KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

7 files changed, 750 insertions, 0 deletions

diff --git a/tools/testing/selftests/acct/.gitignore b/tools/testing/selftests/acct/.gitignore
new file mode 100644
index 000000000..fe0896f54
--- /dev/null
+++ b/tools/testing/selftests/acct/.gitignore
@@ -0,0 +1,5 @@
+acct_syscall
+taskstats_fill_stats_tgid
+cgroupstats
+config
+process_log
diff --git a/tools/testing/selftests/acct/Makefile b/tools/testing/selftests/acct/Makefile
new file mode 100644
index 000000000..93a11a28a
--- /dev/null
+++ b/tools/testing/selftests/acct/Makefile
@@ -0,0 +1,19 @@
+# SPDX-License-Identifier: GPL-2.0
+TEST_GEN_PROGS := acct_syscall
+TEST_GEN_PROGS += taskstats_fill_stats_tgid
+TEST_GEN_PROGS += cgroupstats
+
+NETLINK_HELPER_PROGS := cgroupstats taskstats_fill_stats_tgid
+
+CFLAGS += -Wall
+LDLIBS += -lpthread
+
+include ../lib.mk
+
+$(NETLINK_HELPER_PROGS): %: %.c netlink_helper.c netlink_helper.h
+	$(call msg,CC,,$@)
+	$(Q)$(LINK.c) $< netlink_helper.c $(LDLIBS) -o $@
+
+$(addprefix $(OUTPUT)/,$(NETLINK_HELPER_PROGS)): $(OUTPUT)/%: %.c netlink_helper.c netlink_helper.h
+	$(call msg,CC,,$@)
+	$(Q)$(LINK.c) $< netlink_helper.c $(LDLIBS) -o $@
diff --git a/tools/testing/selftests/acct/acct_syscall.c b/tools/testing/selftests/acct/acct_syscall.c
new file mode 100644
index 000000000..421adbdc2
--- /dev/null
+++ b/tools/testing/selftests/acct/acct_syscall.c
@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/* kselftest for acct() system call
+ *  The acct() system call enables or disables process accounting.
+ */
+
+#include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include <sys/wait.h>
+
+#include "kselftest.h"
+
+int main(void)
+{
+	char filename[] = "process_log";
+	FILE *fp;
+	pid_t child_pid;
+	int sz;
+
+	// Setting up kselftest framework
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	// Check if test is run a root
+	if (geteuid()) {
+		ksft_exit_skip("This test needs root to run!\n");
+		return 1;
+	}
+
+	// Create file to log closed processes
+	fp = fopen(filename, "w");
+
+	if (!fp) {
+		ksft_test_result_error("%s.\n", strerror(errno));
+		ksft_finished();
+		return 1;
+	}
+
+	acct(filename);
+
+	// Handle error conditions
+	if (errno) {
+		ksft_test_result_error("%s.\n", strerror(errno));
+		fclose(fp);
+		ksft_finished();
+		return 1;
+	}
+
+	// Create child process and wait for it to terminate.
