Re: [PATCH] fs/binfmt_elf: Fix memsz > filesz handling

[David Gow <davidgow@google.com>]

On Mon, Nov 7, 2022 at 11:59 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Sun, Nov 06, 2022 at 02:16:57AM +0000, Pedro Falcato wrote:
> David, has there been any work on adding a way to instantiate
> userspace VMAs in a KUnit test? I tried to write this myself, but I
> couldn't figure out how to make the userspace memory mappings appear.
> Here's my fumbling attempt:
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=devel/kunit/usercopy
>
> I really wish KUnit had userspace mapping support -- I have a bunch of
> unit tests that need to get built up around checking for regressions
> here, etc.

Hi Kees,

Sorry the the delayed response!

Alas, my attempts to get this to work haven't been much more
successful than yours. It's definitely something we'd like to support,
but I confess to not knowing enough about the mm code to know exactly
what would be involved.

The workaround is to load tests as modules, and use something like
Vitor's original patch here:
https://lore.kernel.org/all/20200721174036.71072-1-vitor@massaru.org/

Basically, using the existing mm of the module loader. Adapting those
changes to your branch (and fixing a couple of back-to-front KUnit
assertions) does work for me when built as a module, in an x86_64 vm:

root@slicestar:~# modprobe usercopy_kunit
[   52.986290]     # Subtest: usercopy
[   52.986701]     1..1
[   53.246058]     ok 1 - usercopy_test
[   53.246628] ok 1 - usercopy

But getting it to work with built-in tests hasn't been successful so
far. I wondered if we could just piggy-back on init_mm or similar, but
that doesn't seem to work either.

So, in the short-term, this is only possible for modules. If that's
useful enough, we can get Vitor's support patch (or something similar)
in, and just mark any tests module-only (or have them skip if there's
no mm). Because kunit.py only runs built-in tests, though, it's
definitely less convenient.

Cheers,
-- David