[PATCH v7 0/5] kunit: Add support for suppressing warning backtraces

[PATCH v7 0/5] kunit: Add support for suppressing warning backtraces

[Albert Esteve]

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

One option to address the problem would be to add messages such as
"expected warning backtraces start/end here" to the kernel log.
However, that would again require filter scripts, might result in
missing real problematic warning backtraces triggered while the test
is running, and the irrelevant backtrace(s) would still clog the
kernel log.

Solve the problem by providing a means to identify and suppress specific
warning backtraces while executing test code. Support suppressing multiple
backtraces while at the same time limiting changes to generic code to the
absolute minimum.

Overview:
Patch#1 Introduces the suppression infrastructure.
Patch#2 Mitigates the impact of suppression.
Patch#3 Adds selftests to validate the functionality.
Patch#4 Demonstrates real-world usage in the DRM subsystem.
Patch#5 Documents the new API and usage guidelines.

Design Notes:
The objective is to suppress unwanted WARN*() generated messages.

Although most major architectures share common bug handling via `lib/bug.c`
and `report_bug()`, some minor or legacy architectures still rely on their
own platform-specific handling. This divergence must be considered in any
such feature. Additionally, a key challenge in implementing this feature is
the fragmentation of `WARN*()` message emission: part of the output is
produced in the macro itself (via __warn_printk()), and part in the exception
handler.

Lessons from the Previous Attempt:
In earlier iterations, suppression logic was added inside the
`__report_bug()` function to intercept WARN*() output. To implement the
check in the bug handler code, two strategies were considered:

* Strategy #1: Use `kallsyms` to infer the originating function. This
  approach proved unreliable due to compiler-induced transformations
  such as inlining, cloning, and code fragmentation.

* Strategy #2: Store function name `__func__` in `struct bug_entry` in
  the `__bug_table`. However, `__func__` is a compiler-generated symbol,
  which complicates relocation and linking in position-independent code.
  Additionally, architectures not using the unified `BUG()` path would
  still require ad-hoc handling.

A per-macro solution was also attempted (v5-v6), injecting checks
directly into the `WARN*()` macros in `include/asm-generic/bug.h`.
While this offered full control, it required modifying the generic
bug header and was considered too invasive and damaging the critical
path, and thus incorrect [1].

Current Proposal: Check in `warn_slowpath_fmt()` and `__report_bug()`.
Suppression is checked at two points in the warning path:
- In `warn_slowpath_fmt()` (kernel/panic.c), for architectures without
  __WARN_FLAGS. The check runs before any output, fully suppressing
  both message and backtrace.
- In `__report_bug()` (lib/bug.c), for architectures that define
  __WARN_FLAGS. The check runs before `__warn()` is called, suppressing
  the backtrace and stack dump. On this path, the `WARN()` format message
  may still appear in the kernel log since `__warn_printk()` executes
  before the trap.
This approach avoids modifying include/asm-generic/bug.h entirely,
requires no architecture-specific code, and limits changes to generic
code to the absolute minimum.

A helper function, `__kunit_is_suppressed_warning()`, walks an RCU-
protected list of active suppressions, matching by current task. The
suppression state is dynamically allocated via kunit_kzalloc() and
tied to the KUnit test lifecycle via kunit_add_action(), ensuring
automatic cleanup at test exit.

To minimize runtime impact when no suppressions are active, an atomic
counter tracks the number of active suppressions.
`__kunit_is_suppressed_warning()` checks this counter first and returns
immediately when it is zero, avoiding the RCU-protected list traversal
in the common case.

This series is based on the RFC patch and subsequent discussion at
https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
and offers a more comprehensive solution of the problem discussed there.

[1] https://lore.kernel.org/all/CAGegRW76X8Fk_5qqOBw_aqBwAkQTsc8kXKHEuu9ECeXzdJwMSw@mail.gmail.com/

Changes since RFC:
- Introduced CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Minor cleanups and bug fixes
- Added support for all affected architectures
- Added support for counting suppressed warnings
- Added unit tests using those counters
- Added patch to suppress warning backtraces in dev_addr_lists tests

Changes since v1:
- Rebased to v6.9-rc1
- Added Tested-by:, Acked-by:, and Reviewed-by: tags
  [I retained those tags since there have been no functional changes]
- Introduced KUNIT_SUPPRESS_BACKTRACE configuration option, enabled by
  default.

Changes since v2:
- Rebased to v6.9-rc2
- Added comments to drm warning suppression explaining why it is needed.
- Added patch to move conditional code in arch/sh/include/asm/bug.h
  to avoid kerneldoc warning
- Added architecture maintainers to Cc: for architecture specific patches
- No functional changes

Changes since v3:
- Rebased to v6.14-rc6
- Dropped net: "kunit: Suppress lock warning noise at end of dev_addr_lists tests"
  since 3db3b62955cd6d73afde05a17d7e8e106695c3b9
- Added __kunit_ and KUNIT_ prefixes.
- Tested on interessed architectures.

Changes since v4:
- Rebased to v6.15-rc7
- Dropped all code in __report_bug()
- Moved all checks in WARN*() macros.
- Dropped all architecture specific code.
- Made __kunit_is_suppressed_warning nice to noinstr functions.

Changes since v5:
- Rebased to v7.0-rc3
- Added RCU protection for the suppressed warnings list.
- Added static key and branching optimization.
- Removed custom `strcmp` implementation and reworked
  __kunit_is_suppressed_warning() entrypoint function.

Changes since v6:
- Moved suppression checks from WARN*() macros to warn_slowpath_fmt()
  and __report_bug().
- Replaced stack-allocated suppression struct with kunit_kzalloc() heap
  allocation tied to the KUnit test lifecycle.
- Changed suppression strategy from function-name matching to task-scoped:
  all warnings on the current task are suppressed between START and END,
  rather than only warnings originating from a specific named function.
- Simplified macro API: removed KUNIT_DECLARE_SUPPRESSED_WARNING(),
  the START macro now takes (test) and handles allocation internally.
- Removed static key and branching optiomization, as by the time it
  was executed, callers are already in warn slowpaths.
- Link to v6: https://lore.kernel.org/r/20260317-kunit_add_support-v6-0-dd22aeb3fe5d@redhat.com

Alessandro Carminati (2):
  bug/kunit: Core support for suppressing warning backtraces
  bug/kunit: Suppressing warning backtraces reduced impact on WARN*()
    sites

Guenter Roeck (3):
  Add unit tests to verify that warning backtrace suppression works.
  drm: Suppress intentional warning backtraces in scaling unit tests
  kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst |  30 ++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   |  16 ++++
 include/asm-generic/bug.h               |  48 +++++++----
 include/kunit/bug.h                     |  62 ++++++++++++++
 include/kunit/test.h                    |   1 +
 lib/kunit/Kconfig                       |   9 ++
 lib/kunit/Makefile                      |   9 +-
 lib/kunit/backtrace-suppression-test.c  | 105 ++++++++++++++++++++++++
 lib/kunit/bug.c                         |  54 ++++++++++++
 9 files changed, 316 insertions(+), 18 deletions(-)
 create mode 100644 include/kunit/bug.h
 create mode 100644 lib/kunit/backtrace-suppression-test.c
 create mode 100644 lib/kunit/bug.c

--
2.34.1

---
Alessandro Carminati (2):
      bug/kunit: Core support for suppressing warning backtraces
      bug/kunit: Reduce runtime impact of warning backtrace suppression

Guenter Roeck (3):
      kunit: Add backtrace suppression self-tests
      drm: Suppress intentional warning backtraces in scaling unit tests
      kunit: Add documentation for warning backtrace suppression API

