[PATCH] printf: mark errptr() noinline

[PATCH] printf: mark errptr() noinline

[Tamir Duberstein]

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>

Re: [PATCH] printf: mark errptr() noinline

[Greg KH]

On Sun, Apr 05, 2026 at 01:31:50PM -0400, Tamir Duberstein wrote:
> 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>
> 
> 

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>

Re: [PATCH] printf: mark errptr() noinline

[Steven Rostedt]

On Sun, 05 Apr 2026 13:31:50 -0400
Tamir Duberstein <tamird@kernel.org> wrote:

> 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>

Another solution which I would be fine with is:

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index e130da35808f..c07e8eadfdd0 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -692,6 +692,7 @@ config PROFILE_ANNOTATED_BRANCHES
 config PROFILE_ALL_BRANCHES
 	bool "Profile all if conditionals" if !FORTIFY_SOURCE
 	select TRACE_BRANCH_PROFILING
+	depends on !KUNIT
 	help
 	  This tracer profiles all branch conditions. Every if ()
 	  taken in the kernel is recorded whether it hit or miss.


-- Steve

Re: [PATCH] printf: mark errptr() noinline

[Tamir Duberstein]

On Mon, Apr 6, 2026 at 11:14 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Sun, 05 Apr 2026 13:31:50 -0400
> Tamir Duberstein <tamird@kernel.org> wrote:
>
> > 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>
>
> Another solution which I would be fine with is:
>
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index e130da35808f..c07e8eadfdd0 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -692,6 +692,7 @@ config PROFILE_ANNOTATED_BRANCHES
>  config PROFILE_ALL_BRANCHES
>         bool "Profile all if conditionals" if !FORTIFY_SOURCE
>         select TRACE_BRANCH_PROFILING
> +       depends on !KUNIT
>         help
>           This tracer profiles all branch conditions. Every if ()
>           taken in the kernel is recorded whether it hit or miss.
>
>
> -- Steve

Thanks Steve. IMO that is a very big hammer and not warranted in this
case. There's been talk of encouraging distros to enable CONFIG_KUNIT
by default [0], which would probably interact poorly with the change
you propose.

Link: https://lore.kernel.org/all/CABVgOS=KZrM2dWyp1HzVS0zh7vquLxmTY2T2Ti53DQADrW+sJg@mail.gmail.com/
[0]

Re: [PATCH] printf: mark errptr() noinline

[Steven Rostedt]

On Mon, 6 Apr 2026 11:21:39 -0400
Tamir Duberstein <tamird@kernel.org> wrote:

> Thanks Steve. IMO that is a very big hammer and not warranted in this
> case. There's been talk of encouraging distros to enable CONFIG_KUNIT
> by default [0], which would probably interact poorly with the change
> you propose.
> 

Branch profiling is really just a niche that is enabled specifically for
seeing all branches taken in the kernel. It hooks to all "if" statements!
As you can imagine, it causes a rather large overhead in performance.

This option is only used by developers doing special analysis of their code
(namely me ;-).

The only real concern I would have is if the kunit test developers would
want to use the branch profiling on their code, in which case my suggestion
would prevent that.

-- Steve

Re: [PATCH] printf: mark errptr() noinline

[Tamir Duberstein]

On Sun, Apr 5, 2026 at 1:32 PM Tamir Duberstein <tamird@kernel.org> wrote:
>
> 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>

Adding tag per stable instructions:

Cc: stable@vger.kernel.org

Re: [PATCH] printf: mark errptr() noinline

[David Laight]

On Sun, 05 Apr 2026 13:31:50 -0400
Tamir Duberstein <tamird@kernel.org> wrote:

> 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.

Why not convert that check to a run-time one?
It isn't as though IS_ERR() is usually used with constants.

(There are a lot of other tests which would be better as compile-time
ones; since the run-time code just checks the compiler generated
the correct constant. So a compile failure saves you having to run
the tests; but that isn't true here.)