+
+	child_pid = fork();
+
+	if (child_pid < 0) {
+		ksft_test_result_error("Creating a child process to log failed\n");
+		acct(NULL);
+		return 1;
+	} else if (child_pid > 0) {
+		wait(NULL);
+		fseek(fp, 0L, SEEK_END);
+		sz = ftell(fp);
+
+		acct(NULL);
+
+		if (sz <= 0) {
+			ksft_test_result_fail("Terminated child process not logged\n");
+			ksft_exit_fail();
+			return 1;
+		}
+
+		ksft_test_result_pass("Successfully logged terminated process.\n");
+		fclose(fp);
+		ksft_exit_pass();
+		return 0;
+	}
+
+	return 1;
+}
diff --git a/tools/testing/selftests/acct/cgroupstats.c b/tools/testing/selftests/acct/cgroupstats.c
new file mode 100644
index 000000000..0b421a4ca
--- /dev/null
+++ b/tools/testing/selftests/acct/cgroupstats.c
@@ -0,0 +1,231 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/cgroupstats.h>
+#include <linux/genetlink.h>
+#include <linux/netlink.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "netlink_helper.h"
+#include "kselftest.h"
+
+static int send_cgroupstats_cmd(int fd, int family_id, uint32_t cgroup_fd,
+				int flags)
+{
+	struct {
+		struct nlmsghdr nlh;
+		struct genlmsghdr genl;
+		char buf[256];
+	} req = { 0 };
+	struct nlattr *na;
+
+	req.nlh.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
+	req.nlh.nlmsg_type = family_id;
+	req.nlh.nlmsg_flags = NLM_F_REQUEST | flags;
+	req.nlh.nlmsg_seq = 2;
+	req.nlh.nlmsg_pid = getpid();
+
+	req.genl.cmd = CGROUPSTATS_CMD_GET;
+	req.genl.version = 1;
+
+	na = (struct nlattr *)((char *)&req + NLMSG_ALIGN(req.nlh.nlmsg_len));
+	na->nla_type = CGROUPSTATS_CMD_ATTR_FD;
+	na->nla_len = NLA_HDRLEN + sizeof(cgroup_fd);
+	memcpy(nla_data(na), &cgroup_fd, sizeof(cgroup_fd));
+	req.nlh.nlmsg_len = NLMSG_ALIGN(req.nlh.nlmsg_len) + NLA_ALIGN(na->nla_len);
+
+	return send_request(fd, &req, req.nlh.nlmsg_len);
+}
+
+/*
+ * Receive and decode a cgroupstats response.
+ *
+ * Returns:
+ *   0             — success, stats filled from CGROUPSTATS_CMD_NEW reply
+ *   <0            — NLMSG_ERROR errno (e.g. -EBADF, -EINVAL)
+ */
+static int recv_cgroupstats_response(int fd, struct cgroupstats *stats)
+{
+	char resp[8192];
+	struct nlmsghdr *nlh;
+	struct genlmsghdr *genl;
+	struct nlattr *na;
+	int len;
+	int rem;
+
+	memset(stats, 0, sizeof(*stats));
+
+	len = recv(fd, resp, sizeof(resp), 0);
+	if (len < 0)
+		return -errno;
+
+	for (nlh = (struct nlmsghdr *)resp; NLMSG_OK(nlh, len);
+	     nlh = NLMSG_NEXT(nlh, len)) {
+		if (nlh->nlmsg_type == NLMSG_ERROR) {
+			struct nlmsgerr *err = NLMSG_DATA(nlh);
+
+			return err->error;
+		}
+
+		genl = (struct genlmsghdr *)NLMSG_DATA(nlh);
+		if (genl->cmd != CGROUPSTATS_CMD_NEW)
+			continue;
+
+		rem = nlh->nlmsg_len - NLMSG_HDRLEN - GENL_HDRLEN;
+		na = (struct nlattr *)((char *)genl + GENL_HDRLEN);
+		while (nla_ok(na, rem)) {
+			if (na->nla_type == CGROUPSTATS_TYPE_CGROUP_STATS) {
+				memcpy(stats, nla_data(na), sizeof(*stats));
+				return 0;
+			}
+			na = nla_next(na, &rem);
+		}
+	}
+
+	return -EIO;
+}
+
+/* mkdtemp() modifies the template in place, so this cannot be const. */
+static char cg_mountpoint[32];
+static bool cg_mounted;
+
+static int setup_cgroup_v1(void)
+{
+	strcpy(cg_mountpoint, "/tmp/cgstats_test_XXXXXX");
+
+	if (!mkdtemp(cg_mountpoint))
+		return -errno;
+
+	if (mount("cgstats_test", cg_mountpoint, "cgroup", 0,
+		  "none,name=cgstats_test") < 0) {
+		int ret = -errno;
+
+		rmdir(cg_mountpoint);
+		return ret;
+	}
+
+	cg_mounted = true;
+	return 0;
+}
+
+static void cleanup_cgroup_v1(void)
+{
+	if (!cg_mounted)
+		return;
+	umount2(cg_mountpoint, MNT_DETACH);
+	rmdir(cg_mountpoint);
+	cg_mounted = false;
+}
+
+int main(void)
+{
+	struct cgroupstats stats;
+	uint64_t total_tasks;
+	int family_id;
+	int nl_fd;
+	int cg_fd;
+	int ret;
+
+	ksft_print_header();
+
+	nl_fd = netlink_open();
+	if (nl_fd < 0)
+		ksft_exit_skip("failed to open generic netlink socket: %s\n",
+			       strerror(-nl_fd));
+
+	family_id = get_family_id(nl_fd, TASKSTATS_GENL_NAME);
+	if (family_id < 0)
+		ksft_exit_skip("taskstats generic netlink family unavailable: %s\n",
+			       strerror(-family_id));
+
+	ksft_set_plan(3);
+
+	/*
+	 * Test 1: mount a private cgroup v1 hierarchy, query it, and
+	 * verify the response contains sane task counts. If the test
+	 * environment cannot create a private cgroup v1 mount, skip this
+	 * case and continue with the unprivileged regression checks below.
