On Tue, 2026-08-04 at 22:04 +0000, Alejandro Hernandez Samaniego via lists.openembedded.org wrote:

RUST_BUILD_SYS is currently listed in BB_BASEHASH_IGNORE_VARS, which
tells bitbake that rust recipes should hash-match regardless of the
build machine's triplet. In practice that is not true:

rustc computes each crate's Strict Version Hash (SVH) using inputs
that include the *stage0/stage1 bootstrap compiler* fingerprint, which
in turn depends on the build host arch. That seed cascades through
every dependent crate's mangled symbols and the packing order of
rodata sections, so an sstate blob populated on one worker arch and
reused on a differently-arched worker produces byte-different (but
semantically identical) artifacts and fails the reproducibility
selftest causing autobuilder intermittent issues when the sstate
matches for the incorrect architecture.

Remove RUST_BUILD_SYS from BB_BASEHASH_IGNORE_VARS so the task hash
tracks the build triplet and mixed-arch autobuilder pools get an
sstate miss instead of a silently-wrong hit. RUST_HOST_SYS and
RUST_TARGET_SYS stay excluded because they're already covered by the
target/host arch hash inputs.

While this is not ideal, it should unblock the reproducible test case,
another solution would be to patch rust sources manually and attempt
to upstream that change.

This also has the side-effect that multiple variants of the conflicting
rust recipes sstate artifacts are created, but they'll be correctly used
now in each architecture.

Also add an explanatory comment next to the RUST_*_SYS[vardepvalue]
declarations in rust-common.bbclass so future readers don't re-add
the exclusion.

Verified locally: Tier 1 sighash test toggling BUILD_ARCH now produces
different task hashes for rust/libstd-rs/rpm-sequoia/python3-crypto-
graphy/cargo/librsvg, where previously the hashes matched despite the
build machine change.

[YOCTO #15554]

Assisted-by: AI - OpenAI
Signed-off-by: Alejandro Hernandez <alhe@linux.microsoft.com>
---
 meta/classes-recipe/rust-common.bbclass | 10 ++++++++++
 meta/conf/bitbake.conf                  |  2 +-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/meta/classes-recipe/rust-common.bbclass b/meta/classes-recipe/rust-common.bbclass
index 6bc42016d1..98b46c4e3c 100644
--- a/meta/classes-recipe/rust-common.bbclass
+++ b/meta/classes-recipe/rust-common.bbclass
@@ -114,6 +114,16 @@ RUST_HOST_SYS[vardepvalue] = "${RUST_HOST_SYS}"
 RUST_TARGET_SYS = "${@rust_base_triple(d, 'TARGET')}"
 RUST_TARGET_SYS[vardepvalue] = "${RUST_TARGET_SYS}"
 
+# Note: RUST_BUILD_SYS is intentionally NOT on BB_BASEHASH_IGNORE_VARS
+# (see meta/conf/bitbake.conf). rustc's crate SVH (Strict Version Hash)
+# is seeded by the stage0/stage1 bootstrap compiler whose fingerprint
+# depends on the BUILD host arch; that seed cascades through every
+# dependent crate's mangled symbols and rodata packing order. Excluding
+# RUST_BUILD_SYS from task hashes let a mixed-arch autobuilder pool
+# populate sstate on one worker arch and get a cache hit on a differently-
+# arched worker, producing byte-different (but semantically identical)
+# artifacts and failing reproducibility tests. See Yocto bug #15554.
+
 # wrappers to get around the fact that Rust needs a single
 # binary but Yocto's compiler and linker commands have
 # arguments. Technically the archiver is always one command but
diff --git a/meta/conf/bitbake.conf b/meta/conf/bitbake.conf
index bdf37d0da2..ee23459506 100644
--- a/meta/conf/bitbake.conf
+++ b/meta/conf/bitbake.conf
@@ -969,7 +969,7 @@ BB_HASHEXCLUDE_COMMON ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH BBSERVER DL_DI
     SSTATE_HASHEQUIV_OWNER CCACHE_TOP_DIR BB_HASHSERVE GIT_CEILING_DIRECTORIES \
     OMP_NUM_THREADS BB_CURRENTTASK"
 BB_BASEHASH_IGNORE_VARS ?= "${BB_HASHEXCLUDE_COMMON} PSEUDO_INCLUDE_PATHS BUILDHISTORY_DIR \
-    SSTATE_DIR SOURCE_DATE_EPOCH RUST_BUILD_SYS RUST_HOST_SYS RUST_TARGET_SYS"
+    SSTATE_DIR SOURCE_DATE_EPOCH RUST_HOST_SYS RUST_TARGET_SYS"
 BB_HASHCONFIG_IGNORE_VARS ?= "${BB_HASHEXCLUDE_COMMON} DATE TIME SSH_AGENT_PID \
     SSH_AUTH_SOCK PSEUDO_BUILD BB_ENV_PASSTHROUGH_ADDITIONS DISABLE_SANITY_CHECKS \
     PARALLEL_MAKE BB_NUMBER_THREADS BB_ORIGENV BB_INVALIDCONF BBINCLUDED \

Just to be clear, the sstatetests fail for this change for good reason,
it breaks the way "native" works in our system. We work on the
principle that "native" things work the same way regardless of
architecture and that the sstate hashes remain the same.

If you change this as above all of the rust targets will rebuild
depending on the build architecture, despite the fact they're meant to
be build architecture independent. It might manage to fool the test but
the output would still not be build architecture independent and hence
doesn't fix the real issue, just hides it from the tests.

Cheers,

Richard

Yes, that makes sense, the good thing is I think the root cause is correct, but if we instead patch rusts sources
we'll end up with the same problem since the packages would also be arch independent, correct?

Is the correct thing here to add packages to an ignore/allow list?


Alejandro

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