Re: [PATCH 1/2] riscv: Add memmove string operation.

[Nick Hu <nickhu@andestech.com>]

Hi Paul,

On Thu, Aug 15, 2019 at 11:27:51AM -0700, Paul Walmsley wrote:
> On Thu, 15 Aug 2019, Nick Hu wrote:
> 
> > On Wed, Aug 14, 2019 at 10:03:39AM -0700, Paul Walmsley wrote:
> >
> > > Thanks for the explanation.  What do you think about Palmer's idea to 
> > > define a generic C set of KASAN string operations, derived from the newlib 
> > > code?
> > 
> > That sounds good to me. But it should be another topic. We need to investigate
> > it further about replacing something generic and fundamental in lib/string.c
> > with newlib C functions.  Some blind spots may exist.  So I suggest, let's
> > consider KASAN for now.
> 
> OK.  Here is the problem for us as maintainers.  You, Palmer, and I all 
> agree that a C-language version would be better.  We'd rather not merge a 
> pure assembly-language version unless it had significant advantages, and 
> right now we're not anticipating that.  So that suggests that a C-language 
> memmove() is the right way to go.
> 
> But if we merge a C-language memmove() into arch/riscv, other kernel 
> developers would probably ask us why we're doing that, since there's 
> nothing RISC-V-specific about it.  So do you think you might reconsider 
> sending patches to add a generic C-language memmove()?
> 
> 
> - Paul

About pushing mem*() generic, let's start with the reason why in the first place
KASAN needs re-implement its own string operations:

In mm/kasan/common.c:

	#undef memset
	void *memset(void *addr, int c, size_t len)
	{
		check_memory_region((unsigned long)addr, len, true, _RET_IP_);

		return __memset(addr, c, len);
	}

KASAN would call the string operations with the prefix '__', which should be
just an alias to the proper one.

In the past, every architecture that supports KASAN does this in assembly.
E.g. ARM64:

In arch/arm64/lib/memset.S:

	ENTRY(__memset)
	ENTRY(memset)
	...
	...
	EXPORT_SYMBOL(memset)
	EXPORT_SYMBOL(__memset) // export this as an alias

In arch/arm64/include/asm/string.h

	#define __HAVE_ARCH_MEMSET
	extern void *memset(void *, int, __kernel_size_t);
	extern void *__memset(void *, int, __kernel_size_t);

Now, if we are going to replace the current string operations with newlib ones
and let KASAN use them, we must provide something like this:

In lib/string.c:
        void *___memset(...)
        {
                ...
        }

In include/linux/string.h:

	#ifndef __HAVE_ARCH_MEMCPY 
	#ifdef CONFIG_KASAN
	static inline void* __memset(...)
	{
		___memset(...);
        }
	extern void memset(...); // force those who include this header uses the
					memset wrapped by KASAN
	#else
	static inline void *memset(...)
	{
		___memset(...);
	}
	#endif
	#endif

Does this look OK to you?

Nick