Re: [PATCH v3 2/2] x86/purgatory: use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS

[Vaibhav Rustagi]

On Tue, Jul 23, 2019 at 2:24 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> KBUILD_CFLAGS is very carefully built up in the top level Makefile,
> particularly when cross compiling or using different build tools.
> Resetting KBUILD_CFLAGS via := assignment is an antipattern.
>
> The comment above the reset mentions that -pg is problematic.  Other
> Makefiles use `CFLAGS_REMOVE_file.o = $(CC_FLAGS_FTRACE)` when
> CONFIG_FUNCTION_TRACER is set. Prefer that pattern to wiping out all of
> the important KBUILD_CFLAGS then manually having to re-add them. Seems
> also that __stack_chk_fail references are generated when using
> CONFIG_STACKPROTECTOR or CONFIG_STACKPROTECTOR_STRONG.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")
> Reported-by: Vaibhav Rustagi <vaibhavrustagi@xxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> ---
> Alternatively, we could put these in all in one variable and remove it
> without any conditional checks (I think that's ok to do so with
> CFLAGS_REMOVE).
>
> Changes v2 -> v3:
> * Prefer $(CC_FLAGS_FTRACE) which is exported to -pg.
> * Also check CONFIG_STACKPROTECTOR and CONFIG_STACKPROTECTOR_STRONG.
> * Cc stable.
> Changes v1 -> v2:
> Rather than manually add -mno-sse, -mno-mmx, -mno-sse2, prefer to filter
> -pg flags.
>
>  arch/x86/purgatory/Makefile | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index 91ef244026d2..6ef0ced59b9c 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -20,11 +20,27 @@ KCOV_INSTRUMENT := n
>
>  # Default KBUILD_CFLAGS can have -pg option set when FTRACE is enabled. That
>  # in turn leaves some undefined symbols like __fentry__ in purgatory and not
> -# sure how to relocate those. Like kexec-tools, use custom flags.
> -
> -KBUILD_CFLAGS := -fno-strict-aliasing -Wall -Wstrict-prototypes -fno-zero-initialized-in-bss -fno-builtin -ffreestanding -c -Os -mcmodel=large
> -KBUILD_CFLAGS += -m$(BITS)
> -KBUILD_CFLAGS += $(call cc-option,-fno-PIE)
> +# sure how to relocate those.
> +ifdef CONFIG_FUNCTION_TRACER
> +CFLAGS_REMOVE_sha256.o += $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_purgatory.o += $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_string.o += $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_kexec-purgatory.o += $(CC_FLAGS_FTRACE)
> +endif
> +
> +ifdef CONFIG_STACKPROTECTOR
> +CFLAGS_REMOVE_sha256.o += -fstack-protector
> +CFLAGS_REMOVE_purgatory.o += -fstack-protector
> +CFLAGS_REMOVE_string.o += -fstack-protector
> +CFLAGS_REMOVE_kexec-purgatory.o += -fstack-protector
> +endif
> +
> +ifdef CONFIG_STACKPROTECTOR_STRONG
> +CFLAGS_REMOVE_sha256.o += -fstack-protector-strong
> +CFLAGS_REMOVE_purgatory.o += -fstack-protector-strong
> +CFLAGS_REMOVE_string.o += -fstack-protector-strong
> +CFLAGS_REMOVE_kexec-purgatory.o += -fstack-protector-strong
> +endif
>
>  $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
>                 $(call if_changed,ld)
> --
> 2.22.0.709.g102302147b-goog
>

Tested-by: Vaibhav Rustagi <vaibhavrustagi@xxxxxxxxxx>

I tested the v3 patch series with clang compiled kernel for below
scenarios:

1. kexec'ing into a new kernel.
2. Purposely crashing the running kernel to generate kdump logs.

Thanks,
Vaibhav