vfs: Add name to file handle conversion support

The syscall also return mount id which can be used
to lookup file system specific information such as uuid
in /proc/<pid>/mountinfo

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 107 insertions, 0 deletions

diff --git a/fs/fhandle.c b/fs/fhandle.c
new file mode 100644
index 000000000000..9f79e743a840
--- /dev/null
+++ b/fs/fhandle.c
@@ -0,0 +1,107 @@
+#include <linux/syscalls.h>
+#include <linux/slab.h>
+#include <linux/fs.h>
+#include <linux/file.h>
+#include <linux/mount.h>
+#include <linux/namei.h>
+#include <linux/exportfs.h>
+#include <asm/uaccess.h>
+#include "internal.h"
+
+static long do_sys_name_to_handle(struct path *path,
+                                  struct file_handle __user *ufh,
+                                  int __user *mnt_id)
+{
+        long retval;
+        struct file_handle f_handle;
+        int handle_dwords, handle_bytes;
+        struct file_handle *handle = NULL;
+
+        /*
+         * We need t make sure wether the file system
+         * support decoding of the file handle
+         */
+        if (!path->mnt->mnt_sb->s_export_op ||
+            !path->mnt->mnt_sb->s_export_op->fh_to_dentry)
+                return -EOPNOTSUPP;
+
+        if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle)))
+                return -EFAULT;
+
+        if (f_handle.handle_bytes > MAX_HANDLE_SZ)
+                return -EINVAL;
+
+        handle = kmalloc(sizeof(struct file_handle) + f_handle.handle_bytes,
+                         GFP_KERNEL);
+        if (!handle)
+                return -ENOMEM;
+
+        /* convert handle size to  multiple of sizeof(u32) */
+        handle_dwords = f_handle.handle_bytes >> 2;
+
+        /* we ask for a non connected handle */
+        retval = exportfs_encode_fh(path->dentry,
+                                    (struct fid *)handle->f_handle,
+                                    &handle_dwords,  0);
+        handle->handle_type = retval;
+        /* convert handle size to bytes */
+        handle_bytes = handle_dwords * sizeof(u32);
+        handle->handle_bytes = handle_bytes;
+        if ((handle->handle_bytes > f_handle.handle_bytes) ||
+            (retval == 255) || (retval == -ENOSPC)) {
+                /* As per old exportfs_encode_fh documentation
+                 * we could return ENOSPC to indicate overflow
+                 * But file system returned 255 always. So handle
+                 * both the values
+                 */
+                /*
+                 * set the handle size to zero so we copy only
+                 * non variable part of the file_handle
+                 */
+                handle_bytes = 0;
+                retval = -EOVERFLOW;
+        } else
+                retval = 0;
+        /* copy the mount id */
+        if (copy_to_user(mnt_id, &path->mnt->mnt_id, sizeof(*mnt_id)) ||
+            copy_to_user(ufh, handle,
+                         sizeof(struct file_handle) + handle_bytes))
+                retval = -EFAULT;
+        kfree(handle);
+        return retval;
+}
+
+/**
+ * sys_name_to_handle_at: convert name to handle
+ * @dfd: directory relative to which name is interpreted if not absolute
+ * @name: name that should be converted to handle.
+ * @handle: resulting file handle
+ * @mnt_id: mount id of the file system containing the file
+ * @flag: flag value to indicate whether to follow symlink or not
+ *
+ * @handle->handle_size indicate the space available to store the
+ * variable part of the file handle in bytes. If there is not
+ * enough space, the field is updated to return the minimum
+ * value required.
+ */
+SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
+                struct file_handle __user *, handle, int __user *, mnt_id,
+                int, flag)
+{
+        struct path path;
+        int lookup_flags;
+        int err;
+
+        if ((flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
+                return -EINVAL;
+
+        lookup_flags = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
+        if (flag & AT_EMPTY_PATH)
+                lookup_flags |= LOOKUP_EMPTY;
+        err = user_path_at(dfd, name, lookup_flags, &path);
+        if (!err) {
+                err = do_sys_name_to_handle(&path, handle, mnt_id);
+                path_put(&path);
+        }
+        return err;
+}