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

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 3cf7b533b3d1304b44b5eefe7ab55cc09398b2c2..ea1b413afd473cf9ad7cc8b3033f5a0c3abc0169 100644 (file)
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -398,3 +398,4 @@
 384    i386    arch_prctl              sys_arch_prctl                  __ia32_compat_sys_arch_prctl
 385    i386    io_pgetevents           sys_io_pgetevents               __ia32_compat_sys_io_pgetevents
 386    i386    rseq                    sys_rseq                        __ia32_sys_rseq
+387    i386    open_tree               sys_open_tree                   __ia32_sys_open_tree

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index f0b1709a5ffb2b0901d7f2492252876d17bc25a0..0545bed581dc82a382c9fe4d6124843c6ba303d8 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
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact

diff --git a/fs/file_table.c b/fs/file_table.c
index e49af4caf15d92e3a33bbd02e4e551d778d8180e..e03c8d121c6cdfe309a63cb2c56be11915c0a162 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 364c20b5ea2d034a940946fac8be39015c01775d..17029b30e196eb6af56712f21452a309651500ac 100644 (file)
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -85,6 +85,7 @@ 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 8a7e1a7d1d067b307323f33d2c07b8c8a882ea21..ded1a970ec4008bdc8f7896715531bd8aeb0a60c 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -20,12 +20,14 @@
 #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>
 #include <linux/bootmem.h>
 #include <linux/task_work.h>
 #include <linux/sched/task.h>
+#include <uapi/linux/mount.h>
 
 #include "pnode.h"
 #include "internal.h"
@@ -1779,6 +1781,16 @@ struct vfsmount *collect_mounts(const struct path *path)
        return &tree->mnt;
 }
 
+void dissolve_on_fput(struct vfsmount *mnt)
+{
+       namespace_lock();
+       lock_mount_hash();
+       mntget(mnt);
+       umount_tree(real_mount(mnt), UMOUNT_CONNECTED);
+       unlock_mount_hash();
+       namespace_unlock();
+}
+
 void drop_collected_mounts(struct vfsmount *mnt)
 {
        namespace_lock();
@@ -2138,6 +2150,30 @@ 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.
  */
@@ -2145,7 +2181,7 @@ 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)
@@ -2159,38 +2195,21 @@ 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();
@@ -2204,6 +2223,75 @@ out:
        return err;
 }
 
+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 (error)
+               goto out;
+
+       if (detached) {
+               struct mount *mnt = __do_loopback(&path, flags & AT_RECURSIVE);
+               if (IS_ERR(mnt)) {
+                       error = PTR_ERR(mnt);
+                       goto out2;
+               }
+               mntput(path.mnt);
+               path.mnt = &mnt->mnt;
+       }
+
+       file = dentry_open(&path, O_PATH, current_cred());
+       if (IS_ERR(file)) {
+               error = PTR_ERR(file);
+               goto out3;
+       }
+
+       if (detached)
+               file->f_mode |= FMODE_NEED_UNMOUNT;
+       path_put(&path);
+       fd_install(fd, file);
+       return fd;
+
+out3:
+       if (detached)
+               dissolve_on_fput(path.mnt);
+out2:
+       path_put(&path);
+out:
+       put_unused_fd(fd);
+       return error;
+}
+
 static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
 {
        int error = 0;

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4323b8fe353d146be9a7cdf77910325ce144c7ee..6dc32507762f12cf27d710a60390e2a398882c61 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -157,10 +157,13 @@ 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 2ff814c92f7fb1955f3074ba86ae6cb7e6a7baac..6978f3c76d419e5a88f453385b884011d470cb46 100644 (file)
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -906,6 +906,7 @@ 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);
 
 /*
  * Architecture-specific system calls

diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 6448cdd9a350d3a0c6513c7a76aad29a68689bd4..594b85f7cb862f42473e1f8284ca1ef790fee5c6 100644 (file)
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -90,5 +90,7 @@
 #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
new file mode 100644 (file)
index 0000000..e8db291
--- /dev/null
+++ b/include/uapi/linux/mount.h
@@ -0,0 +1,10 @@
+#ifndef _UAPI_LINUX_MOUNT_H
+#define _UAPI_LINUX_MOUNT_H
+
+/*
+ * 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 */