[PATCH] powerpc/Makefile: Always pass --synthetic to nm if supported

From: Michael Ellerman
Date: Tue Aug 20 2019 - 03:23:36 EST

Back in 2004 we added logic to arch/ppc64/Makefile to pass
the --synthetic option to nm, if it was supported by nm.

Then in 2005 when arch/ppc64 and arch/ppc were merged, the logic to
add --synthetic was moved inside an #ifdef CONFIG_PPC64 block within
arch/powerpc/Makefile, and has remained there since.

That was fine, though crufty, until recently when a change to
init/Kconfig added a config time check that uses $(NM). On powerpc
that leads to an infinite loop because Kconfig uses $(NM) to calculate
some values, then the powerpc Makefile changes $(NM), which Kconfig
notices and restarts.

The original commit that added --synthetic simply said:
On new toolchains we need to use nm --synthetic or we miss code

And the nm man page says that the --synthetic option causes nm to:
Include synthetic symbols in the output. These are special symbols
created by the linker for various purposes.

So it seems safe to always pass --synthetic if nm supports it, ie. on
32-bit and 64-bit, it just means 32-bit kernels might have more
symbols reported (and in practice I see no extra symbols). Making it
unconditional avoids the #ifdef CONFIG_PPC64, which in turn avoids the
infinite loop.

Debugged-by: Peter Collingbourne <pcc@xxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
arch/powerpc/Makefile | 2 --
1 file changed, 2 deletions(-)

See the original discussion here: https://lore.kernel.org/lkml/CAMn1gO6P_VfDRjGZb67ZS4Kh0wjTEQi0cbOkmibTokHQOgP7qw@xxxxxxxxxxxxxx/

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index c345b79414a9..403f7e193833 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -39,13 +39,11 @@ endif
uname := $(shell uname -m)
KBUILD_DEFCONFIG := $(if $(filter ppc%,$(uname)),$(uname),ppc64)_defconfig

-ifdef CONFIG_PPC64
new_nm := $(shell if $(NM) --help 2>&1 | grep -- '--synthetic' > /dev/null; then echo y; else echo n; fi)

ifeq ($(new_nm),y)
NM := $(NM) --synthetic

# BITS is used as extension for files which are available in a 32 bit
# and a 64 bit version to simplify shared Makefiles.