Re: [PATCH -V18 04/13] vfs: Allow handle based open on symlinks

["Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>]

On Mon, 23 Aug 2010 09:06:04 +1000, Neil Brown <neilb@suse.de> wrote:
> On Sat, 21 Aug 2010 01:13:52 -0600
> Andreas Dilger <andreas.dilger@oracle.com> wrote:
> 
> > On 2010-08-20, at 18:09, Neil Brown <neilb@suse.de> wrote:
> > > How about a new AT flag:  AT_FILE_HANDLE
> > > 
> > > Meaning is that the 'dirfd' is used only to identify a filesystem (vfsmnt) and
> > > the 'name' pointer actually points to a filehandle fragment interpreted in
> > > that filesystem.
> > > 
> > > One problem is that there is no way to pass the length...
> > > Options:
> > >   fragment is at most 64 bytes nul padded at the end
> > >   fragment is hex encoded and nul terminated
> > >   ??
> > > 
> > > I think I prefer the hex encoding, but I'm hoping someone else has a better
> > > idea.
> > 
> > That makes it ugly for the kernel to stringify and parse the file handles. 
> 
> We already parse filenames into components separated by '/'.  Is HEX decoding
> that much more ugly.
> 
> Filehandles are currently passed between the kernel and mountd as HEX
> strings, so at least there is some precedent.
> 
> > 
> > How about for AT_FILE_HANDLE THE FIRST __u32 (maybe with an extra __u32 for alignment) is the length and the rest of the binary file handle follows this?  In fact, doesn't the handle itself already encode the length in the header?
> 
> That part of a filehandle that nfsd gives to the filesystem is one byte out
> of a 4-byte header, plus the tail of the filehandle after the part that
> identifies the filesystem.
> This 'one byte' does imply the length, but it doesn't necessarily encode it.
> Rather it is a 'type'.  So it cannot really be used to determine the length
> at the point when the filehandle would need to be copied from userspace into
> the kernel.
> 
> 
> I don't think there is any precedent for passing a 4-byte length followed by
> a binary string, while there is plenty of precedent for passing a
> nul-terminated ASCII string.
> 
> [[ Following this approach I would like to avoid any filehandle-specific
> syscalls altogether.
> Just use a *at syscall with AT_FILE_HANDLE for filehandle lookup, and use
> getxattr('system:linux.file_handle') to get the filehandle for a given path.
> 
> Ofcourse we would need to at *at versions of the *xattr syscalls, but that is
> probably a good idea anyway.
> ]]

There are at* syscalls that doesn't take the additional flags as the
argument, like openat, readlinkat. How will handle based open and
readlink work with the above interface ?

-aneesh