zdtm: Check dead pidfd is restored correctly

After, C/R of pidfds that point to dead processes their inodes might
change. But if two pidfds point to same dead process they should
continue to do so after C/R.

This test ensures that this happens by calling `statx()` on pidfds after
C/R and then comparing their inode numbers.

Support for comparing pidfds by using `statx()` and inode numbers was
introduced alongside pidfs. So if `f_type` of pidfd is not equal to
`PID_FS_MAGIC` then we skip this test.

signed-off-by: Bhavik Sachdev <b.sachdev1904@gmail.com>

test/zdtm/static/Makefile

@@ -54,6 +54,7 @@ TST_NOFILE	:=				\
 		shm-mp				\
 		ptrace_sig			\
 		pidfd_self			\
+		pidfd_dead			\
 		pidfd_child			\
 		pidfd_kill			\
 		pipe00				\

test/zdtm/static/pidfd_dead.c

@@ -0,0 +1,244 @@
+#include <sys/statfs.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+
+#include "zdtmtst.h"
+
+const char *test_doc = "Check C/R of pidfds that point to dead processes\n";
+const char *test_author = "Bhavik Sachdev <b.sachdev1904@gmail.com>";
+
+#ifndef PID_FS_MAGIC
+#define PID_FS_MAGIC 0x50494446
+#endif
+
+/*
+ * main
+ *	`- child
+ *		`- grandchild
+ *
+ * main opens a pidfd for both child and grandchild.
+ * Before C/R we kill both child and grandchild.
+ * We end up with two unique dead pidfds.
+ */
+
+static long get_fs_type(int lfd)
+{
+	struct statfs fst;
+
+	if (fstatfs(lfd, &fst)) {
+		return -1;
+	}
+	return fst.f_type;
+}
+
+static int pidfd_open(pid_t pid, unsigned int flags)
+{
+	return syscall(__NR_pidfd_open, pid, flags);
+}
+
+static int pidfd_send_signal(int pidfd, int sig, siginfo_t* info, unsigned int flags)
+{
+	return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
+}
+
+static int open_pidfd_pair(int pidfd[2], int pid)
+{
+	pidfd[0] = pidfd_open(pid, 0);
+	if (pidfd[0] < 0) {
+		pr_perror("pidfd_open() failed");
+		return 1;
+	}
+
+	pidfd[1] = pidfd_open(pid, 0);
+	if (pidfd[1] < 0) {
+		close(pidfd[0]);
+		pr_perror("pidfd_open() failed");
+		return 1;
+	}
+	return 0;
+}
+
+static int compare_pidfds(int pidfd[2])
+{
+	/*
+	* After linux 6.9 we can compare inode numbers
+	* to determine if two pidfds point to the same process.
+	* While the inode number may change before and after C/R
+	* pidfds pointing to the same pid should have the same inode number.
+	*/
+	struct statx stats[2];
+	statx(pidfd[0], "", AT_EMPTY_PATH, STATX_ALL, &stats[0]);
+	statx(pidfd[1], "", AT_EMPTY_PATH, STATX_ALL, &stats[1]);
+	if (stats[0].stx_ino != stats[1].stx_ino)
+		return 1;
+	return 0;
+}
+
+static int check_for_pidfs(void)
+{
+	long type;
+	int pidfd = pidfd_open(getpid(), 0);
+	if (pidfd < 0) {
+		pr_perror("pidfd open() failed");
+		return -1;
+	}
+	type = get_fs_type(pidfd);
+	close(pidfd);
+	return type == PID_FS_MAGIC;
+}
+
+int main(int argc, char* argv[])
+{
+	#define READ 0
+	#define WRITE 1
+
+	int child, ret, gchild, p[2], status;
