linux-kernel - [PATCH v2 2/2] x86/purgatory: use CFLAGS_REMOVE rather than reset KBUILD

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Message-Id: <20190722213250.238685-2-ndesaulniers@google.com>
Date:   Mon, 22 Jul 2019 14:32:45 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de
Cc:     peterz@...radead.org, clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Vaibhav Rustagi <vaibhavrustagi@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/2] x86/purgatory: use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS

KBUILD_CFLAGS is very carefully built up in the top level Makefile,
particularly when cross compiling or using different build tools.
Resetting KBUILD_CFLAGS via := assignment is an antipattern.

The comment above the reset mentions that -pg is problematic.  Other
Makefiles like arch/x86/xen/vdso/Makefile use
`CFLAGS_REMOVE_file.o = -pg` when CONFIG_FUNCTION_TRACER is set. Prefer
that pattern to wiping out all of the important KBUILD_CFLAGS then
manually having to re-add them.

Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")
Reported-by: Vaibhav Rustagi <vaibhavrustagi@...gle.com>
Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Rather than manually add -mno-sse, -mno-mmx, -mno-sse2, prefer to filter
-pg flags.

 arch/x86/purgatory/Makefile | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index 91ef244026d2..56bcabca283f 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -20,11 +20,13 @@ KCOV_INSTRUMENT := n
 
 # Default KBUILD_CFLAGS can have -pg option set when FTRACE is enabled. That
 # in turn leaves some undefined symbols like __fentry__ in purgatory and not
-# sure how to relocate those. Like kexec-tools, use custom flags.
-
-KBUILD_CFLAGS := -fno-strict-aliasing -Wall -Wstrict-prototypes -fno-zero-initialized-in-bss -fno-builtin -ffreestanding -c -Os -mcmodel=large
-KBUILD_CFLAGS += -m$(BITS)
-KBUILD_CFLAGS += $(call cc-option,-fno-PIE)
+# sure how to relocate those.
+ifdef CONFIG_FUNCTION_TRACER
+CFLAGS_REMOVE_sha256.o = -pg
+CFLAGS_REMOVE_purgatory.o = -pg
+CFLAGS_REMOVE_string.o = -pg
+CFLAGS_REMOVE_kexec-purgatory.o = -pg
+endif
 
 $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
 		$(call if_changed,ld)
-- 
2.22.0.657.g960e92d24f-goog