[PATCH v2] selftests: harness: Support KCOV.

28 Oct 2025

While writing a selftest with kselftest_harness.h, I often want to
check which paths are actually exercised.
Let's support generating KCOV coverage data.
We can specify the output directory via the KCOV_OUTPUT environment
variable, and the number of instructions to collect via the KCOV_SLOTS
environment variable.
# KCOV_OUTPUT=$PWD/kcov KCOV_SLOTS=$((4096 * 2)) \
    ./tools/testing/selftests/net/af_unix/scm_inq
Both variables can also be specified as the make variable.
# make -C tools/testing/selftests/ \
    KCOV_OUTPUT=$PWD/kcov KCOV_SLOTS=$((4096 * 4)) \
    kselftest_override_timeout=60 TARGETS=net/af_unix run_tests
The coverage data can be simply decoded with addr2line:
$ cat kcov/* | sort | uniq | addr2line -e vmlinux | grep unix
  net/unix/af_unix.c:1056
  net/unix/af_unix.c:3138
  net/unix/af_unix.c:3834
  net/unix/af_unix.c:3838
  net/unix/af_unix.c:311 (discriminator 2)
  ...
or more nicely with a script embedded in vock [0]:
$ cat kcov/* | sort | uniq > local.log
  $ python3 ~/kernel/tools/vock/report.py \
    --kernel-src ./ --vmlinux ./vmlinux \
    --mode local --local-log local.log --filter unix
  ...
  ------------------------------- Coverage Report --------------------------------
  📄 net/unix/af_unix.c (276 lines)
   ...
  942 | static int unix_setsockopt(struct socket *sock, int level, int optname,
  943 | 			   sockptr_t optval, unsigned int optlen)
  944 | {
   ...
  961 | 	switch (optname) {
  962 | 	case SO_INQ:
  963 > 		if (sk->sk_type != SOCK_STREAM)
  964 | 			return -EINVAL;
  965 |
  966 > 		if (val > 1 || val < 0)
  967 | 			return -EINVAL;
  968 |
  969 > 		WRITE_ONCE(u->recvmsg_inq, val);
  970 | 		break;
Link: https://github.com/kzall0c/vock/blob/f3d97de9954f9df758c0ab287ca7e24e654288c... #[0]
Signed-off-by: Kuniyuki Iwashima kuniyu@google.com
---
v2: Support TEST()
v1: https://lore.kernel.org/linux-kselftest/20251017084022.3721950-1-kuniyu@goog...
---
 Documentation/dev-tools/kselftest.rst       |  41 ++++++
 tools/testing/selftests/Makefile            |  14 ++-
 tools/testing/selftests/kselftest_harness.h | 133 +++++++++++++++++++-
 3 files changed, 178 insertions(+), 10 deletions(-)

