[v2,2/2] x86/purgatory: use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS

Message ID 20190722213250.238685-2-ndesaulniers@google.com
State New
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Series
  • [v2,1/2] x86/purgatory: do not use __builtin_memcpy and __builtin_memset
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Commit Message

Nick Desaulniers July 22, 2019, 9:32 p.m.
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 like arch/x86/xen/vdso/Makefile use
`CFLAGS_REMOVE_file.o = -pg` 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.

Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")
Reported-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

---
Rather than manually add -mno-sse, -mno-mmx, -mno-sse2, prefer to filter
-pg flags.

 arch/x86/purgatory/Makefile | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

-- 
2.22.0.657.g960e92d24f-goog

Comments

Nick Desaulniers July 22, 2019, 10:10 p.m. | #1
On Mon, Jul 22, 2019 at 2:33 PM Nick Desaulniers
<ndesaulniers@google.com> 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 like arch/x86/xen/vdso/Makefile use

> `CFLAGS_REMOVE_file.o = -pg` 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.

>

> Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")

> Reported-by: Vaibhav Rustagi <vaibhavrustagi@google.com>

> Suggested-by: Peter Zijlstra <peterz@infradead.org>

> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

> ---

> Rather than manually add -mno-sse, -mno-mmx, -mno-sse2, prefer to filter

> -pg flags.

>

>  arch/x86/purgatory/Makefile | 12 +++++++-----

>  1 file changed, 7 insertions(+), 5 deletions(-)

>

> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile

> index 91ef244026d2..56bcabca283f 100644

> --- a/arch/x86/purgatory/Makefile

> +++ b/arch/x86/purgatory/Makefile

> @@ -20,11 +20,13 @@ 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)


Is purgatory/kexec supported for CONFIG_X86_32?  Should I be keeping
`-m$(BITS)`?  arch/x86/purgatory/Makefile mentions
`setup-x86_$(BITS).o` which I assume is broken as there is no
arch/x86/purgatory/setup-x86_32.S?

> -KBUILD_CFLAGS += $(call cc-option,-fno-PIE)

> +# sure how to relocate those.

> +ifdef CONFIG_FUNCTION_TRACER

> +CFLAGS_REMOVE_sha256.o = -pg

> +CFLAGS_REMOVE_purgatory.o = -pg

> +CFLAGS_REMOVE_string.o = -pg

> +CFLAGS_REMOVE_kexec-purgatory.o = -pg

> +endif

>

>  $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE

>                 $(call if_changed,ld)

> --

> 2.22.0.657.g960e92d24f-goog

>



-- 
Thanks,
~Nick Desaulniers
Vaibhav Rustagi July 22, 2019, 10:59 p.m. | #2
On Mon, Jul 22, 2019 at 3:10 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>

> On Mon, Jul 22, 2019 at 2:33 PM Nick Desaulniers

> <ndesaulniers@google.com> 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 like arch/x86/xen/vdso/Makefile use

> > `CFLAGS_REMOVE_file.o = -pg` 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.

> >

> > Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")

> > Reported-by: Vaibhav Rustagi <vaibhavrustagi@google.com>

> > Suggested-by: Peter Zijlstra <peterz@infradead.org>

> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

> > ---

> > Rather than manually add -mno-sse, -mno-mmx, -mno-sse2, prefer to filter

> > -pg flags.

> >

> >  arch/x86/purgatory/Makefile | 12 +++++++-----

> >  1 file changed, 7 insertions(+), 5 deletions(-)

> >

> > diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile

> > index 91ef244026d2..56bcabca283f 100644

> > --- a/arch/x86/purgatory/Makefile

> > +++ b/arch/x86/purgatory/Makefile

> > @@ -20,11 +20,13 @@ 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)

>

> Is purgatory/kexec supported for CONFIG_X86_32?  Should I be keeping

> `-m$(BITS)`?  arch/x86/purgatory/Makefile mentions

> `setup-x86_$(BITS).o` which I assume is broken as there is no

> arch/x86/purgatory/setup-x86_32.S?

>

> > -KBUILD_CFLAGS += $(call cc-option,-fno-PIE)

> > +# sure how to relocate those.

> > +ifdef CONFIG_FUNCTION_TRACER

> > +CFLAGS_REMOVE_sha256.o = -pg

> > +CFLAGS_REMOVE_purgatory.o = -pg

> > +CFLAGS_REMOVE_string.o = -pg

> > +CFLAGS_REMOVE_kexec-purgatory.o = -pg

> > +endif

> >


The changes suggested will cause undefined symbols while loading the new kernel.
On doing 'nm purgatory.ro', I found below undefined symbols:

U bmcp
U __stack_chk_fail

> >  $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE

> >                 $(call if_changed,ld)

> > --

> > 2.22.0.657.g960e92d24f-goog

> >

>

>

> --

> Thanks,

> ~Nick Desaulniers
Nick Desaulniers July 23, 2019, 9:28 p.m. | #3
On Mon, Jul 22, 2019 at 3:59 PM Vaibhav Rustagi
<vaibhavrustagi@google.com> wrote:
> The changes suggested will cause undefined symbols while loading the new kernel.

> On doing 'nm purgatory.ro', I found below undefined symbols:

>

> U bmcp

> U __stack_chk_fail


Thanks for the report, a v3: https://lkml.org/lkml/2019/7/23/864
(Tested v3 with defconfig, defconfig+CONFIG_FUNCTION_TRACER,
+CONFIG_STACKPROTECTOR_STRONG, and allyesconfig)
-- 
Thanks,
~Nick Desaulniers

Patch

diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
index 91ef244026d2..56bcabca283f 100644
--- a/arch/x86/purgatory/Makefile
+++ b/arch/x86/purgatory/Makefile
@@ -20,11 +20,13 @@  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 = -pg
+CFLAGS_REMOVE_purgatory.o = -pg
+CFLAGS_REMOVE_string.o = -pg
+CFLAGS_REMOVE_kexec-purgatory.o = -pg
+endif
 
 $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
 		$(call if_changed,ld)