[PATCH] x86/boot: Fix if_changed build flip/flop

[PATCH] x86/boot: Fix if_changed build flip/flop

[Kees Cook]

The if_changed kbuild function can only be used once per target. If not
it will effectively always trigger, flipping back and forth between the
two commands getting recorded. Instead, merge the two commands into a
single function to get stable build artifacts (i.e. .vmlinux.cmd).

Reported-by: Dirk Gouders <dirk@gouders.net>
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Fixes: 98f78525371b ("x86/boot: Refuse to build with data relocations")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 arch/x86/boot/compressed/Makefile | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index fa42f895fdde..169c2feda14a 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -106,9 +106,13 @@ define cmd_check_data_rel
 	done
 endef
 
+# We need to run two commands under "if_changed", so merge them into a
+# single invocation.
+quiet_cmd_check-and-link-vmlinux = LD      $@
+      cmd_check-and-link-vmlinux = $(cmd_check_data_rel); $(cmd_ld)
+
 $(obj)/vmlinux: $(vmlinux-objs-y) FORCE
-	$(call if_changed,check_data_rel)
-	$(call if_changed,ld)
+	$(call if_changed,check-and-link-vmlinux)
 
 OBJCOPYFLAGS_vmlinux.bin :=  -R .comment -S
 $(obj)/vmlinux.bin: vmlinux FORCE
-- 
2.17.1


-- 
Kees Cook
Pixel Security

Re: [PATCH] x86/boot: Fix if_changed build flip/flop

[Ingo Molnar]

* Kees Cook <keescook@chromium.org> wrote:

> The if_changed kbuild function can only be used once per target. If not
> it will effectively always trigger, flipping back and forth between the
> two commands getting recorded. Instead, merge the two commands into a
> single function to get stable build artifacts (i.e. .vmlinux.cmd).
> 
> Reported-by: Dirk Gouders <dirk@gouders.net>

What actual symptoms does this bug have?

I.e. it would be nice if the changelog started with such an explanation:

  Dirk Gouders reported that .... <bad stuff happens during kernel build> ... 
  and bisected it back to:

    98f78525371b ("x86/boot: Refuse to build with data relocations")

  The root cause of the bug is that the if_changed kbuild function ...

Or something like that.

Thanks,

	Ingo

Re: [PATCH] x86/boot: Fix if_changed build flip/flop

[Masahiro Yamada]

2018-07-16 7:58 GMT+09:00 Ingo Molnar <mingo@kernel.org>:
>
> * Kees Cook <keescook@chromium.org> wrote:
>
>> The if_changed kbuild function can only be used once per target. If not
>> it will effectively always trigger, flipping back and forth between the
>> two commands getting recorded. Instead, merge the two commands into a
>> single function to get stable build artifacts (i.e. .vmlinux.cmd).
>>
>> Reported-by: Dirk Gouders <dirk@gouders.net>
>
> What actual symptoms does this bug have?
>
> I.e. it would be nice if the changelog started with such an explanation:
>
>   Dirk Gouders reported that .... <bad stuff happens during kernel build> ...
>   and bisected it back to:
>
>     98f78525371b ("x86/boot: Refuse to build with data relocations")
>
>   The root cause of the bug is that the if_changed kbuild function ...
>
> Or something like that.
>
> Thanks,
>
>         Ingo




Anyway, the code diff looks good to me.

Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>



-- 
Best Regards
Masahiro Yamada