diff --git a/Documentation/dev-tools/kselftest.rst b/Documentation/dev-tools/kselftest.rst
index 18c2da67fae4..5c2b92ac4a30 100644
--- a/Documentation/dev-tools/kselftest.rst
+++ b/Documentation/dev-tools/kselftest.rst
@@ -200,6 +200,47 @@ You can look at the TAP output to see if you ran into the timeout. Test
 runners which know a test must run under a specific time can then optionally
 treat these timeouts then as fatal.
+KCOV for selftests
+==================
+
+Selftests built with `kselftest_harness.h` natively support generating
+KCOV coverage data.  See :doc:`KCOV: code coverage for fuzzing </dev-tools/kcov>`
+for prerequisites.
+
+You can specify the output directory with the `KCOV_OUTPUT` environment
+variable.  Additionally, you can specify the number of instructions to
+collect with the `KCOV_SLOTS` environment variable ::
+
+  # KCOV_OUTPUT=$PWD/kcov KCOV_SLOTS=$((4096 * 2)) \
+        ./tools/testing/selftests/net/af_unix/scm_inq
+
+In the output directory, a coverage file is generated for each test
+case in the selftest ::
+
+  $ ls kcov/
+  scm_inq.dgram.basic  scm_inq.seqpacket.basic  scm_inq.stream.basic
+
+The default value of `KCOV_SLOTS` is `4096`, and `KCOV_SLOTS` multiplied
+by `sizeof(unsigned long)` must be multiple of `4096`, so the smallest
+value is `512`.
+
+Both `KCOV_OUTPUT` and `KCOV_SLOTS` can be specified as the variables
+on the `make` command line ::
+
+  # make -C tools/testing/selftests/ \
+        kselftest_override_timeout=60 \
+        KCOV_OUTPUT=$PWD/kcov KCOV_SLOTS=$((4096 * 4)) \
+        TARGETS=net/af_unix run_tests
+
+The collected data can be decoded with `addr2line` ::
+
+  $ cat kcov/* | sort | uniq | addr2line -e vmlinux | grep unix
+  net/unix/af_unix.c:1056
+  net/unix/af_unix.c:3138
+  net/unix/af_unix.c:3834
+  net/unix/af_unix.c:3838
+  ...
+
 Packaging selftests
 ===================
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index c46ebdb9b8ef..40e70fb1a347 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -218,12 +218,14 @@ all:
    done; exit $$ret;
run_tests: all
-	@for TARGET in $(TARGETS); do \
-		BUILD_TARGET=$$BUILD/$$TARGET;	\
-		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET run_tests \
-				SRC_PATH=$(shell readlink -e $$(pwd)) \
-				OBJ_PATH=$(BUILD)                   \
-				O=$(abs_objtree);		    \
+	@for TARGET in $(TARGETS); do				\
+		BUILD_TARGET=$$BUILD/$$TARGET;			\
+		$(MAKE) OUTPUT=$$BUILD_TARGET			\
+			KCOV_OUTPUT=$(abspath $(KCOV_OUTPUT))	\
+			-C $$TARGET run_tests			\
+			SRC_PATH=$(shell readlink -e $$(pwd))	\
+			OBJ_PATH=$(BUILD)			\
+			O=$(abs_objtree);			\
    done;
hotplug:
diff --git a/tools/testing/selftests/kselftest_harness.h b/tools/testing/selftests/kselftest_harness.h
index 3f66e862e83e..5b7a01722981 100644
--- a/tools/testing/selftests/kselftest_harness.h
+++ b/tools/testing/selftests/kselftest_harness.h
@@ -56,6 +56,8 @@
 #include <asm/types.h>
 #include <ctype.h>
 #include <errno.h>
+#include <fcntl.h>
+#include <linux/kcov.h>
 #include <linux/unistd.h>
 #include <poll.h>
 #include <stdbool.h>
@@ -63,7 +65,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/ioctl.h>
 #include <sys/mman.h>
+#include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
@@ -175,9 +179,12 @@
    static void test_name(struct __test_metadata *_metadata); \
    static void wrapper_##test_name( \
    	struct __test_metadata *_metadata, \
-		struct __fixture_variant_metadata __attribute__((unused)) *variant) \
+		struct __fixture_variant_metadata __attribute__((unused)) *variant, \
+		char *test_full_name) \
    { \
+		enable_kcov(_metadata); \
    	test_name(_metadata); \
+		disable_kcov(_metadata, test_full_name); \
    } \
    static struct __test_metadata _##test_name##_object = \
    	{ .name = #test_name, \
@@ -401,7 +408,8 @@
    	const FIXTURE_VARIANT(fixture_name) *variant); \
    static void wrapper_##fixture_name##_##test_name( \
    	struct __test_metadata *_metadata, \
-		struct __fixture_variant_metadata *variant) \
+		struct __fixture_variant_metadata *variant, \
+		char *test_full_name) \
    { \
    	/* fixture data is alloced, setup, and torn down per call. */ \
    	FIXTURE_DATA(fixture_name) self_private, *self = NULL; \
@@ -430,7 +438,9 @@
    		if (_metadata->exit_code) \
    			_exit(0); \
    		*_metadata->no_teardown = false; \
+			enable_kcov(_metadata); \
