[PATCH v8 0/4] kunit: Add support for suppressing warning backtraces

[PATCH v8 0/4] 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 suppress warning backtraces
originating from the current kthread while executing test code.
Since each KUnit test runs in its own kthread, this effectively scopes
suppression to the test that enabled it, without requiring any
architecture-specific code.

Overview:
Patch#1 Introduces the suppression infrastructure integrated into
        KUnit's hook mechanism.
Patch#2 Adds selftests to validate the functionality.
Patch#3 Demonstrates real-world usage in the DRM subsystem.
Patch#4 Documents the new API and usage guidelines.

Design Notes:
Suppression is integrated into the existing KUnit hooks infrastructure,
reusing the kunit_running static branch for zero overhead
when no tests are running. The implementation lives entirely in the
kunit module; only a static-inline wrapper and a function pointer
slot are added to built-in code.

Suppression is checked at three 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 `__warn_printk()` (kernel/panic.c), for architectures that define
  __WARN_FLAGS but not their own __WARN_printf (arm64, loongarch,
  parisc, powerpc, riscv, sh). The check suppresses the warning message
  text that is printed before the trap enters __report_bug().
- 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. 

To avoid double-counting on architectures where both `__warn_printk()`
and `__report_bug()` run for the same warning, the hook takes a bool
parameter: true to increment the suppression counter, false to suppress
without counting.

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. Writer-side access to the global
suppression list is serialized with a spinlock; readers use RCU.

Three API forms are provided:
- kunit_warning_suppress(test) { ... }: scoped blocks with automatic
  cleanup. The suppression handle is not accessible outside the block,
  so warning counts (if needed) must be checked inside. Multiple
  suppression blocks are allowed.
- KUNIT_START/END_SUPPRESSED_WARNING(test): manual macros for larger
  blocks or post-suppression count checks. Limited to one pair per
  scope.
- kunit_start/end_suppress_warning(test): direct functions that return
  an explicit handle. Use when the handle needs to be retained, or passed
  across helpers. Multiple suppression blocks are allowed.

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.

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

Changes since v7:
- Integrated suppression into existing KUnit hooks infrastructure
- Removed CONFIG_KUNIT_SUPPRESS_BACKTRACE
- Added suppression check in __warn_printk()
- Added spinlock for writer-side RCU protection
- Replaced explicit rcu_read_lock/unlock with guard(rcu)()
- Added scoped API (kunit_warning_suppress) using __cleanup attribute
- Updated DRM patch to use scoped API
- Expanded self-tests: incremental counting, cross-kthread isolation
- Rewrote documentation covering all three API forms with examples
- Link to v7: https://lore.kernel.org/r/20260420-kunit_add_support-v7-0-e8bc6e0f70de@redhat.com

--
2.34.1

---
Alessandro Carminati (1):
      bug/kunit: Core support for suppressing warning backtraces

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 |  63 ++++++++++-
 drivers/gpu/drm/tests/drm_rect_test.c   |  23 +++-
 include/kunit/test-bug.h                |  25 +++++
 include/kunit/test.h                    | 138 ++++++++++++++++++++++++
 kernel/panic.c                          |  15 ++-
 lib/bug.c                               |  10 ++
 lib/kunit/Makefile                      |   4 +-
 lib/kunit/backtrace-suppression-test.c  | 184 ++++++++++++++++++++++++++++++++
 lib/kunit/bug.c                         | 115 ++++++++++++++++++++
 lib/kunit/hooks-impl.h                  |   2 +
 10 files changed, 571 insertions(+), 8 deletions(-)
---
base-commit: 80234b5ab240f52fa45d201e899e207b9265ef91
change-id: 20260312-kunit_add_support-2f35806b19dd

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

[PATCH v8 1/4] 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 suppress warning backtraces
originating from the current kthread while executing test code. Since
each KUnit test runs in its own kthread, this effectively scopes
suppression to the test that enabled it. Limit changes to generic code
to the absolute minimum.

Implementation details:
Suppression is integrated into the existing KUnit hooks infrastructure
in test-bug.h, reusing the kunit_running static branch for zero
overhead when no tests are running.

