linux-kernel - Re: [PATCH linux-kselftest/test v4] lib/list-test: add a test for the 'list' doubly linked list

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Date:   Wed, 23 Oct 2019 14:25:49 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     David Gow <davidgow@...gle.com>
Cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        KUnit Development <kunit-dev@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH linux-kselftest/test v4] lib/list-test: add a test for the
 'list' doubly linked list

On Tue, Oct 22, 2019 at 3:13 PM David Gow <davidgow@...gle.com> wrote:
>
> On Sat, Oct 19, 2019 at 1:27 AM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> >
> > On Fri, Oct 18, 2019 at 02:55:49PM -0700, David Gow wrote:
> > > +     list4 = kzalloc(sizeof(*list4), GFP_KERNEL);
> > > +     KUNIT_ASSERT_NOT_ERR_OR_NULL(test, list4);
> >
> > Why not just use GFP_KERNEL | GFP_NOFAIL and remove the check?
>
> I've sent a new version of the patch out (v5) which uses __GFP_NOFAIL instead.
>
> The idea had been to exercise KUnit's assertion functionality, in the
> hope that it'd allow the test to fail (but potentially allow other
> tests to still run) in the case of allocation failure. Given that
> we're only allocating enough to store ~4 pointers in total, though,
> that's probably of little use.
>
> > kzalloc() can't return error pointers.  If this were an IS_ERR_OR_NULL()
> > check then it would generate a static checker warning, but static
> > checkers don't know about KUNIT_ASSERT_NOT_ERR_OR_NULL() yet so you're
> > safe.
>
> Alas, KUnit doesn't have a KUNIT_ASSERT_NOT_NULL() macro, and I'd
> assumed it was not dangerous (even if not ideal) to check for error
> pointers, even if kzalloc() can't return them.

Maybe it would be good for us (not in this case, just generally
speaking) to add a KUNIT_ASSERT_NOT_NULL() and friends?

> Perhaps it'd make sense to add a convenient way of checking the
> NULL-ness of pointers to KUnit (it's possible with the
> KUNIT_ASSERT_PTR_EQ(), but requires a bit of casting to make the type
> checker happy) in the future. Once KUnit is properly upstream, it may
> be worth teaching the static analysis tools about these functions to
> avoid having warnings in these sorts of tests.
>
> For now, though, (and for this test in particular), I agree with the
> suggestion of just using __GFP_NOFAIL.
>
> Thanks a lot for the comments,
> -- David