 Documentation/dev-tools/kunit/usage.rst | 30 ++++++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   | 14 +++++
 include/kunit/bug.h                     | 56 ++++++++++++++++++++
 include/kunit/test.h                    |  1 +
 kernel/panic.c                          |  8 ++-
 lib/bug.c                               |  8 +++
 lib/kunit/Kconfig                       |  9 ++++
 lib/kunit/Makefile                      |  9 +++-
 lib/kunit/backtrace-suppression-test.c  | 90 ++++++++++++++++++++++++++++++++
 lib/kunit/bug.c                         | 91 +++++++++++++++++++++++++++++++++
 10 files changed, 312 insertions(+), 4 deletions(-)
---
base-commit: 80234b5ab240f52fa45d201e899e207b9265ef91
change-id: 20260312-kunit_add_support-2f35806b19dd

Best regards,
-- 
Albert Esteve <aesteve@redhat.com>

[PATCH v7 1/5] bug/kunit: Core support for suppressing warning backtraces

[Albert Esteve]

From: Alessandro Carminati <acarmina@redhat.com>

Some unit tests intentionally trigger warning backtraces by passing bad
parameters to kernel API functions. Such unit tests typically check the
return value from such calls, not the existence of the warning backtrace.

Such intentionally generated warning backtraces are neither desirable
nor useful for a number of reasons:
- They can result in overlooked real problems.
- A warning that suddenly starts to show up in unit tests needs to be
  investigated and has to be marked to be ignored, for example by
  adjusting filter scripts. Such filters are ad hoc because there is
  no real standard format for warnings. On top of that, such filter
  scripts would require constant maintenance.

Solve the problem by providing a means to identify and suppress specific
warning backtraces while executing test code. Support suppressing multiple
backtraces while at the same time limiting changes to generic code to the
absolute minimum.

Implementation details:
Suppression is checked at two points in the warning path:
- In warn_slowpath_fmt(), the check runs before any output, fully
  suppressing both message and backtrace.
- In __report_bug(), the check runs before __warn() is called,
  suppressing the backtrace and stack dump. Note that on this path,
  the WARN() format message may still appear in the kernel log since
  __warn_printk() runs before the trap that enters __report_bug().

A helper function, `__kunit_is_suppressed_warning()`, walks an
RCU-protected list of active suppressions, matching by current task.
The suppression state is tied to the KUnit test lifecycle via
kunit_add_action(), ensuring automatic cleanup at test exit.

The list of suppressed warnings is protected with RCU to allow
concurrent read access without locks.

The implementation is deliberately simple and avoids architecture-specific
optimizations to preserve portability.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 include/kunit/bug.h  | 56 +++++++++++++++++++++++++++++++++++
 include/kunit/test.h |  1 +
 kernel/panic.c       |  8 ++++-
 lib/bug.c            |  8 +++++
 lib/kunit/Kconfig    |  9 ++++++
 lib/kunit/Makefile   |  6 ++--
 lib/kunit/bug.c      | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 7 files changed, 169 insertions(+), 3 deletions(-)

diff --git a/include/kunit/bug.h b/include/kunit/bug.h
new file mode 100644
index 0000000000000..e52c9d21d9fe6
--- /dev/null
+++ b/include/kunit/bug.h
@@ -0,0 +1,56 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * KUnit helpers for backtrace suppression
+ *
+ * Copyright (C) 2025 Alessandro Carminati <acarmina@redhat.com>
+ * Copyright (C) 2024 Guenter Roeck <linux@roeck-us.net>
+ */
+
+#ifndef _KUNIT_BUG_H
+#define _KUNIT_BUG_H
+
+#ifndef __ASSEMBLY__
+
+#include <linux/kconfig.h>
+
+struct kunit;
+
+#ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
+
+#include <linux/types.h>
+
+struct task_struct;
+
+struct __suppressed_warning {
+	struct list_head node;
+	struct task_struct *task;
+	int counter;
+};
+
+struct __suppressed_warning *
+__kunit_start_suppress_warning(struct kunit *test);
+void __kunit_end_suppress_warning(struct kunit *test,
+				  struct __suppressed_warning *warning);
+int __kunit_suppressed_warning_count(struct __suppressed_warning *warning);
+bool __kunit_is_suppressed_warning(void);
+
+#define KUNIT_START_SUPPRESSED_WARNING(test) \
+	struct __suppressed_warning *__kunit_suppress =	\
+		__kunit_start_suppress_warning(test)
+
+#define KUNIT_END_SUPPRESSED_WARNING(test) \
+	__kunit_end_suppress_warning(test, __kunit_suppress)
+
+#define KUNIT_SUPPRESSED_WARNING_COUNT() \
+	__kunit_suppressed_warning_count(__kunit_suppress)
+
+#else /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
+
+#define KUNIT_START_SUPPRESSED_WARNING(test)
+#define KUNIT_END_SUPPRESSED_WARNING(test)
+#define KUNIT_SUPPRESSED_WARNING_COUNT() 0
+static inline bool __kunit_is_suppressed_warning(void) { return false; }
+
+#endif /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
+#endif /* __ASSEMBLY__ */
+#endif /* _KUNIT_BUG_H */
diff --git a/include/kunit/test.h b/include/kunit/test.h
index 9cd1594ab697d..4ec07b3fa0204 100644
--- a/include/kunit/test.h
+++ b/include/kunit/test.h
@@ -10,6 +10,7 @@
 #define _KUNIT_TEST_H
 
 #include <kunit/assert.h>
+#include <kunit/bug.h>
 #include <kunit/try-catch.h>
 
 #include <linux/args.h>
diff --git a/kernel/panic.c b/kernel/panic.c
index c78600212b6c1..d7a7a679f56c4 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -39,6 +39,7 @@
 #include <linux/sys_info.h>
 #include <trace/events/error_report.h>
 #include <asm/sections.h>
+#include <kunit/bug.h>
 
 #define PANIC_TIMER_STEP 100
 #define PANIC_BLINK_SPD 18
@@ -1080,9 +1081,14 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 void warn_slowpath_fmt(const char *file, int line, unsigned taint,
 		       const char *fmt, ...)
 {
-	bool rcu = warn_rcu_enter();
+	bool rcu;
 	struct warn_args args;
 
+	if (__kunit_is_suppressed_warning())
+		return;
+
+	rcu = warn_rcu_enter();
+
 	pr_warn(CUT_HERE);
 
 	if (!fmt) {
diff --git a/lib/bug.c b/lib/bug.c
index 623c467a8b76c..606205c8c302f 100644
--- a/lib/bug.c
+++ b/lib/bug.c
@@ -48,6 +48,7 @@
 #include <linux/rculist.h>
 #include <linux/ftrace.h>
 #include <linux/context_tracking.h>
+#include <kunit/bug.h>
 
 extern struct bug_entry __start___bug_table[], __stop___bug_table[];
 
@@ -223,6 +224,13 @@ static enum bug_trap_type __report_bug(struct bug_entry *bug, unsigned long buga
 	no_cut   = bug->flags & BUGFLAG_NO_CUT_HERE;
 	has_args = bug->flags & BUGFLAG_ARGS;
 