Suppression is checked at three points in the warning path:
- In warn_slowpath_fmt(), the check runs before any output, fully
  suppressing both message and backtrace. This covers architectures
  without __WARN_FLAGS.
- In __warn_printk(), the check suppresses the warning message text.
  This covers architectures that define __WARN_FLAGS but not their own
  __WARN_printf (arm64, loongarch, parisc, powerpc, riscv, sh), where
  the message is printed before the trap enters __report_bug().
- In __report_bug(), the check runs before __warn() is called,
  suppressing the backtrace and stack dump.

To avoid double-counting on architectures where both __warn_printk()
and __report_bug() run for the same warning, kunit_is_suppressed_warning()
takes a bool parameter: true to increment the suppression counter
(used in warn_slowpath_fmt and __report_bug), false to check only
(used in __warn_printk).

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. Writer-side access to the global
suppression list is serialized with a spinlock; readers use RCU.

Three API forms are provided:
- kunit_warning_suppress(test) { ... }: scoped, uses __cleanup for
  automatic teardown on scope exit, kunit_add_action() as safety net
  for abnormal exits (e.g. kthread_exit from failed assertions).
  Suppression handle is only accessible inside the block.
- KUNIT_START/END_SUPPRESSED_WARNING(test): manual macros for larger
  blocks or when warning counts need to be checked after suppression
  ends. Limited to one pair per scope.
- kunit_start/end_suppress_warning(test): direct functions returning
  an explicit handle, for retaining the handle within the test,
  or for cross-function usage.

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/test-bug.h |  25 +++++++++
 include/kunit/test.h     | 138 +++++++++++++++++++++++++++++++++++++++++++++++
 kernel/panic.c           |  15 +++++-
 lib/bug.c                |  10 ++++
 lib/kunit/Makefile       |   3 +-
 lib/kunit/bug.c          | 115 +++++++++++++++++++++++++++++++++++++++
 lib/kunit/hooks-impl.h   |   2 +
 7 files changed, 305 insertions(+), 3 deletions(-)

diff --git a/include/kunit/test-bug.h b/include/kunit/test-bug.h
index 47aa8f21ccce8..6237e48ceadfd 100644
--- a/include/kunit/test-bug.h
+++ b/include/kunit/test-bug.h
@@ -23,6 +23,7 @@ DECLARE_STATIC_KEY_FALSE(kunit_running);
 extern struct kunit_hooks_table {
 	__printf(3, 4) void (*fail_current_test)(const char*, int, const char*, ...);
 	void *(*get_static_stub_address)(struct kunit *test, void *real_fn_addr);
+	bool (*is_suppressed_warning)(bool count);
 } kunit_hooks;
 
 /**
@@ -60,9 +61,33 @@ static inline struct kunit *kunit_get_current_test(void)
 		}								\
 	} while (0)
 
+/**
+ * kunit_is_suppressed_warning() - Check if warnings are being suppressed
+ *                                 by the current KUnit test.
+ * @count: if true, increment the suppression counter on match.
+ *
+ * Returns true if the current task has active warning suppression.
+ * Uses the kunit_running static branch for zero overhead when no tests run.
+ *
+ * A single WARN*() may traverse multiple call sites in the warning path
+ * (e.g., __warn_printk() and __report_bug()). Pass @count = true at the
+ * primary suppression point to count each warning exactly once, and
+ * @count = false at secondary points to suppress output without
+ * inflating the count.
+ */
+static inline bool kunit_is_suppressed_warning(bool count)
+{
+	if (!static_branch_unlikely(&kunit_running))
+		return false;
+
+	return kunit_hooks.is_suppressed_warning &&
+	       kunit_hooks.is_suppressed_warning(count);
+}
+
 #else
 
 static inline struct kunit *kunit_get_current_test(void) { return NULL; }
+static inline bool kunit_is_suppressed_warning(bool count) { return false; }
 
