Re: [PATCH v2] kasan: don't emit builtin calls when sanitization is off

[Andrey Konovalov]

On Tue, Jan 23, 2018 at 10:24 AM, Andrey Ryabinin
<aryabinin@xxxxxxxxxxxxx> wrote:
>
>
> On 01/19/2018 08:58 PM, Andrey Konovalov wrote:
>> With KASAN enabled the kernel has two different memset() functions, one
>> with KASAN checks (memset) and one without (__memset). KASAN uses some
>> macro tricks to use the proper version where required. For example memset()
>> calls in mm/slub.c are without KASAN checks, since they operate on poisoned
>> slab object metadata.
>>
>> The issue is that clang emits memset() calls even when there is no memset()
>> in the source code. They get linked with improper memset() implementation
>> and the kernel fails to boot due to a huge amount of KASAN reports during
>> early boot stages.
>>
>
> So how did you observe this? Why am I not seeing this problem?

I only observed this when I tried to cross-compile the kernel for
arm64 with clang. I suspect this can happen on x86 as well.

>
>> The solution is to add -fno-builtin flag for files with KASAN_SANITIZE := n
>> marker.
>>
>
> I'm not sure I understand how is this solves the problem. And what clang does
> instead of memset()? Does it inlines memset()? But according to GCC's man (clang's man is very vague about this)
> -fno-builtin should do exactly the opposite  - prevent inlining builtin functions
> and always generate a function call.

The issue is that the compiler is allowed to replace plain assignment
with memset() calls. And -fno-builtin suppresses that behavior, but
there are no mentions of this in the documentation that I could find.
I also failed to find the exact place in the source code that leads to
this, perhaps Nick can help with that.