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Re: xfsprogs/libhandle : How to get the handle for a symbolic link ?

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: xfsprogs/libhandle : How to get the handle for a symbolic link ?
From: DENIEL Philippe <philippe.deniel@xxxxxx>
Date: Wed, 02 Jun 2010 09:20:31 +0200
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20100601232917.GG1395@dastard>
Organization: CEA-DAM
References: <4C04F386.908@xxxxxx> <20100601232917.GG1395@dastard>
User-agent: Thunderbird (X11/20070728)
Hi Dave,

In fact, path_to_handle does not do the work correctly. Its code is this (extract from xfsprogs-3.0.3 sources) :

           char            *path,          /* input,  path to convert */
           void            **hanp,         /* output, pointer to data */
           size_t          *hlen)          /* output, size of returned
   data */
           int             fd;
           int             result;
           comarg_t        obj;

           fd = open(path, O_RDONLY);
           if (fd < 0)
                   return -1;

           obj.path = path;
           result = obj_to_handle(path, fd, XFS_IOC_PATH_TO_HANDLE,
                                   obj, hanp, hlen);
           return result;

As you see, it performs a open at the beginning, which will results in opening the file pointed by the symlink or returns ENOENT if the path "inside" the symlink does not exist. I tried using open with O_NOFOLLOW option, but it changed nothing, I got ELOOP when opening the file (which is a regular behavior so far).
Any other ideas ?


Dave Chinner a écrit :
On Tue, Jun 01, 2010 at 01:48:22PM +0200, DENIEL Philippe wrote:

I am currently developing a user space nfs server with various
backends. One of this backend module use xfsprogss's libhandle to
implement XFS support. I could do almost everything with
open_by_handle and fd_to_handle, used jointly with ATFILE_SOURCE
functions, but I do have a problem with symbolic links. To build an
xfs object's handle, I get its parent handle (now problem to this)
then I call "openat" to get the fd to the object before calling
fd_to_handle. This works ok, but not for symbolic link : the openat
with follow the link. I added the O_NOFOLLOW flag to openat, but now
openat return ELOOP instead.
I know there is a readlink_by_handle function in libhandle. How
could I build the related handle to be used as argument to it (I
mean, how to build a handle that refers to the symlink itself, not
the object it points to).

Doesn't path_to_handle() do what you want? From the man page:

        "... If the final component of the path name is a symbolic
        link, the handle returned is that of the link itself."



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