 #define kunit_fail_current_test(fmt, ...) do {} while (0)
 
diff --git a/include/kunit/test.h b/include/kunit/test.h
index 9cd1594ab697d..f278ec028019c 100644
--- a/include/kunit/test.h
+++ b/include/kunit/test.h
@@ -1795,4 +1795,142 @@ do {									       \
 // include resource.h themselves if they need it.
 #include <kunit/resource.h>
 
+/*
+ * Warning backtrace suppression API.
+ *
+ * Suppresses WARN*() backtraces on the current task while active. Three forms
+ * are provided, in order of convenience:
+ *
+ * - Scoped: kunit_warning_suppress(test) { ... }
+ *   Suppression is active for the duration of the block. On normal exit,
+ *   the for-loop increment deactivates suppression. On early exit (break,
+ *   return, goto), the __cleanup attribute fires. On kthread_exit() (e.g.,
+ *   a failed KUnit assertion), kunit_add_action() cleans up at test
+ *   teardown. The suppression handle is only accessible inside the block,
+ *   so warning counts must be checked before the block exits.
+ *
+ * - Manual macros: KUNIT_[START|END]_SUPPRESSED_WARNING(test)
+ *   Suppression spans an explicit range in the same scope. kunit_add_action()
+ *   guarantees cleanup even if KUNIT_END_SUPPRESSED_WARNING() is not reached.
+ *   Prefer this form when suppressing warnings across a large block where
+ *   extra indentation is undesirable, or when the warning count needs to be
+ *   checked after suppression ends. Limited to one pair per scope.
+ *
+ * - Direct: kunit_start_suppress_warning() / kunit_end_suppress_warning()
+ *   The underlying functions, returning an explicit handle pointer. Use
+ *   when the handle needs to be retained (e.g., for post-suppression
+ *   count checks) or passed across helper functions.
+ */
+struct kunit_suppressed_warning;
+
+struct kunit_suppressed_warning *
+kunit_start_suppress_warning(struct kunit *test);
+void kunit_end_suppress_warning(struct kunit *test,
+				struct kunit_suppressed_warning *w);
+int kunit_suppressed_warning_count(struct kunit_suppressed_warning *w);
+void __kunit_suppress_auto_cleanup(struct kunit_suppressed_warning **wp);
+bool kunit_has_active_suppress_warning(void);
+
+/**
+ * kunit_warning_suppress() - Suppress WARN*() backtraces for the duration
+ *                            of a block.
+ * @test: The test context object.
+ *
+ * Scoped form of the suppression API. Suppression starts when the block is
+ * entered and ends automatically when the block exits through any path. See
+ * the section comment above for the cleanup guarantees on each exit path.
+ * Fails the test if suppression is already active; nesting is not supported.
+ *
+ * The warning count can be checked inside the block via
+ * KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(). The handle is not accessible
+ * after the block exits.
+ *
+ * Example::
+ *
+ *   kunit_warning_suppress(test) {
+ *       trigger_warning();
+ *       KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+ *   }
+ */
+#define kunit_warning_suppress(test)					\
+	for (struct kunit_suppressed_warning *__kunit_suppress		\
+	     __cleanup(__kunit_suppress_auto_cleanup) =			\
+	     kunit_start_suppress_warning(test);			\
+	     __kunit_suppress;						\
+	     kunit_end_suppress_warning(test, __kunit_suppress),	\
+	     __kunit_suppress = NULL)
+
+/**
+ * KUNIT_START_SUPPRESSED_WARNING() - Begin suppressing WARN*() backtraces.
