On Fri, Dec 10, 2021 at 1:25 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> We could make a special light-weight version of files_lookup_fd_raw(),
> I guess. We don't need the *whole* "look it up again".  We don't need
> to re-check the array bounds, and we don't need to do the nospec
> lookup - we would have triggered a NULL file pointer if that happened
> the first time around.
>
> So all we'd need to do is "check that fdt is the same, and check that
> fdt->fd[fd] is the same".

This is an ENTIRELY UNTESTED patch to do that.

It basically rewrites __fget_files() from scratch: it really wants to
do the fd array lookup by hand, in order to cache the intermediate fdt
pointer, and in order to cache the intermediate speculation-safe fd
array index etc.

It's not a very complicated function, and rewriting it actually cleans
up the loop to not need the ugly goto.

I made it use a helper wrapper function for the rcu locking, so that
the "meat" of the function can just use plain "return NULL" for the
error cases.

However, not only is it entirely untested, this rewrite also means
that gcc has now decided that the result is so simple and clear that
it will inline it into all the callers.

I guess that's a good sign - writing the code in a way that makes the
compiler say "now it's so trivial that it should be inlined" is
certainly not a bad thing. But it makes it hard to really compare the
asm.

I did try a version with "noinline" just to make it more comparable,
and hey, it all looked sane to me there too.

I added more comments about what is going on.

Again - this is UNTESTED. I've looked at the code, I've looked at the
diff, and I've looked at the code it generates. It all looks fine to
me. But I've looked at it so much that I suspect that I'd be entirely
blind to any completely obvious bug by now.

Comments?

Oliver, does this make any difference in the performance department?

                 Linus