Re: [PATCH v2] x86: Fix build of UML with KASAN

[Ingo Molnar]

* Vincent Whitchurch <vincent.whitchurch@xxxxxxxx> wrote:

> Building UML with KASAN fails since commit 69d4c0d32186 ("entry, kasan,
> x86: Disallow overriding mem*() functions") with the following errors:
> 
>  $ tools/testing/kunit/kunit.py run --kconfig_add CONFIG_KASAN=y
>  ...
>  ld: mm/kasan/shadow.o: in function `memset':
>  shadow.c:(.text+0x40): multiple definition of `memset';
>  arch/x86/lib/memset_64.o:(.noinstr.text+0x0): first defined here
>  ld: mm/kasan/shadow.o: in function `memmove':
>  shadow.c:(.text+0x90): multiple definition of `memmove';
>  arch/x86/lib/memmove_64.o:(.noinstr.text+0x0): first defined here
>  ld: mm/kasan/shadow.o: in function `memcpy':
>  shadow.c:(.text+0x110): multiple definition of `memcpy';
>  arch/x86/lib/memcpy_64.o:(.noinstr.text+0x0): first defined here

So the breakage was ~9 months ago, and apparently nobody build-tested UML?

Does UML boot with the fix?

> UML does not use GENERIC_ENTRY and is still supposed to be allowed to
> override the mem*() functions, so use weak aliases in that case.
> 
> Fixes: 69d4c0d32186 ("entry, kasan, x86: Disallow overriding mem*() functions")
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
> ---
> Changes in v2:
> - Use CONFIG_UML instead of CONFIG_GENERIC_ENTRY.
> - Link to v1: https://lore.kernel.org/r/20230609-uml-kasan-v1-1-5fac8d409d4f@xxxxxxxx
> ---
>  arch/x86/lib/memcpy_64.S  | 4 ++++
>  arch/x86/lib/memmove_64.S | 4 ++++
>  arch/x86/lib/memset_64.S  | 4 ++++
>  3 files changed, 12 insertions(+)
> 
> diff --git a/arch/x86/lib/memcpy_64.S b/arch/x86/lib/memcpy_64.S
> index 8f95fb267caa..47b004851cf3 100644
> --- a/arch/x86/lib/memcpy_64.S
> +++ b/arch/x86/lib/memcpy_64.S
> @@ -40,7 +40,11 @@ SYM_TYPED_FUNC_START(__memcpy)
>  SYM_FUNC_END(__memcpy)
>  EXPORT_SYMBOL(__memcpy)
>  
> +#ifdef CONFIG_UML
> +SYM_FUNC_ALIAS_WEAK(memcpy, __memcpy)
> +#else
>  SYM_FUNC_ALIAS(memcpy, __memcpy)
> +#endif
>  EXPORT_SYMBOL(memcpy)

Meh, the extra 3 #ifdefs are rather ugly and don't really express UML's 
expectations here.

So how about introducing a SYM_FUNC_ALIAS_MEMFUNC() variant on x86 in a 
suitable header, which maps to the right thing, with a comment added that 
explains that this is for UML's mem*() functions?

Thanks,

	Ingo