+	/*
+	 * Before the once logic so suppressed warnings do not consume
+	 * the single-fire budget of WARN_ON_ONCE().
+	 */
+	if (warning && __kunit_is_suppressed_warning())
+		return BUG_TRAP_TYPE_WARN;
+
 	if (warning && once) {
 		if (done)
 			return BUG_TRAP_TYPE_WARN;
diff --git a/lib/kunit/Kconfig b/lib/kunit/Kconfig
index 498cc51e493dc..57527418fcf09 100644
--- a/lib/kunit/Kconfig
+++ b/lib/kunit/Kconfig
@@ -15,6 +15,15 @@ menuconfig KUNIT
 
 if KUNIT
 
+config KUNIT_SUPPRESS_BACKTRACE
+	bool "KUnit - Enable backtrace suppression"
+	default y
+	help
+	  Enable backtrace suppression for KUnit. If enabled, backtraces
+	  generated intentionally by KUnit tests are suppressed. Disable
+	  to reduce kernel image size if image size is more important than
+	  suppression of backtraces generated by KUnit tests.
+
 config KUNIT_DEBUGFS
 	bool "KUnit - Enable /sys/kernel/debug/kunit debugfs representation" if !KUNIT_ALL_TESTS
 	default KUNIT_ALL_TESTS
diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
index 656f1fa35abcc..fe177ff3ebdef 100644
--- a/lib/kunit/Makefile
+++ b/lib/kunit/Makefile
@@ -16,8 +16,10 @@ ifeq ($(CONFIG_KUNIT_DEBUGFS),y)
 kunit-objs +=				debugfs.o
 endif
 
-# KUnit 'hooks' are built-in even when KUnit is built as a module.
-obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o
+# KUnit 'hooks' and bug handling are built-in even when KUnit is built
+# as a module.
+obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o \
+					bug.o
 
 obj-$(CONFIG_KUNIT_TEST) +=		kunit-test.o
 obj-$(CONFIG_KUNIT_TEST) +=		platform-test.o
diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
new file mode 100644
index 0000000000000..356c8a5928828
--- /dev/null
+++ b/lib/kunit/bug.c
@@ -0,0 +1,84 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit helpers for backtrace suppression
+ *
+ * Copyright (C) 2025 Alessandro Carminati <acarmina@redhat.com>
+ * Copyright (C) 2024 Guenter Roeck <linux@roeck-us.net>
+ */
+
+#include <kunit/bug.h>
+#include <kunit/resource.h>
+#include <linux/export.h>
+#include <linux/rculist.h>
+#include <linux/sched.h>
+
+#ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
+
+static LIST_HEAD(suppressed_warnings);
+
+static void __kunit_suppress_warning_remove(struct __suppressed_warning *warning)
+{
+	list_del_rcu(&warning->node);
+	synchronize_rcu(); /* Wait for readers to finish */
+}
+
+KUNIT_DEFINE_ACTION_WRAPPER(__kunit_suppress_warning_cleanup,
+			    __kunit_suppress_warning_remove,
+			    struct __suppressed_warning *);
+
+struct __suppressed_warning *
+__kunit_start_suppress_warning(struct kunit *test)
+{
+	struct __suppressed_warning *warning;
+	int ret;
+
+	warning = kunit_kzalloc(test, sizeof(*warning), GFP_KERNEL);
+	if (!warning)
+		return NULL;
+
+	warning->task = current;
+	list_add_rcu(&warning->node, &suppressed_warnings);
+
+	ret = kunit_add_action_or_reset(test,
+					__kunit_suppress_warning_cleanup,
+					warning);
+	if (ret)
+		return NULL;
+
+	return warning;
+}
+EXPORT_SYMBOL_GPL(__kunit_start_suppress_warning);
+
+void __kunit_end_suppress_warning(struct kunit *test,
+				  struct __suppressed_warning *warning)
+{
+	if (!warning)
+		return;
+	kunit_release_action(test, __kunit_suppress_warning_cleanup, warning);
+}
+EXPORT_SYMBOL_GPL(__kunit_end_suppress_warning);
+
+int __kunit_suppressed_warning_count(struct __suppressed_warning *warning)
+{
+	return warning ? warning->counter : 0;
+}
+EXPORT_SYMBOL_GPL(__kunit_suppressed_warning_count);
+
+bool __kunit_is_suppressed_warning(void)
+{
+	struct __suppressed_warning *warning;
+
+	rcu_read_lock();
+	list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
+		if (warning->task == current) {
+			warning->counter++;
+			rcu_read_unlock();
+			return true;
+		}
+	}
+	rcu_read_unlock();
+
+	return false;
+}
+
+#endif /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */

-- 
2.52.0

[PATCH v7 2/5] bug/kunit: Reduce runtime impact of warning backtrace suppression

[Albert Esteve]

From: Alessandro Carminati <acarmina@redhat.com>

KUnit support is not consistently present across distributions, some
include it in their stock kernels, while others do not.
While both KUNIT and KUNIT_SUPPRESS_BACKTRACE can be considered debug
features, the fact that some distros ship with KUnit enabled means it's
important to minimize the runtime impact of this patch.

To that end, this patch adds an atomic counter that tracks the number
of active suppressions. __kunit_is_suppressed_warning() checks this
counter first and returns immediately when no suppressions are active,
avoiding RCU-protected list traversal in the common case.

Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 lib/kunit/bug.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
index 356c8a5928828..a7a88f0670d44 100644
--- a/lib/kunit/bug.c
+++ b/lib/kunit/bug.c
@@ -8,6 +8,7 @@
 
 #include <kunit/bug.h>
 #include <kunit/resource.h>
+#include <linux/atomic.h>
 #include <linux/export.h>
 #include <linux/rculist.h>
 #include <linux/sched.h>
@@ -15,11 +16,13 @@
 #ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
 
 static LIST_HEAD(suppressed_warnings);
+static atomic_t suppressed_warnings_cnt = ATOMIC_INIT(0);
 
 static void __kunit_suppress_warning_remove(struct __suppressed_warning *warning)
 {
 	list_del_rcu(&warning->node);
 	synchronize_rcu(); /* Wait for readers to finish */
+	atomic_dec(&suppressed_warnings_cnt);
 }
 
 KUNIT_DEFINE_ACTION_WRAPPER(__kunit_suppress_warning_cleanup,
@@ -37,6 +40,7 @@ __kunit_start_suppress_warning(struct kunit *test)
 		return NULL;
 
 	warning->task = current;
+	atomic_inc(&suppressed_warnings_cnt);
 	list_add_rcu(&warning->node, &suppressed_warnings);
 
 	ret = kunit_add_action_or_reset(test,
@@ -68,6 +72,9 @@ bool __kunit_is_suppressed_warning(void)
 {
 	struct __suppressed_warning *warning;
 
+	if (!atomic_read(&suppressed_warnings_cnt))
+		return false;
+
 	rcu_read_lock();
 	list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
 		if (warning->task == current) {

-- 
2.52.0

[PATCH v7 3/5] kunit: Add backtrace suppression self-tests

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Add unit tests to verify that warning backtrace suppression works,
covering WARN() and WARN_ON() with direct calls, indirect calls
through helper functions, and multiple warnings in a single window.

