x86/asm: Fix build of UML with KASAN

Building UML with KASAN fails since commit 69d4c0d3 ("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

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: 69d4c0d3 ("entry, kasan, x86: Disallow overriding mem*() functions")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20230918-uml-kasan-v3-1-7ad6db477df6@axis.com

arch/x86/include/asm/linkage.h

@@ -105,6 +105,13 @@
 	CFI_POST_PADDING					\
 	SYM_FUNC_END(__cfi_##name)
 
+/* UML needs to be able to override memcpy() and friends for KASAN. */
+#ifdef CONFIG_UML
+# define SYM_FUNC_ALIAS_MEMFUNC	SYM_FUNC_ALIAS_WEAK
+#else
+# define SYM_FUNC_ALIAS_MEMFUNC	SYM_FUNC_ALIAS
+#endif
+
 /* SYM_TYPED_FUNC_START -- use for indirectly called globals, w/ CFI type */
 #define SYM_TYPED_FUNC_START(name)				\
 	SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_F_ALIGN)	\

arch/x86/lib/memcpy_64.S

@@ -40,7 +40,7 @@ SYM_TYPED_FUNC_START(__memcpy)
 SYM_FUNC_END(__memcpy)
 EXPORT_SYMBOL(__memcpy)
 
-SYM_FUNC_ALIAS(memcpy, __memcpy)
+SYM_FUNC_ALIAS_MEMFUNC(memcpy, __memcpy)
 EXPORT_SYMBOL(memcpy)
 
 SYM_FUNC_START_LOCAL(memcpy_orig)

arch/x86/lib/memmove_64.S

@@ -212,5 +212,5 @@ SYM_FUNC_START(__memmove)
 SYM_FUNC_END(__memmove)
 EXPORT_SYMBOL(__memmove)
 
-SYM_FUNC_ALIAS(memmove, __memmove)
+SYM_FUNC_ALIAS_MEMFUNC(memmove, __memmove)
 EXPORT_SYMBOL(memmove)

arch/x86/lib/memset_64.S

@@ -40,7 +40,7 @@ SYM_FUNC_START(__memset)
 SYM_FUNC_END(__memset)
 EXPORT_SYMBOL(__memset)
 
-SYM_FUNC_ALIAS(memset, __memset)
+SYM_FUNC_ALIAS_MEMFUNC(memset, __memset)
 EXPORT_SYMBOL(memset)
 
 SYM_FUNC_START_LOCAL(memset_orig)