+	 */
+	ret = setup_cgroup_v1();
+	if (ret) {
+		ksft_test_result_skip("cgroupstats query: cannot mount cgroup v1: %s\n",
+				      strerror(-ret));
+	} else {
+		cg_fd = open(cg_mountpoint, O_RDONLY | O_DIRECTORY);
+		if (cg_fd < 0) {
+			ksft_test_result_fail("cgroupstats query: open mountpoint: %s\n",
+					      strerror(errno));
+		} else {
+			ret = send_cgroupstats_cmd(nl_fd, family_id,
+						   (uint32_t)cg_fd, 0);
+			if (ret) {
+				ksft_test_result_fail("cgroupstats query: send: %s\n",
+						      strerror(-ret));
+			} else {
+				ret = recv_cgroupstats_response(nl_fd, &stats);
+				if (ret < 0) {
+					ksft_test_result_fail("cgroupstats query: %s\n",
+							      strerror(-ret));
+				} else {
+					total_tasks = (uint64_t)stats.nr_sleeping +
+						      (uint64_t)stats.nr_running +
+						      (uint64_t)stats.nr_stopped +
+						      (uint64_t)stats.nr_uninterruptible +
+						      (uint64_t)stats.nr_io_wait;
+
+					ksft_print_msg("cgroupstats query: total_tasks=%llu\n",
+						       (unsigned long long)total_tasks);
+
+					ksft_test_result(total_tasks > 0,
+							 "cgroupstats query returns valid stats\n");
+				}
+			}
+			close(cg_fd);
+		}
+	}
+	cleanup_cgroup_v1();
+
+	/*
+	 * Test 2: invalid fd without NLM_F_ACK.  The kernel should
+	 * return -EBADF via NLMSG_ERROR regardless of whether the
+	 * client requested an explicit ACK.
+	 */
+	ret = send_cgroupstats_cmd(nl_fd, family_id, 0xFFFFFFFF, 0);
+	if (ret)
+		ksft_exit_fail_msg("send test 2 failed: %s\n", strerror(-ret));
+
+	ret = recv_cgroupstats_response(nl_fd, &stats);
+	ksft_print_msg("bad fd (no ACK): response=%d (%s)\n",
+		       ret, ret < 0 ? strerror(-ret) : "unexpected success");
+	ksft_test_result(ret == -EBADF,
+			 "cgroupstats rejects bad fd without NLM_F_ACK\n");
+
+	/*
+	 * Test 3: invalid fd with NLM_F_ACK.  Same expectation as
+	 * test 2, but exercised through a different netlink flag
+	 * path in the kernel's ack/error handling.
