &ns_dentry_operations)
-               goto out2;
-
-       if (!recurse && has_locked_children(old, old_path.dentry))
-               goto out2;
-
-       if (recurse)
-               mnt = copy_tree(old, old_path.dentry, CL_COPY_MNT_NS_FILE);
-       else
-               mnt = clone_mnt(old, old_path.dentry, 0);
-
+       mnt = __do_loopback(&old_path, recurse);
        if (IS_ERR(mnt)) {
                err = PTR_ERR(mnt);
                goto out2;
        }
 
-       mnt->mnt.mnt_flags &= ~MNT_LOCKED;
-
        err = graft_tree(mnt, parent, mp);
        if (err) {
                lock_mount_hash();
@@ -2288,6 +2311,96 @@ out:
        return err;
 }
 
+static struct file *open_detached_copy(struct path *path, bool recursive)
+{
+       struct user_namespace *user_ns = current->nsproxy->mnt_ns->user_ns;
+       struct mnt_namespace *ns = alloc_mnt_ns(user_ns, true);
+       struct mount *mnt, *p;
+       struct file *file;
+
+       if (IS_ERR(ns))
+               return ERR_CAST(ns);
+
+       namespace_lock();
+       mnt = __do_loopback(path, recursive);
+       if (IS_ERR(mnt)) {
+               namespace_unlock();
+               free_mnt_ns(ns);
+               return ERR_CAST(mnt);
+       }
+
+       lock_mount_hash();
+       for (p = mnt; p; p = next_mnt(p, mnt)) {
+               p->mnt_ns = ns;
+               ns->mounts++;
+       }
+       ns->root = mnt;
+       list_add_tail(&ns->list, &mnt->mnt_list);
+       mntget(&mnt->mnt);
+       unlock_mount_hash();
+       namespace_unlock();
+
+       mntput(path->mnt);
+       path->mnt = &mnt->mnt;
+       file = dentry_open(path, O_PATH, current_cred());
+       if (IS_ERR(file))
+               dissolve_on_fput(path->mnt);
+       else
+               file->f_mode |= FMODE_NEED_UNMOUNT;
+       return file;
+}
+
+SYSCALL_DEFINE3(open_tree, int, dfd, const char *, filename, unsigned, flags)
+{
+       struct file *file;
+       struct path path;
+       int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
+       bool detached = flags & OPEN_TREE_CLONE;
+       int error;
+       int fd;
+
+       BUILD_BUG_ON(OPEN_TREE_CLOEXEC != O_CLOEXEC);
+
+       if (flags & ~(AT_EMPTY_PATH | AT_NO_AUTOMOUNT | AT_RECURSIVE |
+                     AT_SYMLINK_NOFOLLOW | OPEN_TREE_CLONE |
+                     OPEN_TREE_CLOEXEC))
+               return -EINVAL;
+
+       if ((flags & (AT_RECURSIVE | OPEN_TREE_CLONE)) == AT_RECURSIVE)
+               return -EINVAL;
+
+       if (flags & AT_NO_AUTOMOUNT)
+               lookup_flags &= ~LOOKUP_AUTOMOUNT;
+       if (flags & AT_SYMLINK_NOFOLLOW)
+               lookup_flags &= ~LOOKUP_FOLLOW;
+       if (flags & AT_EMPTY_PATH)
+               lookup_flags |= LOOKUP_EMPTY;
+
+       if (detached && !may_mount())
+               return -EPERM;
+
+       fd = get_unused_fd_flags(flags & O_CLOEXEC);
+       if (fd < 0)
+               return fd;
+
+       error = user_path_at(dfd, filename, lookup_flags, &path);
+       if (unlikely(error)) {
+               file = ERR_PTR(error);
+       } else {
+               if (detached)
+                       file = open_detached_copy(&path, flags & AT_RECURSIVE);
+               else
+                       file = dentry_open(&path, O_PATH, current_cred());
+               path_put(&path);
+       }
+       if (IS_ERR(file)) {
+               put_unused_fd(fd);
+               return PTR_ERR(file);
+       }
+       fd_install(fd, file);
+       return fd;
+}
+
 /*
  * Don't allow locked mount flags to be cleared.
  *

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8b42df0..09b05ec 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -162,10 +162,13 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 #define FMODE_NONOTIFY         ((__force fmode_t)0x4000000)
 
 /* File is capable of returning -EAGAIN if I/O will block */
-#define FMODE_NOWAIT   ((__force fmode_t)0x8000000)
+#define FMODE_NOWAIT           ((__force fmode_t)0x8000000)
+
+/* File represents mount that needs unmounting */
+#define FMODE_NEED_UNMOUNT     ((__force fmode_t)0x10000000)
 
 /* File does not contribute to nr_files count */
-#define FMODE_NOACCOUNT        ((__force fmode_t)0x20000000)
+#define FMODE_NOACCOUNT                ((__force fmode_t)0x20000000)
 
 /*
  * Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index e446806..6c29d58 100644 (file)
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -985,6 +985,7 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
                          unsigned mask, struct statx __user *buffer);
 asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
                         int flags, uint32_t sig);
+asmlinkage long sys_open_tree(int dfd, const char __user *path, unsigned flags);
 asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
                                       siginfo_t __user *info,
                                       unsigned int flags);

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index a2f8658..1d33835 100644 (file)
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -91,5 +91,7 @@
 #define AT_STATX_FORCE_SYNC    0x2000  /* - Force the attributes to be sync'd with the server */
 #define AT_STATX_DONT_SYNC     0x4000  /* - Don't sync attributes with the server */
 
+#define AT_RECURSIVE           0x8000  /* Apply to the entire subtree */
+
 
 #endif /* _UAPI_LINUX_FCNTL_H */

diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index 3f9ec42..fd7ae2e 100644 (file)
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -55,4 +55,10 @@
 #define MS_MGC_VAL 0xC0ED0000
 #define MS_MGC_MSK 0xffff0000
 
+/*
+ * open_tree() flags.
+ */
+#define OPEN_TREE_CLONE                1               /* Clone the target tree and attach the clone */
+#define OPEN_TREE_CLOEXEC      O_CLOEXEC       /* Close the file on execve() */
+
 #endif /* _UAPI_LINUX_MOUNT_H */