Re: [PATCH v2] kasan: Support for r/w instrumentation control

[Dmitry Vyukov]

On Tue, Dec 13, 2016 at 2:59 PM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> wrote:
> On 12/13/2016 12:38 PM, Dmitry Vyukov wrote:
>> On Tue, Dec 13, 2016 at 10:20 AM, Andrey Ryabinin
>> <aryabinin@xxxxxxxxxxxxx> wrote:
>>>
>>>
>>> On 12/13/2016 11:58 AM, Dmitry Vyukov wrote:
>>>
>>>> --- a/Documentation/dev-tools/kasan.rst
>>>> +++ b/Documentation/dev-tools/kasan.rst
>>>> @@ -40,6 +40,14 @@ similar to the following to the respective kernel Makefile:
>>>>
>>>>      KASAN_SANITIZE := n
>>>>
>>>> +Sometimes it may be useful to disable instrumentation of reads, or writes
>>>> +or both for the entire kernel. For example, if binary size is a concern,
>>>> +it may be useful to disable instrumentation of reads to reduce binary size but
>>>> +still catch more harmful bugs on writes. Or, if one is interested only in
>>>> +sanitization of a particular module and performance is a concern, she can
>>>> +disable instrumentation of both reads and writes for kernel code.
>>>> +Instrumentation can be disabled with CONFIG_KASAN_READS and
>>>> CONFIG_KASAN_WRITES.
>>>> +
>>>
>>> I don't understand this. How this can be related to modules? Configs are global.
>>> You can't just disable/enable config per module.
>>
>>
>> Build everything without instrumentation. Then enable instrumentation
>> and do "make lib/test_kasan.ko".
>> Or build everything, copy out bzImage, change config, build everything again.
>
> Yeah, this is soooooo convenient...
>
> Seriously speaking, per-file instrumentation is absolutely irrelevant to this patch and should have been
> addressed from a different angle. E.g. see how UBSAN/GCOV/KCOV do that.


KASAN already has that functionality (i.e. KASAN_SANITIZE_main.o :=
n). But that functionality is intended for cases when we want to
persistently disable instrumentation of some files (e.g. if they cause
crashes of false positives). CONFIG_KASAN_READS/WRITES is intended for
situations when one wants to disable instrumentation wholesale.


> As for this patch, I'd say only one option would be enough - KASAN_DONT_SANITIZE_READS.
> Nobody wants to sanitize only reads without writes, right? Writes are fewer and more dangerous.

I've asked this question in v1. See the case related to modules -- one
can use completely uninstrumented kernel, but load an instrumented
modules.