linkage: add SYM_FUNC_ALIAS{,_LOCAL,_WEAK}()

Currently aliasing an asm function requires adding START and END
annotations for each name, as per Documentation/asm-annotations.rst:

	SYM_FUNC_START_ALIAS(__memset)
	SYM_FUNC_START(memset)
	    ... asm insns ...
	SYM_FUNC_END(memset)
	SYM_FUNC_END_ALIAS(__memset)

This is more painful than necessary to maintain, especially where a
function has many aliases, some of which we may wish to define
conditionally. For example, arm64's memcpy/memmove implementation (which
uses some arch-specific SYM_*() helpers) has:

	SYM_FUNC_START_ALIAS(__memmove)
	SYM_FUNC_START_ALIAS_WEAK_PI(memmove)
	SYM_FUNC_START_ALIAS(__memcpy)
	SYM_FUNC_START_WEAK_PI(memcpy)
	    ... asm insns ...
	SYM_FUNC_END_PI(memcpy)
	EXPORT_SYMBOL(memcpy)
	SYM_FUNC_END_ALIAS(__memcpy)
	EXPORT_SYMBOL(__memcpy)
	SYM_FUNC_END_ALIAS_PI(memmove)
	EXPORT_SYMBOL(memmove)
	SYM_FUNC_END_ALIAS(__memmove)
	EXPORT_SYMBOL(__memmove)
	SYM_FUNC_START(name)

It would be much nicer if we could define the aliases *after* the
standard function definition. This would avoid the need to specify each
symbol name twice, and would make it easier to spot the canonical
function definition.

This patch adds new macros to allow us to do so, which allows the above
example to be rewritten more succinctly as:

	SYM_FUNC_START(__pi_memcpy)
	    ... asm insns ...
	SYM_FUNC_END(__pi_memcpy)

	SYM_FUNC_ALIAS(__memcpy, __pi_memcpy)
	EXPORT_SYMBOL(__memcpy)
	SYM_FUNC_ALIAS_WEAK(memcpy, __memcpy)
	EXPORT_SYMBOL(memcpy)

	SYM_FUNC_ALIAS(__pi_memmove, __pi_memcpy)
	SYM_FUNC_ALIAS(__memmove, __pi_memmove)
	EXPORT_SYMBOL(__memmove)
	SYM_FUNC_ALIAS_WEAK(memmove, __memmove)
	EXPORT_SYMBOL(memmove)

The reduction in duplication will also make it possible to replace some
uses of WEAK with more accurate Kconfig guards, e.g.

	#ifndef CONFIG_KASAN
	SYM_FUNC_ALIAS(memmove, __memmove)
	EXPORT_SYMBOL(memmove)
	#endif

... which should make it easier to ensure that symbols are neither used
nor overidden unexpectedly.

The existing SYM_FUNC_START_ALIAS() and SYM_FUNC_START_LOCAL_ALIAS() are
marked as deprecated, and will be removed once existing users are moved
over to the new scheme.

The tools/perf/ copy of linkage.h is updated to match. A subsequent
patch will depend upon this when updating the x86 asm annotations.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Mark Brown <broonie@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220216162229.1076788-2-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/Documentation/asm-annotations.rst b/Documentation/asm-annotations.rst
index f4bf0f6395fb..4868b58c60fb 100644
--- a/Documentation/asm-annotations.rst
+++ b/Documentation/asm-annotations.rst
@@ -130,8 +130,20 @@ denoting a range of code via ``SYM_*_START/END`` annotations.
   In fact, this kind of annotation corresponds to the now deprecated ``ENTRY``
   and ``ENDPROC`` macros.
 
-* ``SYM_FUNC_START_ALIAS`` and ``SYM_FUNC_START_LOCAL_ALIAS`` serve for those
-  who decided to have two or more names for one function. The typical use is::
+* ``SYM_FUNC_ALIAS``, ``SYM_FUNC_ALIAS_LOCAL``, and ``SYM_FUNC_ALIAS_WEAK`` can
+  be used to define multiple names for a function. The typical use is::
+
+    SYM_FUNC_START(__memset)
+        ... asm insns ...
+    SYN_FUNC_END(__memset)
+    SYM_FUNC_ALIAS(memset, __memset)
+
+  In this example, one can call ``__memset`` or ``memset`` with the same
+  result, except the debug information for the instructions is generated to
+  the object file only once -- for the non-``ALIAS`` case.
+
+* ``SYM_FUNC_START_ALIAS`` and ``SYM_FUNC_START_LOCAL_ALIAS`` are deprecated
+    ways to define two or more names for one function. The typical use is::
 
     SYM_FUNC_START_ALIAS(__memset)
     SYM_FUNC_START(memset)