open_by_handle on a file ?

open_by_handle on a file ?

[DENIEL Philippe]

Hi,

I start using libhandle.so from xfsprogs-3.0.3 package. I meet an issue 
here : I can get a handle from files or directory.
When used on diretories, open_by_handle works fine : I can read entries 
in it by using getdents, create stuff / removing stuff by using the 
ATFILE_FUNCTION (mkdirat, renameat, ...)

Trouble start when I want to open a file with open_by_handle. I have a 
small test program that does this :

        rc = path_to_fshandle( path_dir,  (void **)(&fshandle),
    &fshandlelen) ;
      if( rc < 0 ) exit( -1)
        
      rc = path_to_handle( path_dir,  (void **)(&filehandle), &handlelen) ;
      if( rc < 0 ) exit( -1 )

      fd = open_by_handle( filehandle, handlelen, O_RDONLY ) ;
      printf( "open_by_handle: fd=%d \n", fd ) ;
      if( fd < 0 )
           printf( "----> Error=%d | %s\n", errno, strerror( errno ) ) ;

The open_by_handle failed with errno=20 aka ENOTDIR, which is true, this 
is a file and no directory.
Question is : Can I use open_by_handle to open a regular file ? If yes, 
how should I proceed. If no, how should I do to open a file knowing its 
handle ?

    Thanks in advance for your time and answer.

       Regards

          Philippe


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Re: open_by_handle on a file ?

[Dave Chinner]

On Wed, May 12, 2010 at 05:39:44PM +0200, DENIEL Philippe wrote:
> Hi,
> 
> I start using libhandle.so from xfsprogs-3.0.3 package. I meet an
> issue here : I can get a handle from files or directory.
> When used on diretories, open_by_handle works fine : I can read
> entries in it by using getdents, create stuff / removing stuff by
> using the ATFILE_FUNCTION (mkdirat, renameat, ...)
> 
> Trouble start when I want to open a file with open_by_handle. I have
> a small test program that does this :
> 
>        rc = path_to_fshandle( path_dir,  (void **)(&fshandle),
>    &fshandlelen) ;
>      if( rc < 0 ) exit( -1)
>      rc = path_to_handle( path_dir,  (void **)(&filehandle), &handlelen) ;
>      if( rc < 0 ) exit( -1 )
> 
>      fd = open_by_handle( filehandle, handlelen, O_RDONLY ) ;
>      printf( "open_by_handle: fd=%d \n", fd ) ;
>      if( fd < 0 )
>           printf( "----> Error=%d | %s\n", errno, strerror( errno ) ) ;
> 
> The open_by_handle failed with errno=20 aka ENOTDIR, which is true,
> this is a file and no directory.

I think that is because the path_dir points to a regular file and
so path_to_fshandle() is generating a fshandle that points to a file
instead of a directory. This handle is cached inside libhandle, and
then use for subsequent handle calls like open_by_handle(). The
kernel rejects the request is the fshandle does not point to a
directory.

Try using the mount point or a directory within the mount for
the path_to_fshandle() call and see if that fixes the problem.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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Re: open_by_handle on a file ?

[DENIEL Philippe]

Thank you Dave, you were definitely right. I used the XFS mount point as 
the argument to  path_to_fshandle and it now woks perfectly. :-)
This leads me to another question : now that I can convert a path to 
fhandle and use it to open a file or a directory, I can use getdents and 
the ATFILE_SOURCE functions (mkdirat, linkat, ....) to implement the NFS 
logic (I just have to "open_by_handle" to get the fs to the related fs 
object and operate on it). The idea behind this is implementing a NFS 
server in Userspace with XFS specific capabilites.
In fact, the NFS on which a minor issue remains is LOOKUP (and it could 
become a major issue since LOOKUP is called very often). To lookup on an 
object knowing its parent directory's handle and its name, I can perform 
an "openat" followed by a xfs related "fd_to_handle" and close the fd 
once the operation is done. But it seems a bit "heavy" to me. Is there 
another (lighter) way of getting the handle to an object knowing its 
name and parent directory (may be by a call to xfsctl ?).

  Regards,

    Philippe

Dave Chinner a écrit :
> On Wed, May 12, 2010 at 05:39:44PM +0200, DENIEL Philippe wrote:
>   
>> Hi,
>>
>> I start using libhandle.so from xfsprogs-3.0.3 package. I meet an
>> issue here : I can get a handle from files or directory.
>> When used on diretories, open_by_handle works fine : I can read
>> entries in it by using getdents, create stuff / removing stuff by
>> using the ATFILE_FUNCTION (mkdirat, renameat, ...)
>>
>> Trouble start when I want to open a file with open_by_handle. I have
>> a small test program that does this :
>>
>>        rc = path_to_fshandle( path_dir,  (void **)(&fshandle),
>>    &fshandlelen) ;
>>      if( rc < 0 ) exit( -1)
>>      rc = path_to_handle( path_dir,  (void **)(&filehandle), &handlelen) ;
>>      if( rc < 0 ) exit( -1 )
>>
>>      fd = open_by_handle( filehandle, handlelen, O_RDONLY ) ;
>>      printf( "open_by_handle: fd=%d \n", fd ) ;
>>      if( fd < 0 )
>>           printf( "----> Error=%d | %s\n", errno, strerror( errno ) ) ;
>>
>> The open_by_handle failed with errno=20 aka ENOTDIR, which is true,
>> this is a file and no directory.
>>     
>
> I think that is because the path_dir points to a regular file and
> so path_to_fshandle() is generating a fshandle that points to a file
> instead of a directory. This handle is cached inside libhandle, and
> then use for subsequent handle calls like open_by_handle(). The
> kernel rejects the request is the fshandle does not point to a
> directory.
>
> Try using the mount point or a directory within the mount for
> the path_to_fshandle() call and see if that fixes the problem.
>
> Cheers,
>
> Dave.
>   

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