linux-kernel - Re: [PATCH] selftests/mm: fix FORCE_READ to read input value correctly.

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Message-ID: <76EE266A-7E9B-484F-AF97-85A99D109005@nvidia.com>
Date: Tue, 05 Aug 2025 14:48:25 -0400
From: Zi Yan <ziy@...dia.com>
To: Jann Horn <jannh@...gle.com>
Cc: wang lian <lianux.mm@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 David Hildenbrand <david@...hat.com>, Wei Yang <richard.weiyang@...il.com>,
 Christian Brauner <brauner@...nel.org>, Kairui Song <ryncsn@...il.com>,
 Liam Howlett <liam.howlett@...cle.com>, Mark Brown <broonie@...nel.org>,
 SeongJae Park <sj@...nel.org>, Shuah Khan <shuah@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] selftests/mm: fix FORCE_READ to read input value
 correctly.

On 5 Aug 2025, at 14:38, Jann Horn wrote:

> On Tue, Aug 5, 2025 at 7:51 PM Zi Yan <ziy@...dia.com> wrote:
>> FORCE_READ() converts input value x to its pointer type then reads from
>> address x. This is wrong. If x is a non-pointer, it would be caught it
>> easily. But all FORCE_READ() callers are trying to read from a pointer and
>> FORCE_READ() basically reads a pointer to a pointer instead of the original
>> typed pointer. Almost no access violation was found, except the one from
>> split_huge_page_test.
> [...]
>> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
>> index c20298ae98ea..b55d1809debc 100644
>> --- a/tools/testing/selftests/mm/vm_util.h
>> +++ b/tools/testing/selftests/mm/vm_util.h
>> @@ -23,7 +23,7 @@
>>   * anything with it in order to trigger a read page fault. We therefore must use
>>   * volatile to stop the compiler from optimising this away.
>>   */
>> -#define FORCE_READ(x) (*(volatile typeof(x) *)x)
>> +#define FORCE_READ(x) (*(const volatile typeof(x) *)&(x))
>
> So is the problem with the old code basically that it should have been
> something like
>
> #define FORCE_READ(x) (*(volatile typeof(*(x)) *)(x))
>
> to actually cast the normal pointer to a volatile pointer?

Yeah. That works too. I would rename it to FORCE_READ_PTR to avoid
misuse. :)

Best Regards,
Yan, Zi