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

Neil Brown
2010-08-23T01:53:05Z

On Mon, 23 Aug 2010 06:54:03 +0530
"Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com> wrote:

> 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 ?
>

Bother... you are right.

I had remembered that at the time that all that *at calls were added there was
discussion about how you always need some flags, particularly in the context
of adding O_CLOEXEC and (I thought) a flag to allow non-sequential allocation
of fds.
I had thought that they all got 'flags' arguments as a result, but it seems
not.
For openat you could squeeze something into the current 'flags' arg
(O_FILE_HANDLE), but for readlinkat, symlinkat at least there is no such
option. Sad really.

NeilBrown