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Subject: Re: [RFC PATCH 0/2] improve vmalloc allocation
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Uladzislau Rezki <urezki@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Garnier <thgarnie@google.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joelaf@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@elte.hu>, Tejun Heo <tj@kernel.org>,
        maco@android.com, Shuah Khan <shuah@kernel.org>
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From:   Shuah Khan <shuah@kernel.org>
Message-ID: <024af44a-77e1-1c61-c9b2-64ffbe4f7c49@kernel.org>
Date:   Tue, 23 Oct 2018 13:48:32 -0600
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On 10/23/2018 01:30 PM, Joel Fernandes wrote:
> On Tue, Oct 23, 2018 at 11:13:36AM -0600, Shuah Khan wrote:
>> On 10/23/2018 11:05 AM, Michal Hocko wrote:
>>> On Tue 23-10-18 08:26:40, Matthew Wilcox wrote:
>>>> On Tue, Oct 23, 2018 at 09:02:56AM -0600, Shuah Khan wrote:
>>> [...]
>>>>> The way it can be handled is by adding a test module under lib. test_kmod,
>>>>> test_sysctl, test_user_copy etc.
>>>>
>>>> The problem is that said module can only invoke functions which are
>>>> exported using EXPORT_SYMBOL.  And there's a cost to exporting them,
>>>> which I don't think we're willing to pay, purely to get test coverage.
>>>
>>> Yes, I think we do not want to export internal functionality which might
>>> be still interesting for the testing coverage. Maybe we want something
>>> like EXPORT_SYMBOL_KSELFTEST which would allow to link within the
>>> kselftest machinery but it wouldn't allow the same for general modules
>>> and will not give any API promisses.
>>>
>>
>> I like this proposal. I think we will open up lot of test opportunities with
>> this approach.
>>
>> Maybe we can use this stress test as a pilot and see where it takes us.
> 
> I am a bit worried that such an EXPORT_SYMBOL_KSELFTEST mechanism can be abused by
> out-of-tree module writers to call internal functionality.

That is  valid concern to consider before we go forward with the proposal.

We could wrap EXPORT_SYMBOL_KSELFTEST this in an existing debug option. This could
be fine grained for each sub-system for its debug option. We do have a few of these
now

# CONFIG_STATIC_KEYS_SELFTEST is not set
# CONFIG_BT_SELFTEST is not set
# CONFIG_DRM_DEBUG_SELFTEST is not set
# CONFIG_CHASH_SELFTEST is not set
# CONFIG_DRM_I915_SELFTEST is not set
# CONFIG_CRC32_SELFTEST is not set
# CONFIG_RANDOM32_SELFTEST is not set
# CONFIG_GLOB_SELFTEST is not set
# CONFIG_STRING_SELFTEST is not set
# CONFIG_DEBUG_LOCKING_API_SELFTESTS is not set
# CONFIG_WW_MUTEX_SELFTEST is not set
# CONFIG_ATOMIC64_SELFTEST is not set
# CONFIG_X86_DECODER_SELFTEST is not set
# CONFIG_DEBUG_NMI_SELFTEST is not set

thanks,
-- Shuah