Re: [PATCH v5 11/75] x86/boot/compressed/64: Disable red-zone usage

Arvind Sankar
2020-07-24T17:58:23Z

On Fri, Jul 24, 2020 at 06:02:32PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> The x86-64 ABI defines a red-zone on the stack:
>
> The 128-byte area beyond the location pointed to by %rsp is considered
> to be reserved and shall not be modified by signal or interrupt
> handlers. Therefore, functions may use this area for temporary data
> that is not needed across function calls. In particular, leaf
> functions may use this area for their entire stack frame, rather than
> adjusting the stack pointer in the prologue and epilogue. This area is
> known as the red zone.
>
> This is not compatible with exception handling, because the IRET frame
> written by the hardware at the stack pointer and the functions to handle
> the exception will overwrite the temporary variables of the interrupted
> function, causing undefined behavior. So disable red-zones for the
> pre-decompression boot code.
>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> arch/x86/boot/Makefile | 2 +-
> arch/x86/boot/compressed/Makefile | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index fe605205b4ce..4d6a16a47e9f 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -66,7 +66,7 @@ targets += cpustr.h
>
> # ---------------------------------------------------------------------------
>
> -KBUILD_CFLAGS := $(REALMODE_CFLAGS) -D_SETUP
> +KBUILD_CFLAGS := $(REALMODE_CFLAGS) -D_SETUP -mno-red-zone
> KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> KBUILD_CFLAGS += -fno-asynchronous-unwind-tables

This change seems unnecessary? REALMODE_CFLAGS means it uses -m16, so
this isn't 64-bit code. [As an aside, we can drop the check for -m16
support in arch/x86/Makefile now that we've bumbed to 4.9]

> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 5a828fde7a42..416f52ab39ec 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -32,7 +32,7 @@ KBUILD_CFLAGS := -m$(BITS) -O2
> KBUILD_CFLAGS += -fno-strict-aliasing $(call cc-option, -fPIE, -fPIC)
> KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
> cflags-$(CONFIG_X86_32) := -march=i386
> -cflags-$(CONFIG_X86_64) := -mcmodel=small
> +cflags-$(CONFIG_X86_64) := -mcmodel=small -mno-red-zone
> KBUILD_CFLAGS += $(cflags-y)
> KBUILD_CFLAGS += -mno-mmx -mno-sse
> KBUILD_CFLAGS += $(call cc-option,-ffreestanding)
> --
> 2.27.0
>

This one is necessary.