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

From: Ingo Molnar <mingo@kernel.org>
To: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Richard Weinberger <richard@nod.at>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>,
	Johannes Berg <johannes@sipsolutions.net>,
	linux-um@lists.infradead.org,
	Andrey Ryabinin <ryabinin.a.a@gmail.com>,
	Alexander Potapenko <glider@google.com>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
	kernel@axis.com
Subject: Re: [PATCH v2] x86: Fix build of UML with KASAN
Date: Fri, 15 Sep 2023 11:32:38 +0200	[thread overview]
Message-ID: <ZQQkthfNuV3dOhZe@gmail.com> (raw)
In-Reply-To: <20230915-uml-kasan-v2-1-ef3f3ff4f144@axis.com>

* Vincent Whitchurch <vincent.whitchurch@axis.com> 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@axis.com>
> ---
> 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@axis.com
> ---
>  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

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