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

[Feng Jiang]

On 2026/1/29 06:39, Kees Cook wrote:
> 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 at kylinos.cn>
>> Acked-by: Andy Shevchenko <andy at kernel.org>
>> Tested-by: Joel Stanley <joel at 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?

Thanks for the suggestion! That is a very effective way to catch potential
over-reads in optimized implementations.

I will refactor the correctness tests in v6 to use a vmalloc-allocated page
and ensure the NUL character is positioned at the very end of the allocation
boundary.

I'll send out the v6 patch set shortly with these changes.

-- 
With Best Regards,
Feng Jiang