> On Thu, 2008-02-14 at 18:39 +0100, Miklos Szeredi wrote:
> > > > And I'm not against doing it with the "at*" variants, as Trond
> > > > suggested.
> > >
> > > If you're going to change the syscall, then you should ensure that it
> > > solves _all_ the problems that are known at this time. Ignoring the
> > > automounter issue is just going to force us to redo the syscall in a
> > > couple of months...
> >
> > Sure.
> >
> > Although, an (almost) equivalent userspace code would be:
> >
> > mount_fooat(int fd, const char *path)
> > {
> > char tmpbuf[64];
> > int tmpfd = openat(fd, path);
> >
> > sprintf(tmpbuf, "/proc/self/fd/%i", tmpfd);
> > return mount_foo(tmpbuf, ...);
> > }
> >
> > Or is there something (other than not requiring proc) that the *at
> > variant gives?
>
> The ability to have a daemon handle mounting onto a directory that only
> exists in another process's private namespace.
>
> Say I'm running in my private namespace that contains paths that are not
> shared by the trusted 'init' namespace. If I were to step on an
> autofs-like trap, I'd like for the kernel to be able to notify the
> automounter that is running in the trusted namespace set up by 'init',
> and have it mount the directory onto my namespace. This should happen
> even if the path is not shared.
>
> With mountat() the kernel can still pass the necessary information to
> the automounter by giving it a directory file descriptor 'fd' that
> points to the directory on top of which it wants the mount to occur.
> Then automounter then executes
>
> mountat(AT_FDCWD, dev_name, fd, '.', type, flags, data);
>
> and hey presto, the magic occurs.
I understand perfectly that this is what you want to do. And I'm
saying that the following code snippet should do exactly the same,
without having to add a new syscall:
char tmpbuf[64];
sprintf(tmpbuf, "/proc/self/fd/%i", fd);
mount(dev_name, tmpbuf, type, flags, data);
[ You could actually try to read people's responses, instead of
immediately assuming they don't understand :-/ ]
So again, what is the advantage of a mountat() syscall over just using
the proc trick?
Miklos
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