+	 */
+	ret = send_cgroupstats_cmd(nl_fd, family_id, 0xFFFFFFFF, NLM_F_ACK);
+	if (ret)
+		ksft_exit_fail_msg("send test 3 failed: %s\n", strerror(-ret));
+
+	ret = recv_cgroupstats_response(nl_fd, &stats);
+	ksft_print_msg("bad fd (with ACK): response=%d (%s)\n",
+		       ret, ret < 0 ? strerror(-ret) : "unexpected success");
+	ksft_test_result(ret == -EBADF,
+			 "cgroupstats rejects bad fd with NLM_F_ACK\n");
+
+	close(nl_fd);
+	ksft_finished();
+	return ksft_get_fail_cnt() ? KSFT_FAIL : KSFT_PASS;
+}
diff --git a/tools/testing/selftests/acct/netlink_helper.c b/tools/testing/selftests/acct/netlink_helper.c
new file mode 100644
index 000000000..3ed834f0e
--- /dev/null
+++ b/tools/testing/selftests/acct/netlink_helper.c
@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <errno.h>
+#include <stdint.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/time.h>
+#include <unistd.h>
+#include <linux/genetlink.h>
+
+#include "netlink_helper.h"
+
+int netlink_open(void)
+{
+	struct timeval tv = { .tv_sec = ACCT_RCV_TIMEOUT_SEC };
+	struct sockaddr_nl addr = {
+		.nl_family = AF_NETLINK,
+		.nl_pid = getpid(),
+	};
+	int fd;
+
+	fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
+	if (fd < 0)
+		return -errno;
+
+	if (setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv)) < 0) {
+		int err = -errno;
+
+		close(fd);
+		return err;
+	}
+
+	if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
+		int err = -errno;
+
+		close(fd);
+		return err;
+	}
+
+	return fd;
+}
+
+int send_request(int fd, void *buf, size_t len)
+{
+	struct sockaddr_nl addr = {
+		.nl_family = AF_NETLINK,
+	};
+
+	if (sendto(fd, buf, len, 0, (struct sockaddr *)&addr, sizeof(addr)) < 0)
+		return -errno;
+
+	return 0;
+}
+
+/*
+ * Resolve the generic netlink family ID for @name.
+ * Returns the family ID (>= 0) on success, negative errno on failure.
+ */
+int get_family_id(int fd, const char *name)
+{
+	struct {
+		struct nlmsghdr nlh;
+		struct genlmsghdr genl;
+		char buf[256];
+	} req = { 0 };
+	char resp[8192];
+	struct nlmsghdr *nlh;
+	struct genlmsghdr *genl;
+	struct nlattr *na;
+	int len;
+	int rem;
+	int ret;
+
+	req.nlh.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
+	req.nlh.nlmsg_type = GENL_ID_CTRL;
+	req.nlh.nlmsg_flags = NLM_F_REQUEST;
+	req.nlh.nlmsg_seq = 1;
+	req.nlh.nlmsg_pid = getpid();
+
+	req.genl.cmd = CTRL_CMD_GETFAMILY;
+	req.genl.version = 1;
+
+	na = (struct nlattr *)((char *)&req + NLMSG_ALIGN(req.nlh.nlmsg_len));
+	na->nla_type = CTRL_ATTR_FAMILY_NAME;
+	na->nla_len = NLA_HDRLEN + strlen(name) + 1;
+	memcpy(nla_data(na), name, strlen(name) + 1);
+	req.nlh.nlmsg_len = NLMSG_ALIGN(req.nlh.nlmsg_len) + NLA_ALIGN(na->nla_len);
+
+	ret = send_request(fd, &req, req.nlh.nlmsg_len);
+	if (ret)
+		return ret;
+
+	len = recv(fd, resp, sizeof(resp), 0);
+	if (len < 0)
+		return -errno;
+
+	for (nlh = (struct nlmsghdr *)resp; NLMSG_OK(nlh, len);
+	     nlh = NLMSG_NEXT(nlh, len)) {
+		if (nlh->nlmsg_type == NLMSG_ERROR) {
+			struct nlmsgerr *err = NLMSG_DATA(nlh);
+
+			return err->error ? err->error : -ENOENT;
+		}
+
+		genl = (struct genlmsghdr *)NLMSG_DATA(nlh);
+		rem = nlh->nlmsg_len - NLMSG_HDRLEN - GENL_HDRLEN;
+		na = (struct nlattr *)((char *)genl + GENL_HDRLEN);
+		while (nla_ok(na, rem)) {
+			if (na->nla_type == CTRL_ATTR_FAMILY_ID)
+				return *(uint16_t *)nla_data(na);
+			na = nla_next(na, &rem);
+		}
+	}
+
+	return -ENOENT;
+}
diff --git a/tools/testing/selftests/acct/netlink_helper.h b/tools/testing/selftests/acct/netlink_helper.h
new file mode 100644
index 000000000..0320729c4
--- /dev/null
+++ b/tools/testing/selftests/acct/netlink_helper.h
@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Shared generic netlink helpers for the acct selftests.
