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    SubjectRe: [PATCH -V14 04/11] vfs: Allow handle based open on symlinks
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    On Thu, 8 Jul 2010 02:57:19 +1000, Nick Piggin <npiggin@suse.de> wrote:
    > On Wed, Jul 07, 2010 at 09:54:52PM +0530, Aneesh Kumar K. V wrote:
    > > On Thu, 8 Jul 2010 01:23:03 +1000, Nick Piggin <npiggin@suse.de> wrote:
    > > > On Tue, Jun 15, 2010 at 10:42:54PM +0530, Aneesh Kumar K.V wrote:
    > > > > The patch update may_open to allow handle based open on symlinks.
    > > > > The file handle based API use file descritor returned from open_by_handle_at
    > > > > to do different file system operations. To find the link target name we
    > > > > need to get a file descriptor on symlinks.
    > > >
    > > > This is a pretty big change, isn't it?
    > >
    > > Yes
    > >
    > > > Have you looked through vfs to
    > > > ensure this is actually OK? I was just looking at remount,ro code, for
    > > > example, and it seems to assume only writable open files on ISREG
    > > > files.
    > >
    > > I verified that write and read returns an error on that descriptor. I
    > > will look at rest of code to ensure the it doesn't break assumptions in
    > > the rest of VFS.
    > >
    > > >
    > > > Is this restricted to RDONLY? Really, it should be O_NONE...
    > > >
    > > >
    > >
    > > we need the interface so that we can do fstat(lstat) and readlink using the
    > > handle. If we are against the idea of allowing an fd on symlink, how about
    > > adding stat_by_handle and readlink_by_handle syscall ? That way we can
    > > drop "vfs: Support null pathname in readlink" patch
    >
    > It would be nice to avoid introducing open files to symlinks, but
    > I don't know about really the *right* thing to do though. I think
    > avoiding adding any by-handle syscalls except for open by handle
    > may be a good idea... OTOH if this is really a special case for
    > handles...
    >
    > I haven't followed this thread closely, so can you just go back
    > and explain to me why you need this? Ie. that you can't achieve with
    > a path-to-handle follow/nofollow option.


    We need to be able to read the link target using file handle. The exact
    usecase i have is with respect to implementing READLINK operation on a
    userspace NFS server. The request contain the file handle and the
    response include target name.

    Similarly we need to able to get file attribute based on file handle.
    I was using readlinkat and fstat to obtain both the above information
    using the descriptor returned by open_by_handle on a file handle
    representing symlink

    -aneesh


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