Re: [PATCH v4 2/2] userns: Add KUnit test suite for uid_gid_map

[Kees Cook <kees@kernel.org>]

On Fri, Sep 04, 2026 at 01:37:00PM -0700, Bill Wendling wrote:
> On Fri, Sep 4, 2026 at 1:02 AM Thomas Weißschuh
> <thomas.weissschuh@linutronix.de> wrote:
> >
> > On Thu, Sep 03, 2026 at 01:21:27PM -0700, Bill Wendling wrote:
> > > On Mon, Aug 31, 2026 at 2:22 AM Thomas Weißschuh
> > > <thomas.weissschuh@linutronix.de> wrote:
> > > > On Thu, Aug 27, 2026 at 12:27:30PM -0700, Bill Wendling wrote:
> > > > > On Thu, Aug 27, 2026 at 6:37 AM Thomas Weißschuh
> > > > > <thomas.weissschuh@linutronix.de> wrote:
> > > >
> > > > (...)
> > > >
> > > > > > > +config USER_NAMESPACE_KUNIT_TEST
> > > > > > > +     bool "Test user namespace map insertion" if !KUNIT_ALL_TESTS
> > > > > > > +     depends on KUNIT=y
> > > > > >
> > > > > > Urgh.
> > > > > >
> > > > > ?? What's wrong? It's identical to the conditional for EXEC_KUNIT_TEST:
> > > >
> > > > Sorry for this non-descript review comment.
> > > >
> > > > > config EXEC_KUNIT_TEST
> > > > >      bool "Build execve tests" if !KUNIT_ALL_TESTS
> > > > >      depends on KUNIT=y
> > > > >      default KUNIT_ALL_TESTS
> > > > >      help
> > > > >           This builds the exec KUnit tests, which tests boundary conditions
> > > > >           of various aspects of the exec internals.
> > > >
> > > > The problem is that KUNIT can be built as module, which would prevent this
> > > > test from being built. We have include/kunit/visibility.h to export certain
> > > > symbols only to tests and avoid this issue.
> > > > But I can see that some maintaines don't like this pattern, so maybe they can
> > > > chime in at some point.
> > >
> > > Bradley commented on this earlier (which is why I mentioned EXEC_KUNIT_TEST):
> > >
> > > <comment>
> > > The test is #include'd into user_namespace.c, which is builtin (USER_NS
> > > is a bool), so =m here still compiles the suite into vmlinux. With
> > > KUNIT=m that calls kunit symbols that live in a module, and the link
> > > fails. Make it bool and depend on KUNIT=y, like EXEC_KUNIT_TEST:
> > >
> > >  bool "KUnit test for user namespace map insertion" if !KUNIT_ALL_TESTS
> > >  depends on USER_NS && KUNIT=y
> > > </comment>
> > >
> > > So there's a conflict and, because I'm not a KUnit guru, I'm not sure
> > > which way is "best".
> >
> > It's subjective. So as mentioned before, the preference of the maintainers
> > should go into it. The aproach I prefer requires a bit more setup boilerplate
> > but make the tests usable in more circumstances.
> >
> Because user_namespace.c is always built-in (USER_NS is a bool),
> compiling the test into vmlinux causes linker failures if
> CONFIG_KUNIT=m. Using the "visibility.h" version also strips static
> from insert_extent() and sort_idmaps() or exporting internal user
> namespace functions into the kernel symbol table, which isn't ideal.

The export namespacing should limit it to just KUnit, so that should be
okay, I would think? My guide for this in the past has been "did the
kernel get less fast/safe because of Kunit exports, etc?" I don't
_think_ that's the case here.

So, I guess it's down to a matter of taste by USER_NS maintainers? I've
move Christian to "To:" from "Cc:", as I think he is the defacto USER_NS
maintainer? I don't know, though, there is no MAINTAINERS entry for it. 

> > > Test (3) (which I'll add in my next upload) throws a sanitizer exception.
> >
> > What is the point of testing this specifically for user namespaces?
> > Normally we expect a used subsystem to work as advertised.
> > It is that used subsystem's responsibility to test that it does so.
> > If there is currently no test that validates __counted_by then it surely
> > should be created. But not here.
> >
> This would directly test that the attribute on the struct field is
> caught by UBSAN. I'm not sure how we could more directly test it
> otherwise...

It is tricky to catch the traps sanely with KUnit. All the Oops tests
I've written for it are in LKDTM, and only __bdos is tested in
lib/tests/overflow_kunit.c. I was playing with catching Oops in KUnit
for testing KCFI recently, but haven't come back to finishing it safely:
https://lore.kernel.org/lkml/20260618210946.it.538-kees@kernel.org/

-Kees

-- 
Kees Cook