objtool/klp: Add test for cold function halves

The compiler splits unlikely code into a separate foo.cold symbol.  Both
halves are the same function and both belong in the livepatch: carrying
only the hot part leaves the cold path branching into unpatched code.

GCC needs -freorder-blocks-and-partition to split reliably.  The flag is
probed rather than assumed, and the test skips when the compiler declines
to split at all.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Assisted-by: Claude:claude-opus-5
Link: https://patch.msgid.link/20260916184351.2720310-17-song@kernel.org

2 files changed, 60 insertions, 0 deletions

diff --git a/tools/objtool/tests/generic/fixtures/cold_function.c b/tools/objtool/tests/generic/fixtures/cold_function.c
new file mode 100644
index 000000000000..f6d410983ce2
--- /dev/null
+++ b/tools/objtool/tests/generic/fixtures/cold_function.c
@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Function the compiler may split into a hot part and a foo.cold part. */
+
+static const char __modinfo[]
+	__attribute__((section(".modinfo"), used, aligned(1))) = "\0name=vmlinux";
+
+static void __attribute__((cold, noinline)) slow_path(int x)
+{
+	__asm__ volatile("" :: "r"(x));
+}
+
+int target(int x)
+{
+	if (__builtin_expect(x < 0, 0))
+		slow_path(x);
+#ifdef PATCHED
+	return x + 2;
+#else
+	return x + 1;
+#endif
+}
diff --git a/tools/objtool/tests/generic/test-cold-function.sh b/tools/objtool/tests/generic/test-cold-function.sh
new file mode 100755
index 000000000000..a02652fe5341
--- /dev/null
+++ b/tools/objtool/tests/generic/test-cold-function.sh
@@ -0,0 +1,39 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Both halves of a split function belong to the patch; carrying only the hot
+# part leaves the cold path branching into unpatched code.
+
+. "$(dirname "$0")/../lib.sh"
+
+# Clang does not split functions into a cold part at all, so there is nothing
+# for this test to look at there.  A given gcc may or may not split, which is a
+# version property rather than a compiler choice -- that stays a probe below.
+gcc_only "clang does not split functions into a cold part"
+
+setup
+
+split_flag=-freorder-blocks-and-partition
+cc_supports "$split_flag" || split_flag=
+
+build_pair cold_function.c $split_flag
+
+# Find what the compiler called the cold half -- target.cold, target.cold.0,
+# depending on version -- and name it exactly from here on.
+cold_sym="$(in_symbols orig.o | awk '$NF ~ /^target\.cold/ { print $NF; exit }')"
+[ -n "$cold_sym" ] ||
+	probe_skip "compiler did not split the function into a cold part"
+
+run_diff
+
+assert_patched target
+# Match the name field exactly, and require it to be defined.  Had the cold
+# half been left behind, the branch to it would appear as an undefined
+# .klp.sym.vmlinux.target.cold,0 -- a different name, which happens to contain
+# this one.  Asking about a column instead of the name would not tell them
+# apart: readelf prints SHN_LIVEPATCH as "OS [0xff20]" and llvm-readelf as
+# "OS[0xff20]", so the fields either side of the name shift between the two.
+out_symbols | awk -v n="$cold_sym" '$NF == n && $(NF - 1) != "UND"' | grep -q . ||
+	fail "cold half ($cold_sym) was not carried into the patch"
+
+pass "cold half carried into the patch with its parent"