[PATCH -V16 02/12] vfs: Add name to file handle conversion support

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

The file handle also include 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>
---
 fs/open.c                |  124 ++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h       |    9 +++
 include/linux/syscalls.h |    5 ++-
 3 files changed, 137 insertions(+), 1 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 5463266..7ad8f28 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -29,6 +29,7 @@
 #include <linux/falloc.h>
 #include <linux/fs_struct.h>
 #include <linux/ima.h>
+#include <linux/exportfs.h>
 
 #include "internal.h"
 
@@ -1040,3 +1041,126 @@ int nonseekable_open(struct inode *inode, struct file *filp)
 }
 
 EXPORT_SYMBOL(nonseekable_open);
+
+#ifdef CONFIG_EXPORTFS
+/* limit the handle size to some value */
+#define MAX_HANDLE_SZ 4096
+static long do_sys_name_to_handle(struct path *path,
+				  struct file_handle __user *ufh)
+{
+	long retval;
+	int handle_size;
+	struct file_handle f_handle;
+	struct file_handle *handle = NULL;
+
+	if (copy_from_user(&f_handle, ufh, sizeof(struct file_handle))) {
+		retval = -EFAULT;
+		goto err_out;
+	}
+	if (f_handle.handle_size > MAX_HANDLE_SZ) {
+		retval = -EINVAL;
+		goto err_out;
+	}
+	handle = kmalloc(sizeof(struct file_handle) + f_handle.handle_size,
+			 GFP_KERNEL);
+	if (!handle) {
+		retval = -ENOMEM;
+		goto err_out;
+	}
+
+	/* convert handle size to  multiple of sizeof(u32) */
+	handle_size = f_handle.handle_size >> 2;
+
+	/* we ask for a non connected handle */
+	retval = exportfs_encode_fh(path->dentry,
+				    (struct fid *)handle->f_handle,
+				    &handle_size,  0);
+	/* convert handle size to bytes */
+	handle_size *= sizeof(u32);
+	handle->handle_type = retval;
+	handle->handle_size = handle_size;
+	/* copy the mount id */
+	handle->mnt_id = path->mnt->mnt_id;
+	if (handle_size > f_handle.handle_size) {
+		/*
+		 * set the handle_size to zero so we copy only
+		 * non variable part of the file_handle
+		 */
+		handle_size = 0;
+		retval = -EOVERFLOW;
+	} else
+		retval = 0;
+	if (copy_to_user(ufh, handle,
+			 sizeof(struct file_handle) + handle_size))
+		retval = -EFAULT;
+
+	kfree(handle);
+err_out:
+	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
+ * @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_DEFINE4(name_to_handle_at, int, dfd, const char __user *, name,
+		struct file_handle __user *, handle, int, flag)
+{
+
+	int follow;
+	int fput_needed;
+	long ret = -EINVAL;
+	struct path path, *pp;
+	struct file *file = NULL;
+
+	if ((flag & ~AT_SYMLINK_FOLLOW) != 0)
+		goto err_out;
+
+	if (name == NULL && dfd != AT_FDCWD) {
+		file = fget_light(dfd, &fput_needed);
+		if (file) {
+			pp = &file->f_path;
+			ret = 0;
+		} else
+			ret = -EBADF;
+	} else {
+		follow = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
+		ret = user_path_at(dfd, name, follow, &path);
+		pp = &path;
+	}
+	if (ret)
+		goto err_out;
+	/*
+	 * We need t make sure wether the file system
+	 * support decoding of the file handle
+	 */
+	if (!pp->mnt->mnt_sb->s_export_op ||
+	    !pp->mnt->mnt_sb->s_export_op->fh_to_dentry) {
+		ret = -EOPNOTSUPP;
+		goto out_path;
+	}
+	ret = do_sys_name_to_handle(pp, handle);
+
+out_path:
+	if (file)
+		fput_light(file, fput_needed);
+	else
+		path_put(&path);
+err_out:
+	return ret;
+}
+#else
+SYSCALL_DEFINE4(name_to_handle_at, int, dfd, const char __user *, name,
+		struct file_handle __user *, handle, int, flag)
+{
+	return -ENOSYS;
+}
+#endif
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 471e1ff..0e7cf4c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -949,6 +949,15 @@ struct file {
 	unsigned long f_mnt_write_state;
 #endif
 };
+
+struct file_handle {
+	int mnt_id;
+	int handle_size;
+	int handle_type;
+	/* file identifier */
+	unsigned char f_handle[0];
+};
+
 extern spinlock_t files_lock;
 #define file_list_lock() spin_lock(&files_lock);
 #define file_list_unlock() spin_unlock(&files_lock);
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 7f614ce..9d6d6e3 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -61,6 +61,7 @@ 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>
@@ -823,5 +824,7 @@ 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 flag);
 #endif
-- 
1.7.2.rc1