Re: [PATCH v5 1/8] lib/string_kunit: add correctness test for strlen()

[Kees Cook <kees@kernel.org>]

On Tue, Jan 27, 2026 at 09:25:51AM +0800, Feng Jiang wrote:
> Add a KUnit test for strlen() to verify correctness across
> different string lengths and memory alignments.
> 
> Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
> Acked-by: Andy Shevchenko <andy@kernel.org>
> Tested-by: Joel Stanley <joel@jms.id.au>
> ---
>  lib/tests/string_kunit.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/lib/tests/string_kunit.c b/lib/tests/string_kunit.c
> index f9a8e557ba77..bc5130c6e5e9 100644
> --- a/lib/tests/string_kunit.c
> +++ b/lib/tests/string_kunit.c
> @@ -17,6 +17,9 @@
>  #define STRCMP_TEST_EXPECT_LOWER(test, fn, ...) KUNIT_EXPECT_LT(test, fn(__VA_ARGS__), 0)
>  #define STRCMP_TEST_EXPECT_GREATER(test, fn, ...) KUNIT_EXPECT_GT(test, fn(__VA_ARGS__), 0)
>  
> +#define STRING_TEST_MAX_LEN	128
> +#define STRING_TEST_MAX_OFFSET	16
> +
>  static void string_test_memset16(struct kunit *test)
>  {
>  	unsigned i, j, k;
> @@ -104,6 +107,28 @@ static void string_test_memset64(struct kunit *test)
>  	}
>  }
>  
> +static void string_test_strlen(struct kunit *test)
> +{
> +	const size_t buf_size = STRING_TEST_MAX_LEN + STRING_TEST_MAX_OFFSET + 1;
> +	size_t len, offset;
> +	char *s;
> +
> +	s = kunit_kzalloc(test, buf_size, GFP_KERNEL);

One aspect of "correctness" that we might want to include here is making
sure we don't have any implementations that over-read. To that end,
perhaps this test can put the string at the end of a vmalloc allocation
(so that the end of the string is right up against an unallocated memory
space).

> +	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, s);
> +
> +	memset(s, 'A', buf_size);
> +	s[buf_size - 1] = '\0';
> +
> +	for (offset = 0; offset < STRING_TEST_MAX_OFFSET; offset++) {
> +		for (len = 0; len <= STRING_TEST_MAX_LEN; len++) {
> +			s[offset + len] = '\0';
> +			KUNIT_EXPECT_EQ_MSG(test, strlen(s + offset), len,
> +				"offset:%zu len:%zu", offset, len);
> +			s[offset + len] = 'A';
> +		}
> +	}
> +}

It would require building the string backwards here. Or maybe we just
need a separate test for the over-read concerns?

Thoughts?

-Kees

-- 
Kees Cook