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Subject[PATCH 06/14] VFS: Implement fsmount() to effect a pre-configured mount [ver #6]
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Provide a system call by which a filesystem opened with fsopen() and
configured by a series of writes can be mounted:

int ret = fsmount(int fsfd, int dfd, const char *path,
unsigned int at_flags, unsigned int flags);

where fsfd is the fd returned by fsopen(), dfd, path and at_flags locate
the mountpoint and flags are the applicable MS_* flags. dfd can be
AT_FDCWD or an fd open to a directory.

In the event that fsmount() fails, it may be possible to get an error
message by calling read(). If no message is available, ENODATA will be
reported.

Signed-off-by: David Howells <dhowells@redhat.com>
---

arch/x86/entry/syscalls/syscall_32.tbl | 1
arch/x86/entry/syscalls/syscall_64.tbl | 1
fs/namespace.c | 82 ++++++++++++++++++++++++++++++++
include/linux/syscalls.h | 2 +
kernel/sys_ni.c | 1
5 files changed, 87 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 9bf8d4c62f85..abe6ea95e0e6 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -392,3 +392,4 @@
383 i386 statx sys_statx
384 i386 arch_prctl sys_arch_prctl compat_sys_arch_prctl
385 i386 fsopen sys_fsopen
+386 i386 fsmount sys_fsmount
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 9b198c5fc412..0977c5079831 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -340,6 +340,7 @@
331 common pkey_free sys_pkey_free
332 common statx sys_statx
333 common fsopen sys_fsopen
+334 common fsmount sys_fsmount

#
# x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/fs/namespace.c b/fs/namespace.c
index d6b0b0067f6d..8676658b6b2c 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3188,6 +3188,88 @@ struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
EXPORT_SYMBOL_GPL(kern_mount_data);

/*
+ * Mount a new, prepared superblock (specified by fs_fd) on the location
+ * specified by dfd and dir_name. dfd can be AT_FDCWD, a dir fd or a container
+ * fd. This cannot be used for binding, moving or remounting mounts.
+ */
+SYSCALL_DEFINE5(fsmount, int, fs_fd, int, dfd, const char __user *, dir_name,
+ unsigned int, at_flags, unsigned int, flags)
+{
+ struct fs_context *fc;
+ struct path mountpoint;
+ struct fd f;
+ unsigned int lookup_flags, mnt_flags = 0;
+ long ret;
+
+ if ((at_flags & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |
+ AT_EMPTY_PATH)) != 0)
+ return -EINVAL;
+
+ if (flags & ~(MS_RDONLY | MS_NOSUID | MS_NODEV | MS_NOEXEC |
+ MS_NOATIME | MS_NODIRATIME | MS_RELATIME | MS_STRICTATIME))
+ return -EINVAL;
+
+ if (flags & MS_RDONLY)
+ mnt_flags |= MNT_READONLY;
+ if (flags & MS_NOSUID)
+ mnt_flags |= MNT_NOSUID;
+ if (flags & MS_NODEV)
+ mnt_flags |= MNT_NODEV;
+ if (flags & MS_NOEXEC)
+ mnt_flags |= MNT_NOEXEC;
+ if (flags & MS_NODIRATIME)
+ mnt_flags |= MNT_NODIRATIME;
+
+ if (flags & MS_STRICTATIME) {
+ if (flags & MS_NOATIME)
+ return -EINVAL;
+ } else if (flags & MS_NOATIME) {
+ mnt_flags |= MNT_NOATIME;
+ } else {
+ mnt_flags |= MNT_RELATIME;
+ }
+
+ f = fdget(fs_fd);
+ if (!f.file)
+ return -EBADF;
+
+ ret = -EINVAL;
+ if (f.file->f_op != &fs_fs_fops)
+ goto err_fsfd;
+
+ fc = f.file->private_data;
+
+ ret = -EPERM;
+ if (!may_mount() ||
+ ((fc->sb_flags & MS_MANDLOCK) && !may_mandlock()))
+ goto err_fsfd;
+
+ /* There must be a valid superblock or we can't mount it */
+ ret = -EINVAL;
+ if (!fc->root)
+ goto err_fsfd;
+
+ /* Find the mountpoint. A container can be specified in dfd. */
+ lookup_flags = LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT;
+ if (at_flags & AT_SYMLINK_NOFOLLOW)
+ lookup_flags &= ~LOOKUP_FOLLOW;
+ if (at_flags & AT_NO_AUTOMOUNT)
+ lookup_flags &= ~LOOKUP_AUTOMOUNT;
+ if (at_flags & AT_EMPTY_PATH)
+ lookup_flags |= LOOKUP_EMPTY;
+ ret = user_path_at(dfd, dir_name, lookup_flags, &mountpoint);
+ if (ret < 0)
+ goto err_fsfd;
+
+ ret = do_new_mount_fc(fc, &mountpoint, mnt_flags);
+
+ path_put(&mountpoint);
+err_fsfd:
+ fdput(f);
+ return ret;
+}
+
+/*
* Return true if path is reachable from root
*
* namespace_sem or mount_lock is held
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 7cd1b65a4152..e82dde171ce8 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -942,5 +942,7 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
unsigned mask, struct statx __user *buffer);
asmlinkage long sys_fsopen(const char *fs_name, unsigned int flags,
void *reserved3, void *reserved4, void *reserved5);
+asmlinkage long sys_fsmount(int fsfd, int dfd, const char *path, unsigned int at_flags,
+ unsigned int flags);

#endif
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index de1dc63e7e47..a0fe764bd5dd 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -261,3 +261,4 @@ cond_syscall(sys_pkey_free);

/* fd-based mount */
cond_syscall(sys_fsopen);
+cond_syscall(sys_fsmount);
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