[PATCH][RFC] open_tree(2) (was Re: [PATCH 30/32] vfs: Allow cloning of a mount tree with open(O_PATH|O_CLONE_MOUNT) [ver #8])

From: Al Viro
Date: Sat Jun 02 2018 - 20:56:08 EST

On Sat, Jun 02, 2018 at 06:49:58PM +0100, Al Viro wrote:

> > > Hell, might even add AT_UMOUNT for "open root and detach, to be dissolved on
> > > close", incompatible with AT_CLONE.
> >
> > Cute. Guess you could do:
> >
> > fd = open_mount(..., OPEN_TREE_DETACH);
> > mount_setattr(fd, "",
> > MOUNT_SETATTR_EMPTY_PATH,
> > MOUNT_SETATTR_NOSUID, NULL);
> > move_mount(fd, "", ...);

Hadn't added that yet, but as for the rest of open_tree() - see
vfs.git#mount-open_tree. open() and its flags are not touched at all.
Other changes compared to the previous:
* may_mount() is required for OPEN_TREE_CLONE
* sys_ni.c cruft is dropped - those make no sense until and unless
those syscalls become conditional.

Appears to work, combined with move_mount() it yields eqiuvalents of
mount --{move,bind,rbind}. Combined with mount_setattr(2) (when that
gets added) we'll get mount -o remount,bind,nodev et.al.
(including the currently absent whole-subtree versions) and
mount --make-{r,}{shared,slave,private,unbindable}

It also can be used to get an isolated subtree usable for ....at()
stuff.

The addition of syscall itself is done by the following and I'd really
like linux-abi folks to comment on that puppy

commit 6cfba4dd99b10278c2156c8d4fced2eddedf167f
Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date: Sat Jun 2 19:42:22 2018 -0400

new syscall: open_tree(2)

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@xxxxxxxxxxxxxxxxxx>

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 14a2f996e543..b2b44ecd2b17 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -397,3 +397,4 @@
383 i386 statx sys_statx __ia32_sys_statx
384 i386 arch_prctl sys_arch_prctl __ia32_compat_sys_arch_prctl
385 i386 io_pgetevents sys_io_pgetevents __ia32_compat_sys_io_pgetevents
+391 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 cd36232ab62f..d6f4949378e7 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -342,6 +342,7 @@
331 common pkey_free __x64_sys_pkey_free
332 common statx __x64_sys_statx
333 common io_pgetevents __x64_sys_io_pgetevents
+339 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 7ec0b3e5f05d..7480271a0d21 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -189,6 +189,7 @@ 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;

might_sleep();

@@ -209,14 +210,14 @@ static void __fput(struct file *file)
file->f_op->release(inode, file);
security_file_free(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);
}
@@ -224,6 +225,8 @@ static void __fput(struct file *file)
file->f_path.mnt = NULL;
file->f_inode = NULL;
file_free(file);
+ if (unlikely(mode & FMODE_NEED_UNMOUNT))
+ dissolve_on_fput(mnt);
dput(dentry);
mntput(mnt);
}
diff --git a/fs/internal.h b/fs/internal.h
index 980d005b21b4..b55575b9b55c 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -85,6 +85,7 @@ extern void __mnt_drop_write(struct vfsmount *);
extern void __mnt_drop_write_file(struct file *);
extern void mnt_drop_write_file_path(struct file *);

+extern void dissolve_on_fput(struct vfsmount *);
/*
* fs_struct.c
*/
diff --git a/fs/namespace.c b/fs/namespace.c
index 5f75969adff1..3281fea98cf0 100644
--- 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"
@@ -1839,6 +1841,16 @@ 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_SYNC);
+ unlock_mount_hash();
+ namespace_unlock();
+}
+
void drop_collected_mounts(struct vfsmount *mnt)
{
namespace_lock();
@@ -2198,6 +2210,30 @@ 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.
*/
@@ -2205,7 +2241,7 @@ 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)
@@ -2219,38 +2255,21 @@ 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();
@@ -2264,6 +2283,75 @@ static int do_loopback(struct path *path, const char *old_name,
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 482563fe549c..706b4605bc26 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -154,6 +154,9 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
/* File is capable of returning -EAGAIN if I/O will block */
#define FMODE_NOWAIT ((__force fmode_t)0x8000000)

+/* File represents mount that needs unmounting */
+#define FMODE_NEED_UNMOUNT ((__force fmode_t)0x10000000)
+
/*
* Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
* that indicates that they should check the contents of the iovec are
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 811172fcb916..925483aba03a 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -896,7 +896,7 @@ asmlinkage long sys_pkey_alloc(unsigned long flags, unsigned long init_val);
asmlinkage long sys_pkey_free(int pkey);
asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
unsigned mask, struct statx __user *buffer);
-
+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 6448cdd9a350..594b85f7cb86 100644
--- 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
index 000000000000..b9c3b46210db
--- /dev/null
+++ b/include/uapi/linux/mount.h
@@ -0,0 +1,7 @@
+#ifndef _UAPI_LINUX_MOUNT_H
+#define _UAPI_LINUX_MOUNT_H
+
+#define OPEN_TREE_CLONE 1
+#define OPEN_TREE_CLOEXEC O_CLOEXEC
+
+#endif /* _UAPI_LINUX_MOUNT_H */