If backtrace suppression does _not_ work, the unit tests will likely
trigger unsuppressed backtraces, which should actually help to get
the affected architectures / platforms fixed.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 lib/kunit/Makefile                     |  3 ++
 lib/kunit/backtrace-suppression-test.c | 90 ++++++++++++++++++++++++++++++++++
 2 files changed, 93 insertions(+)

diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
index fe177ff3ebdef..b2f2b8ada7b71 100644
--- a/lib/kunit/Makefile
+++ b/lib/kunit/Makefile
@@ -23,6 +23,9 @@ obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o \
 
 obj-$(CONFIG_KUNIT_TEST) +=		kunit-test.o
 obj-$(CONFIG_KUNIT_TEST) +=		platform-test.o
+ifeq ($(CONFIG_KUNIT_SUPPRESS_BACKTRACE),y)
+obj-$(CONFIG_KUNIT_TEST) +=		backtrace-suppression-test.o
+endif
 
 # string-stream-test compiles built-in only.
 ifeq ($(CONFIG_KUNIT_TEST),y)
diff --git a/lib/kunit/backtrace-suppression-test.c b/lib/kunit/backtrace-suppression-test.c
new file mode 100644
index 0000000000000..2ba5dcb5fef35
--- /dev/null
+++ b/lib/kunit/backtrace-suppression-test.c
@@ -0,0 +1,90 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit test for suppressing warning tracebacks.
+ *
+ * Copyright (C) 2024, Guenter Roeck
+ * Author: Guenter Roeck <linux@roeck-us.net>
+ */
+
+#include <kunit/test.h>
+#include <linux/bug.h>
+
+static void backtrace_suppression_test_warn_direct(struct kunit *test)
+{
+	KUNIT_START_SUPPRESSED_WARNING(test);
+	WARN(1, "This backtrace should be suppressed");
+	KUNIT_END_SUPPRESSED_WARNING(test);
+
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), 1);
+}
+
+static void trigger_backtrace_warn(void)
+{
+	WARN(1, "This backtrace should be suppressed");
+}
+
+static void backtrace_suppression_test_warn_indirect(struct kunit *test)
+{
+	KUNIT_START_SUPPRESSED_WARNING(test);
+	trigger_backtrace_warn();
+	KUNIT_END_SUPPRESSED_WARNING(test);
+
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), 1);
+}
+
+static void backtrace_suppression_test_warn_multi(struct kunit *test)
+{
+	KUNIT_START_SUPPRESSED_WARNING(test);
+	WARN(1, "This backtrace should be suppressed");
+	trigger_backtrace_warn();
+	KUNIT_END_SUPPRESSED_WARNING(test);
+
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), 2);
+}
+
+static void backtrace_suppression_test_warn_on_direct(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_DEBUG_BUGVERBOSE) && !IS_ENABLED(CONFIG_KALLSYMS))
+		kunit_skip(test, "requires CONFIG_DEBUG_BUGVERBOSE or CONFIG_KALLSYMS");
+
+	KUNIT_START_SUPPRESSED_WARNING(test);
+	WARN_ON(1);
+	KUNIT_END_SUPPRESSED_WARNING(test);
+
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), 1);
+}
+
+static void trigger_backtrace_warn_on(void)
+{
+	WARN_ON(1);
+}
+
+static void backtrace_suppression_test_warn_on_indirect(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_DEBUG_BUGVERBOSE))
+		kunit_skip(test, "requires CONFIG_DEBUG_BUGVERBOSE");
+
+	KUNIT_START_SUPPRESSED_WARNING(test);
+	trigger_backtrace_warn_on();
+	KUNIT_END_SUPPRESSED_WARNING(test);
+
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), 1);
+}
+
+static struct kunit_case backtrace_suppression_test_cases[] = {
+	KUNIT_CASE(backtrace_suppression_test_warn_direct),
+	KUNIT_CASE(backtrace_suppression_test_warn_indirect),
+	KUNIT_CASE(backtrace_suppression_test_warn_multi),
+	KUNIT_CASE(backtrace_suppression_test_warn_on_direct),
+	KUNIT_CASE(backtrace_suppression_test_warn_on_indirect),
+	{}
+};
+
+static struct kunit_suite backtrace_suppression_test_suite = {
+	.name = "backtrace-suppression-test",
+	.test_cases = backtrace_suppression_test_cases,
+};
+kunit_test_suites(&backtrace_suppression_test_suite);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("KUnit test to verify warning backtrace suppression");

-- 
2.52.0

[PATCH v7 4/5] drm: Suppress intentional warning backtraces in scaling unit tests

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

The drm_test_rect_calc_hscale and drm_test_rect_calc_vscale unit tests
intentionally trigger warning backtraces by providing bad parameters to
the tested functions. What is tested is the return value, not the existence
of a warning backtrace. Suppress the backtraces to avoid clogging the
kernel log and distraction from real problems.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Maíra Canal <mcanal@igalia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 drivers/gpu/drm/tests/drm_rect_test.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/gpu/drm/tests/drm_rect_test.c b/drivers/gpu/drm/tests/drm_rect_test.c
index 17e1f34b76101..1dd7d819165e7 100644
--- a/drivers/gpu/drm/tests/drm_rect_test.c
+++ b/drivers/gpu/drm/tests/drm_rect_test.c
@@ -409,8 +409,15 @@ static void drm_test_rect_calc_hscale(struct kunit *test)
 	const struct drm_rect_scale_case *params = test->param_value;
 	int scaling_factor;
 
+	/*
+	 * drm_rect_calc_hscale() generates a warning backtrace whenever bad
+	 * parameters are passed to it. This affects all unit tests with an
+	 * error code in expected_scaling_factor.
+	 */
+	KUNIT_START_SUPPRESSED_WARNING(test);
 	scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
 					      params->min_range, params->max_range);
+	KUNIT_END_SUPPRESSED_WARNING(test);
 
 	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
 }
@@ -420,8 +427,15 @@ static void drm_test_rect_calc_vscale(struct kunit *test)
 	const struct drm_rect_scale_case *params = test->param_value;
 	int scaling_factor;
 
+	/*
+	 * drm_rect_calc_vscale() generates a warning backtrace whenever bad
+	 * parameters are passed to it. This affects all unit tests with an
+	 * error code in expected_scaling_factor.
+	 */
+	KUNIT_START_SUPPRESSED_WARNING(test);
 	scaling_factor = drm_rect_calc_vscale(&params->src, &params->dst,
 					      params->min_range, params->max_range);
+	KUNIT_END_SUPPRESSED_WARNING(test);
 
 	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
 }

-- 
2.52.0

[PATCH v7 5/5] kunit: Add documentation for warning backtrace suppression API

[Albert Esteve]

From: Guenter Roeck <linux@roeck-us.net>

Document API functions for suppressing warning backtraces.

Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 Documentation/dev-tools/kunit/usage.rst | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index ebd06f5ea4550..76e85412f240e 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -157,6 +157,34 @@ Alternatively, one can take full control over the error message by using
 	if (some_setup_function())
 		KUNIT_FAIL(test, "Failed to setup thing for testing");
 
+Suppressing warning backtraces
+------------------------------
+
+Some unit tests trigger warning backtraces either intentionally or as side
+effect. Such backtraces are normally undesirable since they distract from
+the actual test and may result in the impression that there is a problem.
+
+Such backtraces can be suppressed with **task scope suppression**: while
+``START`` / ``END`` is active on the current task, the backtrace and stack
+dump from warnings on that task are suppressed. Wrap the call from your test
+in that window, like shown in the following code.
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		KUNIT_START_SUPPRESSED_WARNING(test);
+		trigger_backtrace();
+		KUNIT_END_SUPPRESSED_WARNING(test);
+	}
+
+``KUNIT_SUPPRESSED_WARNING_COUNT()`` returns the number of suppressed backtraces.
+If the suppressed backtrace was triggered on purpose, this can be used to check
+if the backtrace was actually triggered.
+
+.. code-block:: c
+
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), 1);
 
