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

[Uros Bizjak]

On Fri, Sep 4, 2026 at 3:18 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> On September 4, 2026 1:25:22 AM PDT, Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >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
>
> That seems like a much better answer than that horrible hack.
>
> Acked-by: H. Peter Anvin <hpa@xxxxxxxxx>

I have retracted the 2/2 patch, so there is no need for the new --param anymore.

Uros.