+ */
+#ifndef ACSELFTESTS_ACCT_NETLINK_HELPER_H
+#define ACSELFTESTS_ACCT_NETLINK_HELPER_H
+
+#include <stdbool.h>
+#include <linux/netlink.h>
+
+#ifndef NLA_ALIGNTO
+#define NLA_ALIGNTO 4
+#define NLA_ALIGN(len) (((len) + NLA_ALIGNTO - 1) & ~(NLA_ALIGNTO - 1))
+#define NLA_HDRLEN ((int)NLA_ALIGN(sizeof(struct nlattr)))
+#endif
+
+/* Fail an individual test case instead of hanging the whole binary. */
+#define ACCT_RCV_TIMEOUT_SEC 2
+
+static inline void *nla_data(const struct nlattr *na)
+{
+	return (void *)((char *)na + NLA_HDRLEN);
+}
+
+static inline bool nla_ok(const struct nlattr *na, int remaining)
+{
+	return remaining >= (int)sizeof(*na) &&
+	       na->nla_len >= sizeof(*na) &&
+	       na->nla_len <= remaining;
+}
+
+static inline struct nlattr *nla_next(const struct nlattr *na, int *remaining)
+{
+	int aligned_len = NLA_ALIGN(na->nla_len);
+
+	*remaining -= aligned_len;
+	return (struct nlattr *)((char *)na + aligned_len);
+}
+
+int netlink_open(void);
+int send_request(int fd, void *buf, size_t len);
+int get_family_id(int fd, const char *name);
+
+#endif /* ACSELFTESTS_ACCT_NETLINK_HELPER_H */
diff --git a/tools/testing/selftests/acct/taskstats_fill_stats_tgid.c b/tools/testing/selftests/acct/taskstats_fill_stats_tgid.c
new file mode 100644
index 000000000..9a4c1554d
--- /dev/null
+++ b/tools/testing/selftests/acct/taskstats_fill_stats_tgid.c
@@ -0,0 +1,257 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <linux/genetlink.h>
+#include <linux/netlink.h>
+#include <linux/taskstats.h>
+#include <pthread.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <time.h>
+#include <unistd.h>
+
+#include "netlink_helper.h"
+#include "kselftest.h"
+
+#define BUSY_NS (200ULL * 1000 * 1000)
+
+struct worker_ctx {
+	pthread_mutex_t lock;
+	pthread_cond_t cond;
+	bool ready;
+	bool release;
+};
+
+static unsigned long busy_sink;
+
+static uint64_t timespec_diff_ns(const struct timespec *start,
+				 const struct timespec *end)
+{
+	return (uint64_t)(end->tv_sec - start->tv_sec) * 1000000000ULL +
+	       (uint64_t)(end->tv_nsec - start->tv_nsec);
+}
+
+static void burn_cpu_for_ns(uint64_t runtime_ns)
+{
+	struct timespec start, now;
+	unsigned long acc = 0;
+
+	if (clock_gettime(CLOCK_MONOTONIC, &start)) {
+		perror("clock_gettime");
+		exit(EXIT_FAILURE);
+	}
+
+	do {
+		for (int i = 0; i < 100000; i++)
+			acc += i;
+		if (clock_gettime(CLOCK_MONOTONIC, &now)) {
+			perror("clock_gettime");
+			exit(EXIT_FAILURE);
+		}
+	} while (timespec_diff_ns(&start, &now) < runtime_ns);
+
+	busy_sink = acc;
+}
+
+static int get_taskstats(int fd, int family_id, uint16_t attr_type, uint32_t id,
+			 struct taskstats *stats)
+{
+	struct {
+		struct nlmsghdr nlh;
+		struct genlmsghdr genl;
+		char buf[256];
