[PATCH v4 35/58] objtool/klp: Add test for recorded checksum values

[Song Liu <song@kernel.org>]

Whether klp diff treats a function as changed is decided by its checksum,
and until now nothing looked at one.  A test asserting only that the right
functions were cloned cannot tell a correct checksum from one that happens
to differ.

Asserts both directions -- the changed function's checksum moves, the
untouched one's does not -- and that checksumming identical input twice
gives the same answer, since otherwise every rebuild reports spurious
changes.

Assisted-by: Claude:claude-opus-4
Based-on-test-by: Joe Lawrence <joe.lawrence@redhat.com>
Assisted-by: Claude:claude-opus-5
Signed-off-by: Song Liu <song@kernel.org>
---
 .../tests/generic/test-checksum-value.sh      | 37 +++++++++++++++++++
 1 file changed, 37 insertions(+)
 create mode 100755 tools/objtool/tests/generic/test-checksum-value.sh

diff --git a/tools/objtool/tests/generic/test-checksum-value.sh b/tools/objtool/tests/generic/test-checksum-value.sh
new file mode 100755
index 000000000000..feae6a12e98d
--- /dev/null
+++ b/tools/objtool/tests/generic/test-checksum-value.sh
@@ -0,0 +1,37 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# The per-function checksums klp checksum records are what klp diff uses to
+# decide which functions changed.  A checksum covering too little misses a real
+# change and the patch silently omits the function; one covering too much, or
+# unstable across identical input, clones functions nobody patched and drags
+# their dependencies in with them.
+#
+# test-basic covers which functions got cloned, which is downstream of this and
+# passes for either kind of wrong checksum as long as the two errors do not
+# happen to cancel.  This checks the checksums themselves.
+
+. "$(dirname "$0")/../lib.sh"
+
+setup
+build_pair basic.c
+
+assert_input_symbol changed
+assert_input_symbol untouched
+
+run_checksum
+
+# The edited function's checksum has to move, the untouched one's must not.
+assert_checksum_differs changed
+assert_checksum_matches untouched
+
+# And it has to be a function of the code, not of the build: checksumming the
+# same input twice has to give the same answer, or every rebuild reports
+# spurious changes.
+first="$(checksum_of orig.o changed)"
+build_pair basic.c
+run_checksum
+[ "$(checksum_of orig.o changed)" = "$first" ] ||
+	fail "checksum for 'changed' differs between builds of identical source"
+
+pass "checksums track the changed function and are stable across rebuilds"
-- 
2.53.0-Meta