+	int cpidfd[2], gpidfd[2];
+	struct statx stats[2];
+
+	test_init(argc, argv);
+
+	ret = check_for_pidfs();
+	if (ret < 0)
+		return 1;
+
+	if (ret == 0) {
+		test_daemon();
+		test_waitsig();
+		skip("Test requires pidfs. skipping...");
+		pass();
+		return 0;
+	}
+
+	if (pipe(p)) {
+		pr_perror("pipe");
+		return 1;
+	}
+
+	child = test_fork();
+	if (child < 0) {
+		pr_perror("fork");
+		return 1;
+	} else if (child == 0) {
+		int gchild = test_fork();
+		close(p[READ]);
+		if (gchild < 0) {
+			pr_perror("fork");
+			return 1;
+		} else if (gchild == 0) {
+			close(p[WRITE]);
+			while(1)
+				sleep(1000);
+		} else {
+			if (write(p[WRITE], &gchild, sizeof(int)) != sizeof(int)) {
+				pr_perror("write");
+				return 1;
+			}
+			close(p[WRITE]);
+			if (waitpid(gchild, &status, 0) != gchild) {
+				pr_perror("waitpid");
+				return 1;
+			}
+
+			if (!WIFSIGNALED(status)) {
+				fail("Expected grandchild to be terminated by a signal");
+				return 1;
+			}
+
+			if (WTERMSIG(status) != SIGKILL) {
+				fail("Expected grandchild to be terminated by SIGKILL");
+				return 1;
+			}
+
+			return 0;
+		}
+	}
+
+	ret = open_pidfd_pair(cpidfd, child);
+	if (ret)
+		return 1;
+
+	close(p[WRITE]);
+	if (read(p[READ], &gchild, sizeof(int)) != sizeof(int)) {
+		pr_perror("write");
+		return 1;
+	}
+	close(p[READ]);
+
+	ret = open_pidfd_pair(gpidfd, gchild);
+	if (ret)
+		return 1;
+
+	/*
+	* We kill grandchild and child processes only after opening pidfds.
+	*/
+	if (pidfd_send_signal(gpidfd[0], SIGKILL, NULL, 0)) {
+		pr_perror("pidfd_send_signal");
+		goto fail_close;
+	}
+
+	if (waitpid(child, &status, 0) != child) {
+		pr_perror("waitpid");
+		goto fail_close;
+	}
+
+	if (!WIFEXITED(status)) {
+		fail("Expected child to exit normally");
+		goto fail_close;
+	}
+
+	if (WEXITSTATUS(status) != 0) {
+		fail("Expected child to exit with 0");
+		goto fail_close;
+	}
+	usleep(1000);
+
+	if (kill(gchild, 0) != -1 && errno != ESRCH) {
+		fail("Expected grand child to not exist");
+		goto fail_close;
+	}
+
+	if (kill(child, 0) != -1 && errno != ESRCH) {
+		fail("Expected child to not exist");
+		goto fail_close;
+	}
+
+	test_daemon();
+	test_waitsig();
+
+	ret = compare_pidfds(cpidfd);
+	if (ret) {
+		fail("inodes not same for same pid");
+		goto fail_close;
+	}
+
+	ret = compare_pidfds(gpidfd);
+	if (ret) {
+		fail("inodes not same for same pid");
+		goto fail_close;
+	}
+
+	statx(cpidfd[0], "", AT_EMPTY_PATH, STATX_ALL, &stats[0]);
+	statx(gpidfd[0], "", AT_EMPTY_PATH, STATX_ALL, &stats[1]);
+	if (stats[0].stx_ino == stats[1].stx_ino) {
+		fail("pidfds pointing to diff pids should have diff inodes");
+		goto fail_close;
+	}
+
+	pass();
+	close(cpidfd[0]);
+	close(cpidfd[1]);
+	close(gpidfd[0]);
+	close(gpidfd[1]);
+	return 0;
+
+fail_close:
+	close(cpidfd[0]);
+	close(cpidfd[1]);
+	close(gpidfd[0]);
+	close(gpidfd[1]);
+	return 1;
+}