[RFC PATCH] kbuild: Make --build-id linker flag configurable

[Naman Jain <namjain@linux.microsoft.com>]

Build ID hashes include file paths, so building the same source from
different directories produces different binaries. This breaks
reproducible builds.

Add KBUILD_BUILD_ID variable (default: sha1) to allow overriding:

    make KBUILD_BUILD_ID=none

The variable is exported to VDSO Makefiles which also include a
fallback default for standalone invocation.

Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
---
Hi,
Sending this change for RFC, as it is quite possible that this is a
generic problem and I may be missing something.

I am trying to implement reproducible builds for one of my product
kernel. I referred https://reproducible-builds.org/docs/build-path/
and tried to use both -fdebug-prefix-map=OLD=NEW and
-fmacro-prefix-map=OLD=NEW, but still could not achieve bit by bit
binary reproducibility without overwriting build-id to none. 
If I move the kernel to same path in other setup, I was able to create
same binary hash, however, without it, there is some difference in
build-id hash values.

Reproducibility wiki says "In most cases however, post-processing is
required to either remove the build path or to normalize it to a
predefined value.". I have tried that, and it works, but wanted to
conclude if that is my last option here.

Thanks.

---
 Makefile                          | 8 ++++++--
 arch/arm64/kernel/vdso/Makefile   | 5 ++++-
 arch/arm64/kernel/vdso32/Makefile | 5 ++++-
 arch/x86/entry/vdso/Makefile      | 5 ++++-
 4 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/Makefile b/Makefile
index 3373308d2217c..3fcff4af200d7 100644
--- a/Makefile
+++ b/Makefile
@@ -1132,8 +1132,12 @@ KBUILD_AFLAGS   += $(KAFLAGS)
 KBUILD_CFLAGS   += $(KCFLAGS)
 KBUILD_RUSTFLAGS += $(KRUSTFLAGS)
 
-KBUILD_LDFLAGS_MODULE += --build-id=sha1
-LDFLAGS_vmlinux += --build-id=sha1
+# Can be overridden for reproducible builds by using "make KBUILD_BUILD_ID=none"
+KBUILD_BUILD_ID ?= sha1
+export KBUILD_BUILD_ID
+
+KBUILD_LDFLAGS_MODULE += --build-id=$(KBUILD_BUILD_ID)
+LDFLAGS_vmlinux += --build-id=$(KBUILD_BUILD_ID)
 
 KBUILD_LDFLAGS	+= -z noexecstack
 ifeq ($(CONFIG_LD_IS_BFD),y)
diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index 7dec05dd33b70..b3ee5982b4676 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -9,6 +9,9 @@
 # Include the generic Makefile to check the built vdso.
 include $(srctree)/lib/vdso/Makefile.include
 
+# Fallback for standalone builds, normally inherited from top-level Makefile
+KBUILD_BUILD_ID ?= sha1
+
 obj-vdso := vgettimeofday.o note.o sigreturn.o vgetrandom.o vgetrandom-chacha.o
 
 # Build rules
@@ -22,7 +25,7 @@ btildflags-$(CONFIG_ARM64_BTI_KERNEL) += -z force-bti
 # routines, as x86 does (see 6f121e548f83 ("x86, vdso: Reimplement vdso.so
 # preparation in build-time C")).
 ldflags-y := -shared -soname=linux-vdso.so.1 \
-	     -Bsymbolic --build-id=sha1 -n $(btildflags-y)
+	     -Bsymbolic --build-id=$(KBUILD_BUILD_ID) -n $(btildflags-y)
 
 ifdef CONFIG_LD_ORPHAN_WARN
   ldflags-y += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
index 9d0efed91414c..991aed67e65a2 100644
--- a/arch/arm64/kernel/vdso32/Makefile
+++ b/arch/arm64/kernel/vdso32/Makefile
@@ -5,6 +5,9 @@
 
 include $(srctree)/lib/vdso/Makefile.include
 
+# Fallback for standalone builds, normally inherited from top-level Makefile
+KBUILD_BUILD_ID ?= sha1
+
 # Same as cc-*option, but using CC_COMPAT instead of CC
 ifeq ($(CONFIG_CC_IS_CLANG), y)
 CC_COMPAT ?= $(CC)
@@ -87,7 +90,7 @@ VDSO_AFLAGS += -D__ASSEMBLY__
 # From arm vDSO Makefile
 VDSO_LDFLAGS += -Bsymbolic --no-undefined -soname=linux-vdso.so.1
 VDSO_LDFLAGS += -z max-page-size=4096 -z common-page-size=4096
-VDSO_LDFLAGS += -shared --build-id=sha1
+VDSO_LDFLAGS += -shared --build-id=$(KBUILD_BUILD_ID)
 VDSO_LDFLAGS += --orphan-handling=$(CONFIG_LD_ORPHAN_WARN_LEVEL)
 
 
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index f247f5f5cb44d..fefddc91b41ea 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -6,6 +6,9 @@
 # Include the generic Makefile to check the built vDSO:
 include $(srctree)/lib/vdso/Makefile.include
 
+# Fallback for standalone builds, normally inherited from top-level Makefile
+KBUILD_BUILD_ID ?= sha1
+
 # Files to link into the vDSO:
 vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o vgetrandom.o vgetrandom-chacha.o
 vobjs32-y := vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
@@ -155,7 +158,7 @@ quiet_cmd_vdso = VDSO    $@
 		       $(VDSO_LDFLAGS) $(VDSO_LDFLAGS_$(filter %.lds,$(^F))) \
 		       -T $(filter %.lds,$^) $(filter %.o,$^)
 
-VDSO_LDFLAGS = -shared --hash-style=both --build-id=sha1 --no-undefined \
+VDSO_LDFLAGS = -shared --hash-style=both --build-id=$(KBUILD_BUILD_ID) --no-undefined \
 	$(call ld-option, --eh-frame-hdr) -Bsymbolic -z noexecstack
 
 quiet_cmd_vdso_and_check = VDSO    $@

base-commit: 63804fed149a6750ffd28610c5c1c98cce6bd377
-- 
2.43.0