[PATCH] printf: mark errptr() noinline

Tamir Duberstein
2026-04-05T17:32:19Z

Old GCC can miscompile printf_kunit's errptr() test when branch
profiling is enabled. BUILD_BUG_ON(IS_ERR(PTR)) is a constant false
expression, but CONFIG_TRACE_BRANCH_PROFILING and
CONFIG_PROFILE_ALL_BRANCHES make the IS_ERR() path side-effectful.
GCC's IPA splitter can then outline the cold assert arm into
errptr.part.* and leave that clone with an unconditional
__compiletime_assert_*() call, causing a false build failure.

This started showing up after test_hashed() became a macro and moved its
local buffer into errptr(), which changed GCC's inlining and splitting
decisions enough to expose the compiler bug.

Mark errptr() noinline to keep it out of that buggy IPA path while
preserving the BUILD_BUG_ON(IS_ERR(PTR)) check and the macro-based
printf argument checking.

Fixes: 9bfa52dac27a ("printf: convert test_hashed into macro")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604030636.NqjaJvYp-lkp@intel.com/
Signed-off-by: Tamir Duberstein <tamird@kernel.org>
---
lib/tests/printf_kunit.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/lib/tests/printf_kunit.c b/lib/tests/printf_kunit.c
index f6f21b445ece..a8087e8ac826 100644
--- a/lib/tests/printf_kunit.c
+++ b/lib/tests/printf_kunit.c
@@ -749,7 +749,23 @@ static void fourcc_pointer(struct kunit *kunittest)
fourcc_pointer_test(kunittest, try_cb, ARRAY_SIZE(try_cb), "%p4cb");
}

-static void
+/*
+ * GCC < 12.1 can miscompile this test when branch profiling is enabled.
+ *
+ * BUILD_BUG_ON(IS_ERR(PTR)) is a constant false expression, but old GCC can
+ * still trip over it after CONFIG_TRACE_BRANCH_PROFILING and
+ * CONFIG_PROFILE_ALL_BRANCHES rewrite the IS_ERR() unlikely() path into
+ * side-effectful branch counter updates. IPA splitting then outlines the cold
+ * assert arm into errptr.part.* and leaves that clone with an unconditional
+ * __compiletime_assert_*() call, so the build fails even though PTR is not an
+ * ERR_PTR.
+ *
+ * Keep this test out of that buggy IPA path so the BUILD_BUG_ON() can stay in
+ * place without open-coding IS_ERR(). This can be removed once the minimum GCC
+ * includes commit 76fe49423047 ("Fix tree-optimization/101941: IPA splitting
+ * out function with error attribute"), which first shipped in GCC 12.1.
+ */
+static noinline void
errptr(struct kunit *kunittest)
{
test("-1234", "%pe", ERR_PTR(-1234));

---
base-commit: d8a9a4b11a137909e306e50346148fc5c3b63f9d
change-id: 20260405-printf-test-old-gcc-f13fecda6524

Best regards,
--
Tamir Duberstein <tamird@kernel.org>