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    SubjectRe: [PATCH -V16 02/12] vfs: Add name to file handle conversion support
    Date
    On Mon, 12 Jul 2010 10:15:29 +0200, Miklos Szeredi <miklos@szeredi.hu> wrote:
    > On Mon, 12 Jul 2010, Aneesh Kumar K.V wrote:
    > > 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;
    >
    > The mount id is not part of the handle in that it's not used when
    > converting back a handle to a file descriptor. So it shouldn't be
    > included here.
    >
    > The uuid can be looked up based on st_dev.
    >

    That would include another stat call on the file to get the st_dev ? As
    per the last review (Message-id:20100708082143.3701bfc7@notabene.brown)
    http://article.gmane.org/gmane.linux.kernel/1007385 we discussed that
    it would be nice to add st_dev as a part of handle. Later I suggested
    it would be nice to get mount_id instead of st_dev because st_dev is
    not stable (against remounts) for file system that doesn't have a
    backing device. So instead of using something that is partially stable,
    add mnt_id which is explicitly stated to be unstable across remounts.

    If you are against having mount_id as a part of struct file_handle, do
    you think we could add it as a extra argument to syscall ?

    -aneesh


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