+ * @test: The test context object.
+ *
+ * Manual form of the suppression API. Must be paired with
+ * KUNIT_END_SUPPRESSED_WARNING() in the same scope. See the section comment
+ * above for cleanup guarantees. Fails the test if suppression is already
+ * active; nesting is not supported. Limited to one pair per scope; use
+ * sequential kunit_warning_suppress() blocks or the direct function API
+ * when more than one suppression region is needed.
+ *
+ * Example::
+ *
+ *   KUNIT_START_SUPPRESSED_WARNING(test);
+ *   trigger_code_that_should_warn_once();
+ *   KUNIT_END_SUPPRESSED_WARNING(test);
+ *   KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+ */
+#define KUNIT_START_SUPPRESSED_WARNING(test)				\
+	struct kunit_suppressed_warning *__kunit_suppress =		\
+		kunit_start_suppress_warning(test)
+
+/**
+ * KUNIT_END_SUPPRESSED_WARNING() - End suppressing WARN*() backtraces.
+ * @test: The test context object.
+ *
+ * Deactivates suppression started by KUNIT_START_SUPPRESSED_WARNING().
+ * The warning count remains readable via KUNIT_SUPPRESSED_WARNING_COUNT()
+ * after this call.
+ */
+#define KUNIT_END_SUPPRESSED_WARNING(test)				\
+	kunit_end_suppress_warning(test, __kunit_suppress)
+
+/**
+ * KUNIT_SUPPRESSED_WARNING_COUNT() - Returns the suppressed warning count.
+ *
+ * Returns the number of WARN*() calls suppressed since the current
+ * suppression block started, or 0 if the handle is NULL. Usable inside a
+ * kunit_warning_suppress() block or after KUNIT_END_SUPPRESSED_WARNING().
+ */
+#define KUNIT_SUPPRESSED_WARNING_COUNT() \
+	kunit_suppressed_warning_count(__kunit_suppress)
+
+/**
+ * KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT() - Sets an expectation that the
+ *                                           suppressed warning count equals
+ *                                           @expected.
+ * @test: The test context object.
+ * @expected: an expression that evaluates to the expected warning count.
+ *
+ * Sets an expectation that the number of suppressed WARN*() calls equals
+ * @expected. This is semantically equivalent to
+ * KUNIT_EXPECT_EQ(@test, KUNIT_SUPPRESSED_WARNING_COUNT(), @expected).
+ * See KUNIT_EXPECT_EQ() for more information.
+ */
+#define KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, expected) \
+	KUNIT_EXPECT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), expected)
+
+/**
+ * KUNIT_ASSERT_SUPPRESSED_WARNING_COUNT() - Sets an assertion that the
+ *                                           suppressed warning count equals
+ *                                           @expected.
+ * @test: The test context object.
+ * @expected: an expression that evaluates to the expected warning count.
+ *
+ * Sets an assertion that the number of suppressed WARN*() calls equals
+ * @expected. This is the same as KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(),
+ * except it causes an assertion failure (see KUNIT_ASSERT_TRUE()) when the
+ * assertion is not met.
+ */
+#define KUNIT_ASSERT_SUPPRESSED_WARNING_COUNT(test, expected) \
+	KUNIT_ASSERT_EQ(test, KUNIT_SUPPRESSED_WARNING_COUNT(), expected)
+
 #endif /* _KUNIT_TEST_H */
diff --git a/kernel/panic.c b/kernel/panic.c
index c78600212b6c1..697d8ca054bef 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/test-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(true))
+		return;
+
+	rcu = warn_rcu_enter();
+
 	pr_warn(CUT_HERE);
 
