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