 Test Suites
 ~~~~~~~~~~~
@@ -1211,4 +1239,4 @@ For example:
 		dev_managed_string = devm_kstrdup(fake_device, "Hello, World!");
 
 		// Everything is cleaned up automatically when the test ends.
-	}
\ No newline at end of file
+	}

-- 
2.52.0

Re: [PATCH v7 1/5] bug/kunit: Core support for suppressing warning backtraces

[Peter Zijlstra]

On Mon, Apr 20, 2026 at 02:28:03PM +0200, Albert Esteve wrote:
> From: Alessandro Carminati <acarmina@redhat.com>
> 
> Some unit tests intentionally trigger warning backtraces by passing bad
> parameters to kernel API functions. Such unit tests typically check the
> return value from such calls, not the existence of the warning backtrace.
> 
> Such intentionally generated warning backtraces are neither desirable
> nor useful for a number of reasons:
> - They can result in overlooked real problems.
> - A warning that suddenly starts to show up in unit tests needs to be
>   investigated and has to be marked to be ignored, for example by
>   adjusting filter scripts. Such filters are ad hoc because there is
>   no real standard format for warnings. On top of that, such filter
>   scripts would require constant maintenance.
> 
> Solve the problem by providing a means to identify and suppress specific
> warning backtraces while executing test code. Support suppressing multiple
> backtraces while at the same time limiting changes to generic code to the
> absolute minimum.
> 
> Implementation details:
> Suppression is checked at two points in the warning path:
> - In warn_slowpath_fmt(), the check runs before any output, fully
>   suppressing both message and backtrace.
> - In __report_bug(), the check runs before __warn() is called,
>   suppressing the backtrace and stack dump. Note that on this path,
>   the WARN() format message may still appear in the kernel log since
>   __warn_printk() runs before the trap that enters __report_bug().

This is for architectures that implement __WARN_FLAGS but not
__WARN_printf right? (which is arm64, loongarch, parisc, powerpc, riscv,
sh, afaict). ARM64 should eventually get __WARN_printf, but other than
that this should be fixable by moving __WARN_FLAGS() into
__warn_printk() or so. This is the only __warn_printk() user anyway.


> diff --git a/include/kunit/bug.h b/include/kunit/bug.h
> new file mode 100644
> index 0000000000000..e52c9d21d9fe6
> --- /dev/null
> +++ b/include/kunit/bug.h
> @@ -0,0 +1,56 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * KUnit helpers for backtrace suppression
> + *
> + * Copyright (C) 2025 Alessandro Carminati <acarmina@redhat.com>
> + * Copyright (C) 2024 Guenter Roeck <linux@roeck-us.net>
> + */
> +
> +#ifndef _KUNIT_BUG_H
> +#define _KUNIT_BUG_H
> +
> +#ifndef __ASSEMBLY__
> +
> +#include <linux/kconfig.h>
> +
> +struct kunit;
> +
> +#ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
> +
> +#include <linux/types.h>
> +
> +struct task_struct;
> +
> +struct __suppressed_warning {
> +	struct list_head node;
> +	struct task_struct *task;
> +	int counter;
> +};
> +
> +struct __suppressed_warning *
> +__kunit_start_suppress_warning(struct kunit *test);
> +void __kunit_end_suppress_warning(struct kunit *test,
> +				  struct __suppressed_warning *warning);
> +int __kunit_suppressed_warning_count(struct __suppressed_warning *warning);
> +bool __kunit_is_suppressed_warning(void);
> +
> +#define KUNIT_START_SUPPRESSED_WARNING(test) \
> +	struct __suppressed_warning *__kunit_suppress =	\
> +		__kunit_start_suppress_warning(test)
> +
> +#define KUNIT_END_SUPPRESSED_WARNING(test) \
> +	__kunit_end_suppress_warning(test, __kunit_suppress)

We have __cleanup for this?

> +
> +#define KUNIT_SUPPRESSED_WARNING_COUNT() \
> +	__kunit_suppressed_warning_count(__kunit_suppress)
> +
> +#else /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
> +
> +#define KUNIT_START_SUPPRESSED_WARNING(test)
> +#define KUNIT_END_SUPPRESSED_WARNING(test)
> +#define KUNIT_SUPPRESSED_WARNING_COUNT() 0
> +static inline bool __kunit_is_suppressed_warning(void) { return false; }
> +
> +#endif /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
> +#endif /* __ASSEMBLY__ */
> +#endif /* _KUNIT_BUG_H */

> diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
> index 656f1fa35abcc..fe177ff3ebdef 100644
> --- a/lib/kunit/Makefile
> +++ b/lib/kunit/Makefile
> @@ -16,8 +16,10 @@ ifeq ($(CONFIG_KUNIT_DEBUGFS),y)
>  kunit-objs +=				debugfs.o
>  endif
>  
> -# KUnit 'hooks' are built-in even when KUnit is built as a module.
> -obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o
> +# KUnit 'hooks' and bug handling are built-in even when KUnit is built
> +# as a module.
> +obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o \
> +					bug.o
>  
>  obj-$(CONFIG_KUNIT_TEST) +=		kunit-test.o
>  obj-$(CONFIG_KUNIT_TEST) +=		platform-test.o
> diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
> new file mode 100644
> index 0000000000000..356c8a5928828
> --- /dev/null
> +++ b/lib/kunit/bug.c
> @@ -0,0 +1,84 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * KUnit helpers for backtrace suppression
> + *
> + * Copyright (C) 2025 Alessandro Carminati <acarmina@redhat.com>
> + * Copyright (C) 2024 Guenter Roeck <linux@roeck-us.net>
> + */
> +
> +#include <kunit/bug.h>
> +#include <kunit/resource.h>
> +#include <linux/export.h>
> +#include <linux/rculist.h>
> +#include <linux/sched.h>
> +
> +#ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
> +
> +static LIST_HEAD(suppressed_warnings);
> +
> +static void __kunit_suppress_warning_remove(struct __suppressed_warning *warning)
> +{
> +	list_del_rcu(&warning->node);
> +	synchronize_rcu(); /* Wait for readers to finish */
> +}
> +
> +KUNIT_DEFINE_ACTION_WRAPPER(__kunit_suppress_warning_cleanup,
> +			    __kunit_suppress_warning_remove,
> +			    struct __suppressed_warning *);
> +
> +struct __suppressed_warning *
> +__kunit_start_suppress_warning(struct kunit *test)
> +{
> +	struct __suppressed_warning *warning;
> +	int ret;
> +
> +	warning = kunit_kzalloc(test, sizeof(*warning), GFP_KERNEL);
> +	if (!warning)
> +		return NULL;
> +
> +	warning->task = current;
> +	list_add_rcu(&warning->node, &suppressed_warnings);

What if anything serializes this global list?

> +
> +	ret = kunit_add_action_or_reset(test,
> +					__kunit_suppress_warning_cleanup,
> +					warning);
> +	if (ret)
> +		return NULL;
> +
> +	return warning;
> +}
> +EXPORT_SYMBOL_GPL(__kunit_start_suppress_warning);
> +
> +void __kunit_end_suppress_warning(struct kunit *test,
> +				  struct __suppressed_warning *warning)
> +{
> +	if (!warning)
> +		return;
> +	kunit_release_action(test, __kunit_suppress_warning_cleanup, warning);
> +}
> +EXPORT_SYMBOL_GPL(__kunit_end_suppress_warning);
> +
> +int __kunit_suppressed_warning_count(struct __suppressed_warning *warning)
> +{
> +	return warning ? warning->counter : 0;
> +}
> +EXPORT_SYMBOL_GPL(__kunit_suppressed_warning_count);
> +
> +bool __kunit_is_suppressed_warning(void)
> +{
> +	struct __suppressed_warning *warning;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
> +		if (warning->task == current) {
> +			warning->counter++;
> +			rcu_read_unlock();
> +			return true;
> +		}
> +	}
> +	rcu_read_unlock();
> +
> +	return false;
> +}
> +
> +#endif /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
> 
> -- 
> 2.52.0
> 

Re: [PATCH v7 2/5] bug/kunit: Reduce runtime impact of warning backtrace suppression

[Peter Zijlstra]

