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

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

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?

> 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.