+	} req = { 0 };
+	char resp[16384];
+	struct nlmsghdr *nlh;
+	struct genlmsghdr *genl;
+	struct nlattr *na;
+	struct nlattr *nested;
+	int len;
+	int rem;
+	int nrem;
+	int ret;
+
+	memset(stats, 0, sizeof(*stats));
+
+	req.nlh.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
+	req.nlh.nlmsg_type = family_id;
+	req.nlh.nlmsg_flags = NLM_F_REQUEST;
+	req.nlh.nlmsg_seq = 2;
+	req.nlh.nlmsg_pid = getpid();
+
+	req.genl.cmd = TASKSTATS_CMD_GET;
+	req.genl.version = 1;
+
+	na = (struct nlattr *)((char *)&req + NLMSG_ALIGN(req.nlh.nlmsg_len));
+	na->nla_type = attr_type;
+	na->nla_len = NLA_HDRLEN + sizeof(id);
+	memcpy(nla_data(na), &id, sizeof(id));
+	req.nlh.nlmsg_len = NLMSG_ALIGN(req.nlh.nlmsg_len) + NLA_ALIGN(na->nla_len);
+
+	ret = send_request(fd, &req, req.nlh.nlmsg_len);
+	if (ret)
+		return ret;
+
+	len = recv(fd, resp, sizeof(resp), 0);
+	if (len < 0)
+		return -errno;
+
+	for (nlh = (struct nlmsghdr *)resp; NLMSG_OK(nlh, len);
+	     nlh = NLMSG_NEXT(nlh, len)) {
+		if (nlh->nlmsg_type == NLMSG_ERROR) {
+			struct nlmsgerr *err = NLMSG_DATA(nlh);
+
+			return err->error ? err->error : -ENOENT;
+		}
+
+		genl = (struct genlmsghdr *)NLMSG_DATA(nlh);
+		rem = nlh->nlmsg_len - NLMSG_HDRLEN - GENL_HDRLEN;
+		na = (struct nlattr *)((char *)genl + GENL_HDRLEN);
+		while (nla_ok(na, rem)) {
+			if (na->nla_type == TASKSTATS_TYPE_AGGR_PID ||
+			    na->nla_type == TASKSTATS_TYPE_AGGR_TGID) {
+				nested = (struct nlattr *)nla_data(na);
+				nrem = na->nla_len - NLA_HDRLEN;
+				while (nla_ok(nested, nrem)) {
+					if (nested->nla_type == TASKSTATS_TYPE_STATS) {
+						memcpy(stats, nla_data(nested),
+						       sizeof(*stats));
+						return 0;
+					}
+					nested = nla_next(nested, &nrem);
+				}
+			}
+			na = nla_next(na, &rem);
+		}
+	}
+
+	return -ENOENT;
+}
+
+static uint64_t cpu_total(const struct taskstats *stats)
+{
+	return (uint64_t)stats->ac_utime + (uint64_t)stats->ac_stime;
+}
+
+static void print_stats(const char *label, const struct taskstats *stats)
+{
+	ksft_print_msg("%s: cpu_total=%llu nvcsw=%llu nivcsw=%llu\n",
+		       label, (unsigned long long)cpu_total(stats),
+		       (unsigned long long)stats->nvcsw,
+		       (unsigned long long)stats->nivcsw);
+}
+
+static void *worker_thread(void *arg)
+{
+	struct worker_ctx *ctx = arg;
+
+	burn_cpu_for_ns(BUSY_NS);
+
+	pthread_mutex_lock(&ctx->lock);
+	ctx->ready = true;
+	pthread_cond_broadcast(&ctx->cond);
+	while (!ctx->release)
+		pthread_cond_wait(&ctx->cond, &ctx->lock);
+	pthread_mutex_unlock(&ctx->lock);
+
+	return NULL;
+}
+
+int main(void)
+{
+	struct worker_ctx ctx = {
+		.lock = PTHREAD_MUTEX_INITIALIZER,
+		.cond = PTHREAD_COND_INITIALIZER,
+	};
+	struct taskstats before, after;
+	pthread_t thread;
+	pid_t tgid = getpid();
