linux-kernel - [PATCH 04/12] tools/nolibc: use attribute((naked)) if available

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Message-Id: <20240728-nolibc-llvm-v1-4-bc384269bc35@weissschuh.net>
Date: Sun, 28 Jul 2024 12:09:58 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Willy Tarreau <w@....eu>, Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 Thomas Weißschuh <linux@...ssschuh.net>
Subject: [PATCH 04/12] tools/nolibc: use attribute((naked)) if available

The current entrypoint attributes optimize("Os", "omit-frame-pointer")
are intended to avoid all compiler generated code, like function
porologue and epilogue.
This is the exact usecase implemented by the attribute "naked".

Unfortunately this is not implemented by GCC for all targets,
so only use it where available.
This also provides compatibility with clang, which recognizes the
"naked" attribute but not the previously used attribute "optimized".

Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
---
 tools/include/nolibc/compiler.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/include/nolibc/compiler.h b/tools/include/nolibc/compiler.h
index fe3701863634..f77bb7d3e1a8 100644
--- a/tools/include/nolibc/compiler.h
+++ b/tools/include/nolibc/compiler.h
@@ -9,6 +9,15 @@
 #define __entrypoint __attribute__((optimize("Os", "omit-frame-pointer")))
 #define __entrypoint_epilogue() __builtin_unreachable()
 
+#if defined(__has_attribute)
+#  if __has_attribute(naked)
+#    undef  __entrypoint
+#    define __entrypoint __attribute__((naked))
+#    undef __entrypoint_epilogue
+#    define __entrypoint_epilogue()
+#  endif
+#endif /* defined(__has_attribute) */
+
 #if defined(__SSP__) || defined(__SSP_STRONG__) || defined(__SSP_ALL__) || defined(__SSP_EXPLICIT__)
 
 #define _NOLIBC_STACKPROTECTOR

-- 
2.45.2