[PATCH v2 1/2] x86/boot: Disable GCC min-pagesize assumption in boot code

[Uros Bizjak]

GCC treats absolute addresses smaller than the min-pagesize param
(which defaults to 4kB) in the generic address space as presumed
results of pointer arithmetics from NULL. For example, 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 boot and realmode source code.

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 x86 boot
and realmode code with GCC inhibits warnings for addresses below 4kB.

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

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
---
 arch/x86/boot/Makefile        | 3 +++
 arch/x86/realmode/rm/Makefile | 3 +++
 2 files changed, 6 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)
 
diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
index a0fb39abc5c8..75058f4dd7c1 100644
--- a/arch/x86/realmode/rm/Makefile
+++ b/arch/x86/realmode/rm/Makefile
@@ -67,3 +67,6 @@ KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP -D_WAKEUP \
 		   -I$(srctree)/arch/x86/boot
 KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
+ifdef CONFIG_CC_IS_GCC
+KBUILD_CFLAGS	+= $(call cc-option,--param=min-pagesize=0)
+endif
-- 
2.55.0