 	if (!fmt) {
@@ -1102,9 +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(false))
+		return;
+
+	rcu = warn_rcu_enter();
+
 	pr_warn(CUT_HERE);
 
 	va_start(args, fmt);
diff --git a/lib/bug.c b/lib/bug.c
index 623c467a8b76c..a5cebde554ed8 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/test-bug.h>
 
 extern struct bug_entry __start___bug_table[], __stop___bug_table[];
 
@@ -223,6 +224,15 @@ 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;
 
+#ifdef CONFIG_KUNIT
+	/*
+	 * Before the once logic so suppressed warnings do not consume
+	 * the single-fire budget of WARN_ON_ONCE().
+	 */
+	if (warning && kunit_is_suppressed_warning(true))
+		return BUG_TRAP_TYPE_WARN;
+#endif
+
 	if (warning && once) {
 		if (done)
 			return BUG_TRAP_TYPE_WARN;
diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
index 656f1fa35abcc..4592f9d0aa8dd 100644
--- a/lib/kunit/Makefile
+++ b/lib/kunit/Makefile
@@ -10,7 +10,8 @@ kunit-objs +=				test.o \
 					executor.o \
 					attributes.o \
 					device.o \
-					platform.o
+					platform.o \
+					bug.o
 
 ifeq ($(CONFIG_KUNIT_DEBUGFS),y)
 kunit-objs +=				debugfs.o
diff --git a/lib/kunit/bug.c b/lib/kunit/bug.c
new file mode 100644
index 0000000000000..b0b6778d7399a
--- /dev/null
+++ b/lib/kunit/bug.c
@@ -0,0 +1,115 @@
+// 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/resource.h>
+#include <linux/export.h>
+#include <linux/rculist.h>
+#include <linux/sched.h>
+#include <linux/spinlock.h>
+
+#include "hooks-impl.h"
+
+struct kunit_suppressed_warning {
+	struct list_head node;
+	struct task_struct *task;
+	struct kunit *test;
+	int counter;
+};
+
+static LIST_HEAD(suppressed_warnings);
+static DEFINE_SPINLOCK(suppressed_warnings_lock);
+
+static void kunit_suppress_warning_remove(struct kunit_suppressed_warning *w)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&suppressed_warnings_lock, flags);
+	list_del_rcu(&w->node);
+	spin_unlock_irqrestore(&suppressed_warnings_lock, flags);
+	synchronize_rcu(); /* Wait for readers to finish */
+}
+
+KUNIT_DEFINE_ACTION_WRAPPER(kunit_suppress_warning_cleanup,
+			    kunit_suppress_warning_remove,
+			    struct kunit_suppressed_warning *);
+
+bool kunit_has_active_suppress_warning(void)
+{
+	return __kunit_is_suppressed_warning_impl(false);
+}
+EXPORT_SYMBOL_GPL(kunit_has_active_suppress_warning);
+
+struct kunit_suppressed_warning *
+kunit_start_suppress_warning(struct kunit *test)
+{
+	struct kunit_suppressed_warning *w;
+	unsigned long flags;
+	int ret;
+
+	if (kunit_has_active_suppress_warning()) {
+		KUNIT_FAIL(test, "Another suppression block is already active");
+		return NULL;
+	}
+
+	w = kunit_kzalloc(test, sizeof(*w), GFP_KERNEL);
+	if (!w)
+		return NULL;
+
+	w->task = current;
+	w->test = test;
+
+	spin_lock_irqsave(&suppressed_warnings_lock, flags);
+	list_add_rcu(&w->node, &suppressed_warnings);
+	spin_unlock_irqrestore(&suppressed_warnings_lock, flags);
+
+	ret = kunit_add_action_or_reset(test,
+					kunit_suppress_warning_cleanup, w);
+	if (ret)
+		return NULL;
+
+	return w;
+}
+EXPORT_SYMBOL_GPL(kunit_start_suppress_warning);
+
+void kunit_end_suppress_warning(struct kunit *test,
+				struct kunit_suppressed_warning *w)
+{
+	if (!w)
+		return;
+	kunit_release_action(test, kunit_suppress_warning_cleanup, w);
+}
+EXPORT_SYMBOL_GPL(kunit_end_suppress_warning);
+
+void __kunit_suppress_auto_cleanup(struct kunit_suppressed_warning **wp)
+{
+	if (*wp)
+		kunit_end_suppress_warning((*wp)->test, *wp);
+}
+EXPORT_SYMBOL_GPL(__kunit_suppress_auto_cleanup);
+
+int kunit_suppressed_warning_count(struct kunit_suppressed_warning *w)
+{
+	return w ? w->counter : 0;
+}
+EXPORT_SYMBOL_GPL(kunit_suppressed_warning_count);
+
+bool __kunit_is_suppressed_warning_impl(bool count)
+{
+	struct kunit_suppressed_warning *w;
+
+	guard(rcu)();
+	list_for_each_entry_rcu(w, &suppressed_warnings, node) {
+		if (w->task == current) {
+			if (count)
+				w->counter++;
+			return true;
+		}
+	}
+
+	return false;
+}
diff --git a/lib/kunit/hooks-impl.h b/lib/kunit/hooks-impl.h
index 4e71b2d0143ba..d8720f2616925 100644
--- a/lib/kunit/hooks-impl.h
+++ b/lib/kunit/hooks-impl.h
@@ -19,6 +19,7 @@ void __printf(3, 4) __kunit_fail_current_test_impl(const char *file,
 						   int line,
 						   const char *fmt, ...);
 void *__kunit_get_static_stub_address_impl(struct kunit *test, void *real_fn_addr);
+bool __kunit_is_suppressed_warning_impl(bool count);
 
