[PATCH 2/3] x86/boot: disable GCC min-pagesize assumption in boot code

[Uros Bizjak <ubizjak@gmail.com>]

GCC treats absolute addresses smaller than min-pagesize param
(defaulting to 4kB) as assumed results of pointer arithmetics from
NULL. The following code, when compiled with -O2 -Warray-bounds
(included in -Wall):

  int foo (void) { return *(int *)0x123; }

will emit a rather cryptic warning:

warning: array subscript 0 is outside array bounds of ‘int[0]’ [-Warray-bounds=]
    1 | int foo (void) { return *(int *)0x123; }
      |                         ^~~~~~~~~~~~~
cc1: note: source object is likely at address zero

Currently, the warning is supressed by the GCC specific RELOC_HIDE()
macro that obfuscates arithmetic on a variable address so that GCC
doesn't recognize the original var, and make assumptions about it.

The GCC specific RELOC_HIDE() macro was introduced to work around
certain ppc64 specific compiler bug in pre-4.1 GCC. This bug was
fixed long ago, and replacing GCC specific macro with a generic one
triggers the above warning in copy_boot_params().

The early boot environment does not guarantee any minimum page size,
so explicitly setting the minimum page size to zero by adding
--param=min-pagesize=0 to the compiler flags when building the x86
boot code with GCC inhibits warnings for addresses below 4kB.

The option is guarded by CONFIG_CC_IS_GCC since it is GCC-specific.

No functional changes intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
 arch/x86/boot/Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index 3f9fb3698d66..372e67d2a855 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -55,6 +55,9 @@ KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP
 KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
 KBUILD_CFLAGS	+= $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
+ifdef CONFIG_CC_IS_GCC
+KBUILD_CFLAGS	+= $(call cc-option,--param=min-pagesize=0)
+endif
 
 $(obj)/bzImage: asflags-y  := $(SVGA_MODE)
 
-- 
2.52.0