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


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