[PATCH AUTOSEL 4.4 15/20] x86/build: Add -Wnoaddress-of-packed-member to REALMODE_CFLAGS, to silence GCC9 build warning

From: Sasha Levin
Date: Wed Sep 04 2019 - 12:04:14 EST

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

[ Upstream commit 42e0e95474fc6076b5cd68cab8fa0340a1797a72 ]

One of the very few warnings I have in the current build comes from
arch/x86/boot/edd.c, where I get the following with a gcc9 build:

arch/x86/boot/edd.c: In function âquery_eddâ:
arch/x86/boot/edd.c:148:11: warning: taking address of packed member of âstruct boot_paramsâ may result in an unaligned pointer value [-Waddress-of-packed-member]
148 | mbrptr = boot_params.edd_mbr_sig_buffer;
| ^~~~~~~~~~~

This warning triggers because we throw away all the CFLAGS and then make
a new set for REALMODE_CFLAGS, so the -Wno-address-of-packed-member we
added in the following commit is not present:

6f303d60534c ("gcc-9: silence 'address-of-packed-member' warning")

The simplest solution for now is to adjust the warning for this version
of CFLAGS as well, but it would definitely make sense to examine whether
REALMODE_CFLAGS could be derived from CFLAGS, so that it picks up changes
in the compiler flags environment automatically.

Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Acked-by: Borislav Petkov <bp@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
arch/x86/Makefile | 1 +
1 file changed, 1 insertion(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 00e0226634fa9..8b4d022ce0cbc 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -38,6 +38,7 @@ REALMODE_CFLAGS := $(M16_CFLAGS) -g -Os -D__KERNEL__ \

REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -ffreestanding)
REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -fno-stack-protector)
+REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -Wno-address-of-packed-member)
REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align4))