    		fixture_name##_##test_name(_metadata, self, variant->data); \
+			disable_kcov(_metadata, test_full_name); \
    		_metadata->teardown_fn(false, _metadata, self, variant->data); \
    		_exit(0); \
    	} else if (child < 0 || child != waitpid(child, &status, 0)) { \
@@ -470,6 +480,8 @@
    	object->teardown_fn = &wrapper_##fixture_name##_##test_name##_teardown; \
    	object->termsig = signal; \
    	object->timeout = tmout; \
+		object->kcov_fd = -1; \
+		object->kcov_slots = -1; \
    	_##fixture_name##_##test_name##_object = object; \
    	__register_test(object); \
    } \
@@ -908,7 +920,8 @@ __register_fixture_variant(struct __fixture_metadata *f,
 struct __test_metadata {
    const char *name;
    void (*fn)(struct __test_metadata *,
-		   struct __fixture_variant_metadata *);
+		   struct __fixture_variant_metadata *,
+		   char *test_name);
    pid_t pid;	/* pid of test when being run */
    struct __fixture_metadata *fixture;
    void (*teardown_fn)(bool in_parent, struct __test_metadata *_metadata,
@@ -923,6 +936,10 @@ struct __test_metadata {
    const void *variant;
    struct __test_results *results;
    struct __test_metadata *prev, *next;
+	int kcov_fd;
+	int kcov_slots;
+	char *kcov_dir;
+	unsigned long *kcov_mem;
 };
static inline bool __test_passed(struct __test_metadata *metadata)
@@ -1185,6 +1202,114 @@ static bool test_enabled(int argc, char **argv,
    return !has_positive;
 }
+#define KCOV_SLOTS 4096
+
+static void enable_kcov(struct __test_metadata *t)
+{
+	char *slots;
+	int err;
+
+	t->kcov_dir = getenv("KCOV_OUTPUT");
+	if (!t->kcov_dir || *t->kcov_dir == '\0')
+		return;
+
+	slots = getenv("KCOV_SLOTS");
+	if (slots && *slots != '\0')
+		sscanf(slots, "%d", &t->kcov_slots);
+	if (t->kcov_slots <= 0)
+		t->kcov_slots = KCOV_SLOTS;
+
+	t->kcov_fd = open("/sys/kernel/debug/kcov", O_RDWR);
+	if (t->kcov_fd < 0) {
+		ksft_print_msg("ERROR OPENING KCOV FD\n");
+		goto err;
+	}
+
+	err = ioctl(t->kcov_fd, KCOV_INIT_TRACE, t->kcov_slots);
+	if (err) {
+		ksft_print_msg("ERROR INITIALISING KCOV\n");
+		goto err;
+	}
+
+	t->kcov_mem = mmap(NULL, sizeof(unsigned long) * t->kcov_slots,
+			   PROT_READ | PROT_WRITE, MAP_SHARED, t->kcov_fd, 0);
+	if ((void *)t->kcov_mem == MAP_FAILED) {
+		ksft_print_msg("ERROR ALLOCATING MEMORY FOR KCOV\n");
+		goto err;
+	}
+
+	err = ioctl(t->kcov_fd, KCOV_ENABLE, KCOV_TRACE_PC);
+	if (err) {
+		ksft_print_msg("ERROR ENABLING KCOV\n");
+		goto err;
+	}
+
+	__atomic_store_n(&t->kcov_mem[0], 0, __ATOMIC_RELAXED);
+	return;
+err:
+	t->exit_code = KSFT_FAIL;
+	_exit(KSFT_FAIL);
+}
+
+static void disable_kcov(struct __test_metadata *t, char *test_name)
+{
+	int slots, err, dir, fd, i;
+
+	if (t->kcov_fd == -1)
+		return;
+
+	slots = __atomic_load_n(&t->kcov_mem[0], __ATOMIC_RELAXED);
+	if (slots == t->kcov_slots - 1)
+		ksft_print_msg("Set KCOV_SLOTS to a value greater than %d\n", t->kcov_slots);
+
+	err = ioctl(t->kcov_fd, KCOV_DISABLE, 0);
+	if (err) {
+		ksft_print_msg("ERROR DISABLING KCOV\n");
+		goto out;
+	}
+
+	err = mkdir(t->kcov_dir, 0755);
+	if (err == -1 && errno != EEXIST) {
+		ksft_print_msg("ERROR CREATING '%s'\n", t->kcov_dir);
+		goto out;
+	}
+	err = 0;
+
+	dir = open(t->kcov_dir, O_DIRECTORY);
+	if (dir < 0) {
+		ksft_print_msg("ERROR OPENING %s\n", t->kcov_dir);
+		err = dir;
+		goto out;
+	}
+
+	fd = openat(dir, test_name, O_RDWR | O_CREAT | O_TRUNC);
+
+	close(dir);
+
+	if (fd == -1) {
+		ksft_print_msg("ERROR CREATING '%s' at '%s'\n", test_name, t->kcov_dir);
+		err = fd;
+		goto out;
+	}
+
+	for (i = 0; i < slots; i++) {
+		char buf[64];
+		int size;
+
+		size = snprintf(buf, 64, "0x%lx\n", t->kcov_mem[i + 1]);
+		write(fd, buf, size);
+	}
+
+out:
+	munmap(t->kcov_mem, sizeof(t->kcov_mem[0]) * t->kcov_slots);
+	close(t->kcov_fd);
+
+	if (err) {
+		t->exit_code = KSFT_FAIL;
+		_exit(KSFT_FAIL);
+	}
+}
+
 static void __run_test(struct __fixture_metadata *f,
    	       struct __fixture_variant_metadata *variant,
    	       struct __test_metadata *t)
@@ -1216,7 +1341,7 @@ static void __run_test(struct __fixture_metadata *f,
    	t->exit_code = KSFT_FAIL;
    } else if (child == 0) {
    	setpgrp();
-		t->fn(t, variant);
+		t->fn(t, variant, test_name);
    	_exit(t->exit_code);
    } else {
    	t->pid = child;
-- 
2.51.1.838.g19442a804e-goog



    

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[PATCH v2] selftests: harness: Support KCOV.