Re: [PATCH RFC] fhandle: expose u64 mount id to name_to_handle_at(2)

[Aleksa Sarai]

On 2024-05-20, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Mon, 2024-05-20 at 17:35 -0400, Aleksa Sarai wrote:
> > Now that we have stabilised the unique 64-bit mount ID interface in
> > statx, we can now provide a race-free way for name_to_handle_at(2) to
> > provide a file handle and corresponding mount without needing to worry
> > about racing with /proc/mountinfo parsing.
> > 
> > As with AT_HANDLE_FID, AT_HANDLE_UNIQUE_MNT_ID reuses a statx AT_* bit
> > that doesn't make sense for name_to_handle_at(2).
> > 
> > Signed-off-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
> > ---
> >  fs/fhandle.c               | 27 +++++++++++++++++++--------
> >  include/uapi/linux/fcntl.h |  2 ++
> >  2 files changed, 21 insertions(+), 8 deletions(-)
> > 
> > diff --git a/fs/fhandle.c b/fs/fhandle.c
> > index 8a7f86c2139a..6bc7ffccff8c 100644
> > --- a/fs/fhandle.c
> > +++ b/fs/fhandle.c
> > @@ -16,7 +16,8 @@
> >  
> >  static long do_sys_name_to_handle(const struct path *path,
> >  				  struct file_handle __user *ufh,
> > -				  int __user *mnt_id, int fh_flags)
> > +				  void __user *mnt_id, bool unique_mntid,
> > +				  int fh_flags)
> >  {
> >  	long retval;
> >  	struct file_handle f_handle;
> > @@ -69,10 +70,16 @@ static long do_sys_name_to_handle(const struct path *path,
> >  	} else
> >  		retval = 0;
> >  	/* copy the mount id */
> > -	if (put_user(real_mount(path->mnt)->mnt_id, mnt_id) ||
> > -	    copy_to_user(ufh, handle,
> > -			 struct_size(handle, f_handle, handle_bytes)))
> > -		retval = -EFAULT;
> > +	if (unique_mntid)
> > +		retval = put_user(real_mount(path->mnt)->mnt_id_unique,
> > +				  (u64 __user *) mnt_id);
> > +	else
> > +		retval = put_user(real_mount(path->mnt)->mnt_id,
> > +				  (int __user *) mnt_id);
> > +	/* copy the handle */
> > +	if (!retval)
> > +		retval = copy_to_user(ufh, handle,
> > +				struct_size(handle, f_handle, handle_bytes));
> >  	kfree(handle);
> >  	return retval;
> >  }
> > @@ -83,6 +90,7 @@ static long do_sys_name_to_handle(const struct path *path,
> >   * @name: name that should be converted to handle.
> >   * @handle: resulting file handle
> >   * @mnt_id: mount id of the file system containing the file
> > + *          (u64 if AT_HANDLE_UNIQUE_MNT_ID, otherwise int)
> >   * @flag: flag value to indicate whether to follow symlink or not
> >   *        and whether a decodable file handle is required.
> >   *
> > @@ -92,7 +100,7 @@ static long do_sys_name_to_handle(const struct path *path,
> >   * value required.
> >   */
> >  SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
> > -		struct file_handle __user *, handle, int __user *, mnt_id,
> > +		struct file_handle __user *, handle, void __user *, mnt_id,
> > 
> 
> Changing the syscall signature like this is rather nasty. The new flag
> seems like it should safely gate the difference, but I still have some
> concerns about misuse and people passing in too small a buffer for the
> mnt_id.

Yeah, it's a little ugly, but an name_to_handle_at2 feels like overkill
for such a minor change. I'm also not sure there's a huge risk of users
accidentally passing AT_HANDLE_UNIQUE_MNT_ID with an (int *).

> >  		int, flag)
> >  {
> >  	struct path path;
> > @@ -100,7 +108,8 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
> >  	int fh_flags;
> >  	int err;
> >  
> > -	if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID))
> > +	if (flag & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_HANDLE_FID |
> > +		     AT_HANDLE_UNIQUE_MNT_ID))
> >  		return -EINVAL;
> >  
> >  	lookup_flags = (flag & AT_SYMLINK_FOLLOW) ? LOOKUP_FOLLOW : 0;
> > @@ -109,7 +118,9 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
> >  		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, fh_flags);
> > +		err = do_sys_name_to_handle(&path, handle, mnt_id,
> > +					    flag & AT_HANDLE_UNIQUE_MNT_ID,
> > +					    fh_flags);
> >  		path_put(&path);
> >  	}
> >  	return err;
> > diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> > index c0bcc185fa48..fda970f92fba 100644
> > --- a/include/uapi/linux/fcntl.h
> > +++ b/include/uapi/linux/fcntl.h
> > @@ -118,6 +118,8 @@
> >  #define AT_HANDLE_FID		AT_REMOVEDIR	/* file handle is needed to
> >  					compare object identity and may not
> >  					be usable to open_by_handle_at(2) */
> > +#define AT_HANDLE_UNIQUE_MNT_ID	AT_STATX_FORCE_SYNC /* returned mount id is
> > +					the u64 unique mount id */
> >  #if defined(__KERNEL__)
> >  #define AT_GETATTR_NOSEC	0x80000000
> >  #endif
> > 
> > ---
> > base-commit: 584bbf439d0fa83d728ec49f3a38c581bdc828b4
> > change-id: 20240515-exportfs-u64-mount-id-9ebb5c58b53c
> > 
> > Best regards,
> 
> -- 
> Jeff Layton <jlayton@xxxxxxxxxx>
> 

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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