On Mon, Apr 20, 2026 at 02:28:04PM +0200, Albert Esteve wrote:
> From: Alessandro Carminati <acarmina@redhat.com>
> 
> KUnit support is not consistently present across distributions, some
> include it in their stock kernels, while others do not.
> While both KUNIT and KUNIT_SUPPRESS_BACKTRACE can be considered debug
> features, the fact that some distros ship with KUnit enabled means it's
> important to minimize the runtime impact of this patch.
> 
> To that end, this patch adds an atomic counter that tracks the number
> of active suppressions. __kunit_is_suppressed_warning() checks this
> counter first and returns immediately when no suppressions are active,
> avoiding RCU-protected list traversal in the common case.
> 
> Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
>  lib/kunit/bug.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
> index 356c8a5928828..a7a88f0670d44 100644
> --- a/lib/kunit/bug.c
> +++ b/lib/kunit/bug.c
> @@ -8,6 +8,7 @@
>  
>  #include <kunit/bug.h>
>  #include <kunit/resource.h>
> +#include <linux/atomic.h>
>  #include <linux/export.h>
>  #include <linux/rculist.h>
>  #include <linux/sched.h>
> @@ -15,11 +16,13 @@
>  #ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
>  
>  static LIST_HEAD(suppressed_warnings);
> +static atomic_t suppressed_warnings_cnt = ATOMIC_INIT(0);
>  
>  static void __kunit_suppress_warning_remove(struct __suppressed_warning *warning)
>  {
>  	list_del_rcu(&warning->node);
>  	synchronize_rcu(); /* Wait for readers to finish */
> +	atomic_dec(&suppressed_warnings_cnt);
>  }
>  
>  KUNIT_DEFINE_ACTION_WRAPPER(__kunit_suppress_warning_cleanup,
> @@ -37,6 +40,7 @@ __kunit_start_suppress_warning(struct kunit *test)
>  		return NULL;
>  
>  	warning->task = current;
> +	atomic_inc(&suppressed_warnings_cnt);
>  	list_add_rcu(&warning->node, &suppressed_warnings);
>  
>  	ret = kunit_add_action_or_reset(test,
> @@ -68,6 +72,9 @@ bool __kunit_is_suppressed_warning(void)
>  {
>  	struct __suppressed_warning *warning;
>  
> +	if (!atomic_read(&suppressed_warnings_cnt))
> +		return false;
> +
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
>  		if (warning->task == current) {
> 

So the thing you're skipping is:

  rcu_read_lock();
  list_for_each_entry_rcu() {
  }
  rcu_read_unlock();

Which is really cheap. Did you actually have performance numbers for
this?

A possibly better option is to add a static_branch() that could elide
any and all memory access.

Re: [PATCH v7 1/5] bug/kunit: Core support for suppressing warning backtraces

[Peter Zijlstra]

On Mon, Apr 20, 2026 at 02:28:03PM +0200, Albert Esteve wrote:
> +bool __kunit_is_suppressed_warning(void)
> +{
> +	struct __suppressed_warning *warning;
> +
> +	rcu_read_lock();

	guard(rcu)();

> +	list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
> +		if (warning->task == current) {
> +			warning->counter++;
> +			rcu_read_unlock();
> +			return true;
> +		}
> +	}
> +	rcu_read_unlock();
> +
> +	return false;
> +}
> +
> +#endif /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
> 
> -- 
> 2.52.0
> 

Re: [PATCH v7 4/5] drm: Suppress intentional warning backtraces in scaling unit tests

[Peter Zijlstra]

On Mon, Apr 20, 2026 at 02:28:06PM +0200, Albert Esteve wrote:
> From: Guenter Roeck <linux@roeck-us.net>
> 
> The drm_test_rect_calc_hscale and drm_test_rect_calc_vscale unit tests
> intentionally trigger warning backtraces by providing bad parameters to
> the tested functions. What is tested is the return value, not the existence
> of a warning backtrace. Suppress the backtraces to avoid clogging the
> kernel log and distraction from real problems.
> 
> Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
> Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
> Acked-by: Maíra Canal <mcanal@igalia.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
>  drivers/gpu/drm/tests/drm_rect_test.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/gpu/drm/tests/drm_rect_test.c b/drivers/gpu/drm/tests/drm_rect_test.c
> index 17e1f34b76101..1dd7d819165e7 100644
> --- a/drivers/gpu/drm/tests/drm_rect_test.c
> +++ b/drivers/gpu/drm/tests/drm_rect_test.c
> @@ -409,8 +409,15 @@ static void drm_test_rect_calc_hscale(struct kunit *test)
>  	const struct drm_rect_scale_case *params = test->param_value;
>  	int scaling_factor;
>  
> +	/*
> +	 * drm_rect_calc_hscale() generates a warning backtrace whenever bad
> +	 * parameters are passed to it. This affects all unit tests with an
> +	 * error code in expected_scaling_factor.
> +	 */
> +	KUNIT_START_SUPPRESSED_WARNING(test);
>  	scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
>  					      params->min_range, params->max_range);
> +	KUNIT_END_SUPPRESSED_WARNING(test);

Would not something like:

	scoped_kunit_suppress() {
		scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
						      params->min_range, params->max_range);
	}

be better?

Also, how can you stand all this screaming in the code?

Re: [PATCH v7 1/5] bug/kunit: Core support for suppressing warning backtraces

[Albert Esteve]

On Mon, Apr 20, 2026 at 4:40 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Apr 20, 2026 at 02:28:03PM +0200, Albert Esteve wrote:
> > From: Alessandro Carminati <acarmina@redhat.com>
> >
> > Some unit tests intentionally trigger warning backtraces by passing bad
> > parameters to kernel API functions. Such unit tests typically check the
> > return value from such calls, not the existence of the warning backtrace.
> >
> > Such intentionally generated warning backtraces are neither desirable
> > nor useful for a number of reasons:
> > - They can result in overlooked real problems.
> > - A warning that suddenly starts to show up in unit tests needs to be
> >   investigated and has to be marked to be ignored, for example by
> >   adjusting filter scripts. Such filters are ad hoc because there is
> >   no real standard format for warnings. On top of that, such filter
> >   scripts would require constant maintenance.
> >
> > Solve the problem by providing a means to identify and suppress specific
> > warning backtraces while executing test code. Support suppressing multiple
> > backtraces while at the same time limiting changes to generic code to the
> > absolute minimum.
> >
> > Implementation details:
> > Suppression is checked at two points in the warning path:
> > - In warn_slowpath_fmt(), the check runs before any output, fully
> >   suppressing both message and backtrace.
> > - In __report_bug(), the check runs before __warn() is called,
> >   suppressing the backtrace and stack dump. Note that on this path,
> >   the WARN() format message may still appear in the kernel log since
> >   __warn_printk() runs before the trap that enters __report_bug().
>
> This is for architectures that implement __WARN_FLAGS but not
> __WARN_printf right? (which is arm64, loongarch, parisc, powerpc, riscv,
> sh, afaict). ARM64 should eventually get __WARN_printf, but other than
> that this should be fixable by moving __WARN_FLAGS() into
> __warn_printk() or so. This is the only __warn_printk() user anyway.

Right. On that path, __warn_printk() prints the message before
__WARN_FLAGS() triggers the trap into __report_bug(), so the
suppression check in __report_bug() only catches the backtrace.

Adding the suppression check to __warn_printk() as well should close
this gap, since it's the only caller in the generic __WARN_printf
path. Something like:

```
diff --git a/kernel/panic.c b/kernel/panic.c
index d7a7a679f56c4..cd73038b7c0bd 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -1108,9 +1108,14 @@ EXPORT_SYMBOL(warn_slowpath_fmt);
#else
void __warn_printk(const char *fmt, ...)
{
- bool rcu = warn_rcu_enter();
+ bool rcu;
va_list args;
+ if (__kunit_is_suppressed_warning())
+ return;
+
+ rcu = warn_rcu_enter();
+
pr_warn(CUT_HERE);
va_start(args, fmt);
```
Note that the suppression counter tracking may need a small adjustment
to avoid double-counting, since __report_bug() will still see the same
warning through the trap path, but the approach itself should be
straightforward.

>
>
> > diff --git a/include/kunit/bug.h b/include/kunit/bug.h
> > new file mode 100644
> > index 0000000000000..e52c9d21d9fe6
> > --- /dev/null
> > +++ b/include/kunit/bug.h
> > @@ -0,0 +1,56 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * KUnit helpers for backtrace suppression
> > + *
> > + * Copyright (C) 2025 Alessandro Carminati <acarmina@redhat.com>
> > + * Copyright (C) 2024 Guenter Roeck <linux@roeck-us.net>
> > + */
> > +
> > +#ifndef _KUNIT_BUG_H
> > +#define _KUNIT_BUG_H
> > +
> > +#ifndef __ASSEMBLY__
> > +
> > +#include <linux/kconfig.h>
> > +
> > +struct kunit;
> > +
> > +#ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
> > +
> > +#include <linux/types.h>
> > +
> > +struct task_struct;
> > +
> > +struct __suppressed_warning {
> > +     struct list_head node;
> > +     struct task_struct *task;
> > +     int counter;
> > +};
> > +
> > +struct __suppressed_warning *
> > +__kunit_start_suppress_warning(struct kunit *test);
> > +void __kunit_end_suppress_warning(struct kunit *test,
> > +                               struct __suppressed_warning *warning);
> > +int __kunit_suppressed_warning_count(struct __suppressed_warning *warning);
> > +bool __kunit_is_suppressed_warning(void);
> > +
> > +#define KUNIT_START_SUPPRESSED_WARNING(test) \
> > +     struct __suppressed_warning *__kunit_suppress = \
> > +             __kunit_start_suppress_warning(test)
> > +
> > +#define KUNIT_END_SUPPRESSED_WARNING(test) \
> > +     __kunit_end_suppress_warning(test, __kunit_suppress)
>
> We have __cleanup for this?

I was unfamiliar with this attribute. I have looked into it a bit, and
it could be a good fit. I saw a good example in seqlock. I could try
to use it for the next version.

>
> > +
> > +#define KUNIT_SUPPRESSED_WARNING_COUNT() \
> > +     __kunit_suppressed_warning_count(__kunit_suppress)
> > +
> > +#else /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
> > +
> > +#define KUNIT_START_SUPPRESSED_WARNING(test)
> > +#define KUNIT_END_SUPPRESSED_WARNING(test)
> > +#define KUNIT_SUPPRESSED_WARNING_COUNT() 0
> > +static inline bool __kunit_is_suppressed_warning(void) { return false; }
> > +
> > +#endif /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
> > +#endif /* __ASSEMBLY__ */
> > +#endif /* _KUNIT_BUG_H */
>
> > diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
> > index 656f1fa35abcc..fe177ff3ebdef 100644
> > --- a/lib/kunit/Makefile
> > +++ b/lib/kunit/Makefile
> > @@ -16,8 +16,10 @@ ifeq ($(CONFIG_KUNIT_DEBUGFS),y)
> >  kunit-objs +=                                debugfs.o
> >  endif
> >
> > -# KUnit 'hooks' are built-in even when KUnit is built as a module.
> > -obj-$(if $(CONFIG_KUNIT),y) +=               hooks.o
> > +# KUnit 'hooks' and bug handling are built-in even when KUnit is built
> > +# as a module.
> > +obj-$(if $(CONFIG_KUNIT),y) +=               hooks.o \
> > +                                     bug.o
> >
> >  obj-$(CONFIG_KUNIT_TEST) +=          kunit-test.o
> >  obj-$(CONFIG_KUNIT_TEST) +=          platform-test.o
> > diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
> > new file mode 100644
> > index 0000000000000..356c8a5928828
> > --- /dev/null
> > +++ b/lib/kunit/bug.c
> > @@ -0,0 +1,84 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * KUnit helpers for backtrace suppression
> > + *
> > + * Copyright (C) 2025 Alessandro Carminati <acarmina@redhat.com>
> > + * Copyright (C) 2024 Guenter Roeck <linux@roeck-us.net>
> > + */
> > +
> > +#include <kunit/bug.h>
> > +#include <kunit/resource.h>
> > +#include <linux/export.h>
> > +#include <linux/rculist.h>
> > +#include <linux/sched.h>
> > +
> > +#ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
> > +
> > +static LIST_HEAD(suppressed_warnings);
> > +
> > +static void __kunit_suppress_warning_remove(struct __suppressed_warning *warning)
> > +{
> > +     list_del_rcu(&warning->node);
> > +     synchronize_rcu(); /* Wait for readers to finish */
> > +}
> > +
> > +KUNIT_DEFINE_ACTION_WRAPPER(__kunit_suppress_warning_cleanup,
> > +                         __kunit_suppress_warning_remove,
> > +                         struct __suppressed_warning *);
> > +
> > +struct __suppressed_warning *
> > +__kunit_start_suppress_warning(struct kunit *test)
> > +{
> > +     struct __suppressed_warning *warning;
> > +     int ret;
> > +
> > +     warning = kunit_kzalloc(test, sizeof(*warning), GFP_KERNEL);
> > +     if (!warning)
> > +             return NULL;
> > +
> > +     warning->task = current;
> > +     list_add_rcu(&warning->node, &suppressed_warnings);
>
> What if anything serializes this global list?

I considered adding a spinlock, but since the KUnit executor runs
tests sequentially and it had not been a concern in previous versions,
I left it out to keep the changeset minimal unless someone complains.
If we want a hard guarantee, I'm happy to add the spinlock for the
next version.

>
> > +
> > +     ret = kunit_add_action_or_reset(test,
> > +                                     __kunit_suppress_warning_cleanup,
> > +                                     warning);
> > +     if (ret)
> > +             return NULL;
> > +
> > +     return warning;
> > +}
> > +EXPORT_SYMBOL_GPL(__kunit_start_suppress_warning);
> > +
> > +void __kunit_end_suppress_warning(struct kunit *test,
> > +                               struct __suppressed_warning *warning)
> > +{
> > +     if (!warning)
> > +             return;
> > +     kunit_release_action(test, __kunit_suppress_warning_cleanup, warning);
> > +}
> > +EXPORT_SYMBOL_GPL(__kunit_end_suppress_warning);
> > +
> > +int __kunit_suppressed_warning_count(struct __suppressed_warning *warning)
> > +{
> > +     return warning ? warning->counter : 0;
> > +}
> > +EXPORT_SYMBOL_GPL(__kunit_suppressed_warning_count);
> > +
> > +bool __kunit_is_suppressed_warning(void)
> > +{
> > +     struct __suppressed_warning *warning;
> > +
> > +     rcu_read_lock();
> > +     list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
> > +             if (warning->task == current) {
> > +                     warning->counter++;
> > +                     rcu_read_unlock();
> > +                     return true;
> > +             }
> > +     }
> > +     rcu_read_unlock();
> > +
> > +     return false;
> > +}
> > +
> > +#endif /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
> >
> > --
> > 2.52.0
> >
>

Re: [PATCH v7 1/5] bug/kunit: Core support for suppressing warning backtraces