+	int family_id;
+	int fd;
+	int ret;
+
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	if (geteuid())
+		ksft_exit_skip("taskstats_fill_stats_tgid needs root\n");
+
+	fd = netlink_open();
+	if (fd < 0)
+		ksft_exit_skip("failed to open generic netlink socket: %s\n",
+			       strerror(-fd));
+
+	family_id = get_family_id(fd, TASKSTATS_GENL_NAME);
+	if (family_id < 0)
+		ksft_exit_skip("taskstats generic netlink family unavailable: %s\n",
+			       strerror(-family_id));
+
+	/* Create worker thread that burns 200ms of CPU */
+	if (pthread_create(&thread, NULL, worker_thread, &ctx) != 0)
+		ksft_exit_fail_msg("pthread_create failed: %s\n", strerror(errno));
+
+	/* Wait for worker to finish generating activity */
+	pthread_mutex_lock(&ctx.lock);
+	while (!ctx.ready)
+		pthread_cond_wait(&ctx.cond, &ctx.lock);
+	pthread_mutex_unlock(&ctx.lock);
+
+	/*
+	 * Snapshot A: TGID stats while worker is alive and sleeping.
+	 * Contains main thread + worker contributions.
+	 */
+	ret = get_taskstats(fd, family_id, TASKSTATS_CMD_ATTR_TGID, tgid, &before);
+	if (ret)
+		ksft_exit_fail_msg("TGID query before exit failed: %s\n",
+				   strerror(-ret));
+
+	/* Release worker so it can exit, then join (deterministic wait).
+	 *
+	 * Kernel exit path ordering guarantees:
+	 *   do_exit()
+	 *     taskstats_exit() -> fill_tgid_exit()  (accumulates worker into signal->stats)
+	 *     exit_notify()                         (releases the thread)
+	 *     do_task_dead() -> __schedule()        (wakes joiner)
+	 *
+	 * So pthread_join() returns only after fill_tgid_exit() has completed.
+	 */
+	pthread_mutex_lock(&ctx.lock);
+	ctx.release = true;
+	pthread_cond_broadcast(&ctx.cond);
+	pthread_mutex_unlock(&ctx.lock);
+
+	pthread_join(thread, NULL);
+
+	/*
+	 * Snapshot B: TGID stats after worker has exited.
+	 * fill_stats_for_tgid() does:
+	 *   memcpy(signal->stats)   <- includes fill_tgid_exit accumulation
+	 *   + scan live threads      <- only main thread now
+	 */
+	ret = get_taskstats(fd, family_id, TASKSTATS_CMD_ATTR_TGID, tgid, &after);
+	if (ret)
+		ksft_exit_fail_msg("TGID query after exit failed: %s\n",
+				   strerror(-ret));
+
+	print_stats("TGID before worker exit", &before);
+	print_stats("TGID after  worker exit", &after);
+
+	/*
+	 * The worker burned 200ms of CPU before the first snapshot.
+	 * If the kernel correctly retained its contribution via
+	 * fill_tgid_exit(), then the TGID CPU total after exit must be at
+	 * least as large as the TGID CPU total before exit.
+	 */
+	ksft_test_result(cpu_total(&after) >= cpu_total(&before),
+			 "TGID CPU stats should not regress after thread exit\n");
+
+	close(fd);
+	ksft_finished();
+	return ksft_get_fail_cnt() ? KSFT_FAIL : KSFT_PASS;
+}