 /* Code to set all of the function pointers. */
 static inline void kunit_install_hooks(void)
@@ -26,6 +27,7 @@ static inline void kunit_install_hooks(void)
 	/* Install the KUnit hook functions. */
 	kunit_hooks.fail_current_test = __kunit_fail_current_test_impl;
 	kunit_hooks.get_static_stub_address = __kunit_get_static_stub_address_impl;
+	kunit_hooks.is_suppressed_warning = __kunit_is_suppressed_warning_impl;
 }
 
 #endif /* _KUNIT_HOOKS_IMPL_H */

-- 
2.53.0

[PATCH v8 2/4] 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.

Tests cover all three API forms:
- Scoped: kunit_warning_suppress() with in-block count verification
  and post-block inactivity check.
- Manual macros: KUNIT_START/END_SUPPRESSED_WARNING() with WARN()
  and WARN_ON(), both direct and through helper functions, as well
  as multiple warnings in a single block.
- Direct functions: kunit_start/end_suppress_warning() with
  sequential independent suppression blocks and per-block counts.

Furthermore, tests verify incremental warning counting, that
kunit_has_active_suppress_warning() transitions correctly around
suppression boundaries, and that suppression active in the test
kthread does not leak to a separate kthread.

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>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 lib/kunit/Makefile                     |   1 +
 lib/kunit/backtrace-suppression-test.c | 184 +++++++++++++++++++++++++++++++++
 2 files changed, 185 insertions(+)

diff --git a/lib/kunit/Makefile b/lib/kunit/Makefile
index 4592f9d0aa8dd..2e8a6b71a2ab0 100644
--- a/lib/kunit/Makefile
+++ b/lib/kunit/Makefile
@@ -22,6 +22,7 @@ obj-$(if $(CONFIG_KUNIT),y) +=		hooks.o
 
 obj-$(CONFIG_KUNIT_TEST) +=		kunit-test.o
 obj-$(CONFIG_KUNIT_TEST) +=		platform-test.o
+obj-$(CONFIG_KUNIT_TEST) +=		backtrace-suppression-test.o
 
