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Date:   Thu, 6 Apr 2023 16:07:35 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Alexander Potapenko <glider@google.com>
Cc:     linux-hardening@vger.kernel.org, Kees Cook <kees@outflux.net>,
        Andy Shevchenko <andy@kernel.org>,
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        =?iso-8859-1?Q?Jos=E9_Exp=F3sito?= <jose.exposito89@gmail.com>,
        linux-kernel@vger.kernel.org, kunit-dev@googlegroups.com
Subject: Re: [PATCH 3/9] string: Add Kunit tests for strcat() family
References: <20230405235832.never.487-kees@kernel.org>
 <20230406000212.3442647-3-keescook@chromium.org>
 <CAG_fn=V-3yxPihQdt+OJuOfF6sBuNvQ2OQfYNZak1xbx6viU2w@mail.gmail.com>
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On Thu, Apr 06, 2023 at 11:11:09AM +0200, Alexander Potapenko wrote:
> > +static void strncat_test(struct kunit *test)
> > +{
> > +       char dest[8];
> > +
> > +       /* Destination is terminated. */
> > +       memset(dest, 0, sizeof(dest));
> > +       KUNIT_EXPECT_EQ(test, strlen(dest), 0);
> > +       /* Empty copy of size 0 does nothing. */
> > +       KUNIT_EXPECT_TRUE(test, strncat(dest, "", 0) == dest);
> > +       KUNIT_EXPECT_STREQ(test, dest, "");
> > +       /* Empty copy of size 1 does nothing too. */
> > +       KUNIT_EXPECT_TRUE(test, strncat(dest, "", 1) == dest);
> > +       KUNIT_EXPECT_STREQ(test, dest, "");
> > +       /* Copy of max 0 characters should do nothing. */
> > +       KUNIT_EXPECT_TRUE(test, strncat(dest, "asdf", 0) == dest);
> > +       KUNIT_EXPECT_STREQ(test, dest, "");
> > +
> > +       /* 4 characters copied in, even if max is 8. */
> > +       KUNIT_EXPECT_TRUE(test, strncat(dest, "four\000123", 8) == dest);
> > +       KUNIT_EXPECT_STREQ(test, dest, "four");
> > +       KUNIT_EXPECT_EQ(test, dest[5], '\0');
> 
> Maybe also add a test case for strncat(dest, "four", 4) that checks
> that the fourth byte of dest is not 0?

I think I don't understand what state you want to test for? The line
above (STREQ is checking dest is "four". Maybe I should check for
dest[6] being 0 as well as dest[5]. But if that's not what you mean, I'm
not sure. Is it something here:

char dest[16];
memset(dest, 0, sizeof(dest));
// dest == ""
strncat(dest, "four", 4);
// dest == "four"
strncat(dest, "four", 4);
// dest == "fourfour"

strncat's "n" is how much to reach from source -- dest will always be
terminated.

-- 
Kees Cook