[Albert Esteve]

On Mon, Apr 20, 2026 at 4:45 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Apr 20, 2026 at 02:28:03PM +0200, Albert Esteve wrote:
> > +bool __kunit_is_suppressed_warning(void)
> > +{
> > +     struct __suppressed_warning *warning;
> > +
> > +     rcu_read_lock();
>
>         guard(rcu)();

Nice. I saw plenty of examples using explicit calls, but I had missed
the guard. Thanks for pointing it out.

>
> > +     list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
> > +             if (warning->task == current) {
> > +                     warning->counter++;
> > +                     rcu_read_unlock();
> > +                     return true;
> > +             }
> > +     }
> > +     rcu_read_unlock();
> > +
> > +     return false;
> > +}
> > +
> > +#endif /* CONFIG_KUNIT_SUPPRESS_BACKTRACE */
> >
> > --
> > 2.52.0
> >
>

Re: [PATCH v7 2/5] bug/kunit: Reduce runtime impact of warning backtrace suppression

[Albert Esteve]

On Mon, Apr 20, 2026 at 4:45 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Apr 20, 2026 at 02:28:04PM +0200, Albert Esteve wrote:
> > From: Alessandro Carminati <acarmina@redhat.com>
> >
> > KUnit support is not consistently present across distributions, some
> > include it in their stock kernels, while others do not.
> > While both KUNIT and KUNIT_SUPPRESS_BACKTRACE can be considered debug
> > features, the fact that some distros ship with KUnit enabled means it's
> > important to minimize the runtime impact of this patch.
> >
> > To that end, this patch adds an atomic counter that tracks the number
> > of active suppressions. __kunit_is_suppressed_warning() checks this
> > counter first and returns immediately when no suppressions are active,
> > avoiding RCU-protected list traversal in the common case.
> >
> > Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
> > Signed-off-by: Albert Esteve <aesteve@redhat.com>
> > ---
> >  lib/kunit/bug.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
> > index 356c8a5928828..a7a88f0670d44 100644
> > --- a/lib/kunit/bug.c
> > +++ b/lib/kunit/bug.c
> > @@ -8,6 +8,7 @@
> >
> >  #include <kunit/bug.h>
> >  #include <kunit/resource.h>
> > +#include <linux/atomic.h>
> >  #include <linux/export.h>
> >  #include <linux/rculist.h>
> >  #include <linux/sched.h>
> > @@ -15,11 +16,13 @@
> >  #ifdef CONFIG_KUNIT_SUPPRESS_BACKTRACE
> >
> >  static LIST_HEAD(suppressed_warnings);
> > +static atomic_t suppressed_warnings_cnt = ATOMIC_INIT(0);
> >
> >  static void __kunit_suppress_warning_remove(struct __suppressed_warning *warning)
> >  {
> >       list_del_rcu(&warning->node);
> >       synchronize_rcu(); /* Wait for readers to finish */
> > +     atomic_dec(&suppressed_warnings_cnt);
> >  }
> >
> >  KUNIT_DEFINE_ACTION_WRAPPER(__kunit_suppress_warning_cleanup,
> > @@ -37,6 +40,7 @@ __kunit_start_suppress_warning(struct kunit *test)
> >               return NULL;
> >
> >       warning->task = current;
> > +     atomic_inc(&suppressed_warnings_cnt);
> >       list_add_rcu(&warning->node, &suppressed_warnings);
> >
> >       ret = kunit_add_action_or_reset(test,
> > @@ -68,6 +72,9 @@ bool __kunit_is_suppressed_warning(void)
> >  {
> >       struct __suppressed_warning *warning;
> >
> > +     if (!atomic_read(&suppressed_warnings_cnt))
> > +             return false;
> > +
> >       rcu_read_lock();
> >       list_for_each_entry_rcu(warning, &suppressed_warnings, node) {
> >               if (warning->task == current) {
> >
>
> So the thing you're skipping is:
>
>   rcu_read_lock();
>   list_for_each_entry_rcu() {
>   }
>   rcu_read_unlock();
>
> Which is really cheap. Did you actually have performance numbers for
> this?

No, I do not have performance numbers. I kept the counter and the
separate patch for consistency with the previous version of the
series. But you have a good point, the skipped part is really cheap.

>
> A possibly better option is to add a static_branch() that could elide
> any and all memory access.
>

Previous version had static_branch and I removed it because I
understood from the discussion that the gains would not be significant
as performance gains are irrelevant in warn slowpath. But I think it
would make sense for a disabled feature. I will rework this for the
next version, remove the counter and use static_branch as suggested.

Re: [PATCH v7 4/5] drm: Suppress intentional warning backtraces in scaling unit tests

[Albert Esteve]

On Mon, Apr 20, 2026 at 4:47 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Apr 20, 2026 at 02:28:06PM +0200, Albert Esteve wrote:
> > From: Guenter Roeck <linux@roeck-us.net>
> >
> > The drm_test_rect_calc_hscale and drm_test_rect_calc_vscale unit tests
> > intentionally trigger warning backtraces by providing bad parameters to
> > the tested functions. What is tested is the return value, not the existence
> > of a warning backtrace. Suppress the backtraces to avoid clogging the
> > kernel log and distraction from real problems.
> >
> > Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
> > Acked-by: Dan Carpenter <dan.carpenter@linaro.org>
> > Acked-by: Maíra Canal <mcanal@igalia.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: David Airlie <airlied@gmail.com>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> > Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
> > Signed-off-by: Albert Esteve <aesteve@redhat.com>
> > ---
> >  drivers/gpu/drm/tests/drm_rect_test.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/tests/drm_rect_test.c b/drivers/gpu/drm/tests/drm_rect_test.c
> > index 17e1f34b76101..1dd7d819165e7 100644
> > --- a/drivers/gpu/drm/tests/drm_rect_test.c
> > +++ b/drivers/gpu/drm/tests/drm_rect_test.c
> > @@ -409,8 +409,15 @@ static void drm_test_rect_calc_hscale(struct kunit *test)
> >       const struct drm_rect_scale_case *params = test->param_value;
> >       int scaling_factor;
> >
> > +     /*
> > +      * drm_rect_calc_hscale() generates a warning backtrace whenever bad
> > +      * parameters are passed to it. This affects all unit tests with an
> > +      * error code in expected_scaling_factor.
> > +      */
> > +     KUNIT_START_SUPPRESSED_WARNING(test);
> >       scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
> >                                             params->min_range, params->max_range);
> > +     KUNIT_END_SUPPRESSED_WARNING(test);
>
> Would not something like:
>
>         scoped_kunit_suppress() {
>                 scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
>                                                       params->min_range, params->max_range);
>         }
>
> be better?

Since KUnit already has a few macros in its API it didn't occur to me.
Good idea, I like it. And I guess the scope approach matches well with
your __cleanup comment in the first patch. If no one opposes, I will
work toward that pattern for the next version.

>
> Also, how can you stand all this screaming in the code?
>

Again, KUnit already contains many macros, so this use didn't register
as such. Now I will not be able to unsee it.