 # 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..0e6fb685d2cbb
--- /dev/null
+++ b/lib/kunit/backtrace-suppression-test.c
@@ -0,0 +1,184 @@
+// 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>
+#include <linux/completion.h>
+#include <linux/kthread.h>
+
+static void backtrace_suppression_test_warn_direct(struct kunit *test)
+{
+	kunit_warning_suppress(test) {
+		WARN(1, "This backtrace should be suppressed");
+		/*
+		 * Count must be checked inside the scope; the handle
+		 * is not accessible after the block exits.
+		 */
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+}
+
+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_SUPPRESSED_WARNING_COUNT(test, 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_SUPPRESSED_WARNING_COUNT(test, 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_SUPPRESSED_WARNING_COUNT(test, 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_SUPPRESSED_WARNING_COUNT(test, 1);
+}
+
+static void backtrace_suppression_test_count(struct kunit *test)
+{
+	KUNIT_START_SUPPRESSED_WARNING(test);
+
+	KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 0);
+
+	WARN(1, "suppressed");
+	KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+
+	WARN(1, "suppressed again");
+	KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 2);
+
+	KUNIT_END_SUPPRESSED_WARNING(test);
+}
+
+static void backtrace_suppression_test_active_state(struct kunit *test)
+{
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+
+	KUNIT_START_SUPPRESSED_WARNING(test);
+	KUNIT_EXPECT_TRUE(test, kunit_has_active_suppress_warning());
+	KUNIT_END_SUPPRESSED_WARNING(test);
+
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+
+	kunit_warning_suppress(test) {
+		KUNIT_EXPECT_TRUE(test, kunit_has_active_suppress_warning());
+	}
+
+	KUNIT_EXPECT_FALSE(test, kunit_has_active_suppress_warning());
+}
+
+static void backtrace_suppression_test_multi_scope(struct kunit *test)
+{
+	struct kunit_suppressed_warning *sw1, *sw2;
+
+	if (!IS_ENABLED(CONFIG_DEBUG_BUGVERBOSE))
+		kunit_skip(test, "requires CONFIG_DEBUG_BUGVERBOSE");
+
+	sw1 = kunit_start_suppress_warning(test);
+	trigger_backtrace_warn_on();
+	WARN(1, "suppressed by sw1");
+	kunit_end_suppress_warning(test, sw1);
+
+	sw2 = kunit_start_suppress_warning(test);
+	WARN(1, "suppressed by sw2");
+	kunit_end_suppress_warning(test, sw2);
+
+	KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(sw1), 2);
+	KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(sw2), 1);
+}
+
+struct cross_kthread_data {
+	bool was_active;
+	struct completion done;
+};
+
+static int cross_kthread_fn(void *data)
+{
+	struct cross_kthread_data *d = data;
+
+	d->was_active = kunit_has_active_suppress_warning();
+	complete(&d->done);
+	return 0;
+}
+
+static void backtrace_suppression_test_cross_kthread(struct kunit *test)
+{
+	struct cross_kthread_data data;
+	struct task_struct *task;
+
+	init_completion(&data.done);
+
+	KUNIT_START_SUPPRESSED_WARNING(test);
+
+	task = kthread_run(cross_kthread_fn, &data, "kunit-cross-test");
+	KUNIT_ASSERT_FALSE(test, IS_ERR(task));
+	wait_for_completion(&data.done);
+
+	KUNIT_END_SUPPRESSED_WARNING(test);
+
+	KUNIT_EXPECT_FALSE(test, data.was_active);
+}
+
+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),
+	KUNIT_CASE(backtrace_suppression_test_count),
+	KUNIT_CASE(backtrace_suppression_test_active_state),
+	KUNIT_CASE(backtrace_suppression_test_multi_scope),
+	KUNIT_CASE(backtrace_suppression_test_cross_kthread),
+	{}
+};
+
+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");

-- 
2.53.0

[PATCH v8 3/4] 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>
Acked-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 drivers/gpu/drm/tests/drm_rect_test.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/tests/drm_rect_test.c b/drivers/gpu/drm/tests/drm_rect_test.c
index 17e1f34b76101..818e16e80c8f9 100644
--- a/drivers/gpu/drm/tests/drm_rect_test.c
+++ b/drivers/gpu/drm/tests/drm_rect_test.c
@@ -409,8 +409,16 @@ static void drm_test_rect_calc_hscale(struct kunit *test)
 	const struct drm_rect_scale_case *params = test->param_value;
 	int scaling_factor;
 
-	scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
-					      params->min_range, params->max_range);
+	/*
+	 * 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_warning_suppress(test) {
+		scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
+						      params->min_range,
+						      params->max_range);
+	}
 
 	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
 }
@@ -420,8 +428,15 @@ static void drm_test_rect_calc_vscale(struct kunit *test)
 	const struct drm_rect_scale_case *params = test->param_value;
 	int scaling_factor;
 
-	scaling_factor = drm_rect_calc_vscale(&params->src, &params->dst,
-					      params->min_range, params->max_range);
+	/*
+	 * 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_warning_suppress(test) {
+		scaling_factor = drm_rect_calc_vscale(&params->src, &params->dst,
+						      params->min_range, params->max_range);
+	}
 
