[PATCH v2] printf: Compile the kunit test with DISABLE_BRANCH_PROFILING

[Petr Mladek]

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.

Workaround the problem by disabling the branch profiling for
printf_kunit.o. It is a straightforward and acceptable solution.

Fixes: 9bfa52dac27a ("printf: convert test_hashed into macro")
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202604030636.NqjaJvYp-lkp@xxxxxxxxx/
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
Changes against v1:

  + Disable the branch profiling for the whole printf_kunit.o
    instead of using "noinline".

 lib/tests/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/tests/Makefile b/lib/tests/Makefile
index 05f74edbc62b..fbb2aad26994 100644
--- a/lib/tests/Makefile
+++ b/lib/tests/Makefile
@@ -40,6 +40,7 @@ obj-$(CONFIG_MEMCPY_KUNIT_TEST) += memcpy_kunit.o
 obj-$(CONFIG_MIN_HEAP_KUNIT_TEST) += min_heap_kunit.o
 CFLAGS_overflow_kunit.o = $(call cc-disable-warning, tautological-constant-out-of-range-compare)
 obj-$(CONFIG_OVERFLOW_KUNIT_TEST) += overflow_kunit.o
+CFLAGS_printf_kunit.o += -DDISABLE_BRANCH_PROFILING
 obj-$(CONFIG_PRINTF_KUNIT_TEST) += printf_kunit.o
 obj-$(CONFIG_RANDSTRUCT_KUNIT_TEST) += randstruct_kunit.o
 obj-$(CONFIG_SCANF_KUNIT_TEST) += scanf_kunit.o
-- 
2.53.0