[PATCH] kbuild: disable pahole multithreading for reproducible builds

[Thomas Weißschuh]

A BTF type_id is a numeric identifier allocated by pahole through
libbpfd. Ids are incremented for each allocation.
Running pahole multithreaded makes the sequence of allocations
non-deterministic which also makes the type_id itself non-deterministic.
As the type_id end up in the binary this breaks reproducibility.

Therefore, if the kernel is built reproducibly as indicated by
KBUILD_BUILD_TIMESTAMP, disable threading in pahole.

Fixes: b4f72786429c ("scripts/pahole-flags.sh: Parse DWARF and generate BTF with multithreading.")
Fixes: 72d091846de9 ("kbuild: avoid too many execution of scripts/pahole-flags.sh")
Signed-off-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
---
 scripts/Makefile.btf | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
index 82377e470aed..2356929e62e4 100644
--- a/scripts/Makefile.btf
+++ b/scripts/Makefile.btf
@@ -10,7 +10,10 @@ endif
 
 pahole-flags-$(call test-ge, $(pahole-ver), 121)	+= --btf_gen_floats
 
+# threaded execution randomizes BTF type_id allocation, breaking reproducibility
+ifeq ($(KBUILD_BUILD_TIMESTAMP),)
 pahole-flags-$(call test-ge, $(pahole-ver), 122)	+= -j
+endif
 
 pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)		+= --lang_exclude=rust
 

---
base-commit: 8e938e39866920ddc266898e6ae1fffc5c8f51aa
change-id: 20240322-pahole-reprodicible-02e904e45686

Best regards,
-- 
Thomas Weißschuh <linux@xxxxxxxxxxxxxx>