I'd also test -4095 and -4096...

> 
> 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

While that might make a difference, I'm not sure that it has to.
You aren't changing anything directly related to the failing expansion.

	David

>  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>
> 
> 

Re: [PATCH] printf: mark errptr() noinline

[Petr Mladek]

On Mon 2026-04-06 12:32:32, Steven Rostedt wrote:
> On Mon, 6 Apr 2026 11:21:39 -0400
> Tamir Duberstein <tamird@kernel.org> wrote:
> 
> > Thanks Steve. IMO that is a very big hammer and not warranted in this
> > case. There's been talk of encouraging distros to enable CONFIG_KUNIT
> > by default [0], which would probably interact poorly with the change
> > you propose.
> > 
> 
> Branch profiling is really just a niche that is enabled specifically for
> seeing all branches taken in the kernel. It hooks to all "if" statements!
> As you can imagine, it causes a rather large overhead in performance.
> 
> This option is only used by developers doing special analysis of their code
> (namely me ;-).
> 
> The only real concern I would have is if the kunit test developers would
> want to use the branch profiling on their code, in which case my suggestion
> would prevent that.

I wonder if it might be possible to disable the branch profiling just
for the printf_kunit.c as a compromise.

Would "#undef if" in printf_kunit.c help?

Or I see that DISABLE_BRANCH_PROFILING is an official
way to disable the feature.

I wonder if the following change would solve the problem.
I am sorry, I could not test it easily.

diff --git a/lib/tests/Makefile b/lib/tests/Makefile
index 05f74edbc62b..45d69769ccdf 100644
--- a/lib/tests/Makefile
+++ b/lib/tests/Makefile
@@ -41,6 +41,7 @@ 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
 obj-$(CONFIG_PRINTF_KUNIT_TEST) += printf_kunit.o
+CFLAGS_printf_kunit.o += -DDISABLE_BRANCH_PROFILING
 obj-$(CONFIG_RANDSTRUCT_KUNIT_TEST) += randstruct_kunit.o
 obj-$(CONFIG_SCANF_KUNIT_TEST) += scanf_kunit.o
 obj-$(CONFIG_SEQ_BUF_KUNIT_TEST) += seq_buf_kunit.o


Best Regards,
Petr

Re: [PATCH] printf: mark errptr() noinline

[Tamir Duberstein]

On Tue, Apr 7, 2026 at 7:27 AM Petr Mladek <pmladek@suse.com> wrote:
>
> On Mon 2026-04-06 12:32:32, Steven Rostedt wrote:
> > On Mon, 6 Apr 2026 11:21:39 -0400
> > Tamir Duberstein <tamird@kernel.org> wrote:
> >
> > > Thanks Steve. IMO that is a very big hammer and not warranted in this
> > > case. There's been talk of encouraging distros to enable CONFIG_KUNIT
> > > by default [0], which would probably interact poorly with the change
> > > you propose.
> > >
> >
> > Branch profiling is really just a niche that is enabled specifically for
> > seeing all branches taken in the kernel. It hooks to all "if" statements!
> > As you can imagine, it causes a rather large overhead in performance.
> >
> > This option is only used by developers doing special analysis of their code
> > (namely me ;-).
> >
> > The only real concern I would have is if the kunit test developers would
> > want to use the branch profiling on their code, in which case my suggestion
> > would prevent that.
>
> I wonder if it might be possible to disable the branch profiling just
> for the printf_kunit.c as a compromise.
>
> Would "#undef if" in printf_kunit.c help?
>
> Or I see that DISABLE_BRANCH_PROFILING is an official
> way to disable the feature.
>
> I wonder if the following change would solve the problem.
> I am sorry, I could not test it easily.

Yes, we can disable it for the whole file. I decided against that
because narrow workarounds are better than broad ones IMO, but it is
ultimately up to your preference.

FWIW I did test that this patch fixes the problem in GCC 8.5.0.