 	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
 }

-- 
2.53.0

[PATCH v8 4/4] 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>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 Documentation/dev-tools/kunit/usage.rst | 63 ++++++++++++++++++++++++++++++++-
 1 file changed, 62 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
index ebd06f5ea4550..25724f7e72969 100644
--- a/Documentation/dev-tools/kunit/usage.rst
+++ b/Documentation/dev-tools/kunit/usage.rst
@@ -157,6 +157,67 @@ 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 a 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.
+
+Backtraces can be suppressed with **task-scoped suppression**: while
+suppression is active on the current task, the backtrace and stack dump from
+``WARN*()``, ``WARN_ON*()``, and related macros on that task are suppressed.
+Three API forms are available, in order of convenience.
+
+- Scoped suppression is the simplest form. Wrap the code that triggers
+  warnings in a ``kunit_warning_suppress()`` block:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		kunit_warning_suppress(test) {
+			trigger_backtrace();
+			KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+		}
+	}
+
+.. note::
+   The warning count must be checked inside the block; the suppression handle
+   is not accessible after the block exits.
+
+- Manual macros are useful when the suppressed region is large enough that
+  extra indentation is undesirable, or when the warning count needs to be
+  checked after suppression ends. ``KUNIT_START_SUPPRESSED_WARNING()`` must
+  appear before ``KUNIT_END_SUPPRESSED_WARNING()`` in the same scope.
+  Limited to one pair per scope.
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		KUNIT_START_SUPPRESSED_WARNING(test);
+		trigger_backtrace();
+		KUNIT_END_SUPPRESSED_WARNING(test);
+
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+
+- Direct functions return an explicit handle pointer. Use them when the handle
+  needs to be retained or passed across helper functions:
+
+.. code-block:: c
+
+	static void some_test(struct kunit *test)
+	{
+		struct kunit_suppressed_warning *w;
+
+		w = kunit_start_suppress_warning(test);
+		trigger_backtrace();
+		kunit_end_suppress_warning(test, w);
+
+		KUNIT_EXPECT_EQ(test, kunit_suppressed_warning_count(w), 1);
+	}
 
 Test Suites
 ~~~~~~~~~~~
@@ -1211,4 +1272,4 @@ 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
+	}

-- 
2.53.0

Re: [PATCH v8 3/4] drm: Suppress intentional warning backtraces in scaling unit tests

[Maxime Ripard]

[-- Attachment #1: Type: text/plain, Size: 3011 bytes --]

Hi,

On Mon, May 04, 2026 at 09:41:27AM +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>
> Acked-by: David Gow <david@davidgow.net>
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
>  drivers/gpu/drm/tests/drm_rect_test.c | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tests/drm_rect_test.c b/drivers/gpu/drm/tests/drm_rect_test.c
> index 17e1f34b76101..818e16e80c8f9 100644
> --- a/drivers/gpu/drm/tests/drm_rect_test.c
> +++ b/drivers/gpu/drm/tests/drm_rect_test.c
> @@ -409,8 +409,16 @@ static void drm_test_rect_calc_hscale(struct kunit *test)
>  	const struct drm_rect_scale_case *params = test->param_value;
>  	int scaling_factor;
>  
> -	scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
> -					      params->min_range, params->max_range);
> +	/*
> +	 * 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_warning_suppress(test) {
> +		scaling_factor = drm_rect_calc_hscale(&params->src, &params->dst,
> +						      params->min_range,
> +						      params->max_range);
> +	}
>  
>  	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
>  }
> @@ -420,8 +428,15 @@ static void drm_test_rect_calc_vscale(struct kunit *test)
>  	const struct drm_rect_scale_case *params = test->param_value;
>  	int scaling_factor;
>  
> -	scaling_factor = drm_rect_calc_vscale(&params->src, &params->dst,
> -					      params->min_range, params->max_range);
> +	/*
> +	 * 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_warning_suppress(test) {
> +		scaling_factor = drm_rect_calc_vscale(&params->src, &params->dst,
> +						      params->min_range, params->max_range);
> +	}
>  
>  	KUNIT_EXPECT_EQ(test, scaling_factor, params->expected_scaling_factor);
>  }

For both I think we should add KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT calls, no?

Maxime

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