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>

8 files changed, 139 insertions, 2 deletions

diff --git a/fs/Kconfig b/fs/Kconfig
index 3db9caa57edc..7cb53aafac1e 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -47,7 +47,7 @@ config FS_POSIX_ACL
         def_bool n
 
 config EXPORTFS
-        tristate
+        bool
 
 config FILE_LOCKING
         bool "Enable POSIX file locking API" if EXPERT
diff --git a/fs/Makefile b/fs/Makefile
index a7f7cef0c0c8..ba01202844c5 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -48,6 +48,8 @@ obj-$(CONFIG_FS_POSIX_ACL)	+= posix_acl.o xattr_acl.o
 obj-$(CONFIG_NFS_COMMON)        += nfs_common/
 obj-$(CONFIG_GENERIC_ACL)       += generic_acl.o
 
+obj-$(CONFIG_FHANDLE)           += fhandle.o
+
 obj-y                           += quota/
 
 obj-$(CONFIG_PROC_FS)           += proc/
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;
+}
diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
index 65afdfd31b7b..33a42f24b275 100644
--- a/include/linux/exportfs.h
+++ b/include/linux/exportfs.h
@@ -8,6 +8,9 @@ struct inode;
 struct super_block;
 struct vfsmount;
 
+/* limit the handle size to NFSv4 handle size now */
+#define MAX_HANDLE_SZ 128
+
 /*
  * The fileid_type identifies how the file within the filesystem is encoded.
  * In theory this is freely set and parsed by the filesystem, but we try to
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b7178b05cf3a..3f64630c0e10 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -978,6 +978,13 @@ struct file {
 #endif
 };
 
+struct file_handle {
+        __u32 handle_bytes;
+        int handle_type;
+        /* file identifier */
+        unsigned char f_handle[0];
+};
+
 #define get_file(x)     atomic_long_inc(&(x)->f_count)
 #define fput_atomic(x)  atomic_long_add_unless(&(x)->f_count, -1, 1)
 #define file_count(x)   atomic_long_read(&(x)->f_count)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 98664db1be47..970112613fb4 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -62,6 +62,7 @@ struct robust_list_head;
 struct getcpu_cache;
 struct old_linux_dirent;
 struct perf_event_attr;
+struct file_handle;
 
 #include <linux/types.h>
 #include <linux/aio_abi.h>
@@ -832,5 +833,7 @@ asmlinkage long sys_mmap_pgoff(unsigned long addr, unsigned long len,
                         unsigned long prot, unsigned long flags,
                         unsigned long fd, unsigned long pgoff);
 asmlinkage long sys_old_mmap(struct mmap_arg_struct __user *arg);
-
+asmlinkage long sys_name_to_handle_at(int dfd, const char __user *name,
+                                      struct file_handle __user *handle,
+                                      int __user *mnt_id, int flag);
 #endif
diff --git a/init/Kconfig b/init/Kconfig
index be788c0957d4..e72fa17fe559 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -287,6 +287,18 @@ config BSD_PROCESS_ACCT_V3
           for processing it. A preliminary version of these tools is available
           at <http://www.gnu.org/software/acct/>.
 
+config FHANDLE
+        bool "open by fhandle syscalls"
+        select EXPORTFS
+        help
+          If you say Y here, a user level program will be able to map
+          file names to handle and then later use the handle for
+          different file system operations. This is useful in implementing
+          userspace file servers, which now track files using handles instead
+          of names. The handle would remain the same even if file names
+          get renamed. Enables open_by_handle_at(2) and name_to_handle_at(2)
+          syscalls.
+
 config TASKSTATS
         bool "Export task/process statistics through netlink (EXPERIMENTAL)"
         depends on NET
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index c782fe9924c7..4e013439ac28 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -186,3 +186,6 @@ cond_syscall(sys_perf_event_open);
 /* fanotify! */
 cond_syscall(sys_fanotify_init);
 cond_syscall(sys_fanotify_mark);
+
+/* open by handle */
+cond_syscall(sys_name_to_handle_at);