Add tests to assert that PIDFD_SELF correctly refers to the current
process.

This is only practically meaningful to pidfd_send_signal() and
pidfd_getfd(), but also explicitly test that we disallow this feature for
setns() where it would make no sense.

We cannot reasonably wait on ourself using waitid(P_PIDFD, ...) so while in
theory PIDFD_SELF would work here, we'd be left blocked if we tried it.

We defer testing of mm-specific functionality which uses pidfd, namely
process_madvise() and process_mrelease() to mm testing (though note the
latter can not be sensibly tested as it would require the testing process
to be dying).

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 tools/testing/selftests/pidfd/pidfd.h         |  5 +++
 .../selftests/pidfd/pidfd_getfd_test.c        | 38 +++++++++++++++++++
 .../selftests/pidfd/pidfd_setns_test.c        |  6 +++
 tools/testing/selftests/pidfd/pidfd_test.c    | 13 +++++++
 4 files changed, 62 insertions(+)

diff --git a/tools/testing/selftests/pidfd/pidfd.h b/tools/testing/selftests/pidfd/pidfd.h
index 88d6830ee004..099ee7178193 100644
--- a/tools/testing/selftests/pidfd/pidfd.h
+++ b/tools/testing/selftests/pidfd/pidfd.h
@@ -50,6 +50,11 @@
 #define PIDFD_NONBLOCK O_NONBLOCK
 #endif
 
+/* System header file may not have this available. */
+#ifndef PIDFD_SELF
+#define PIDFD_SELF -200
+#endif
+
 /*
  * The kernel reserves 300 pids via RESERVED_PIDS in kernel/pid.c
  * That means, when it wraps around any pid < 300 will be skipped.
diff --git a/tools/testing/selftests/pidfd/pidfd_getfd_test.c b/tools/testing/selftests/pidfd/pidfd_getfd_test.c
index cd51d547b751..cdf375fd61b2 100644
--- a/tools/testing/selftests/pidfd/pidfd_getfd_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_getfd_test.c
@@ -15,6 +15,7 @@
 #include <sys/prctl.h>
 #include <sys/wait.h>
 #include <unistd.h>
+#include <sys/mman.h>
 #include <sys/socket.h>
 #include <linux/kcmp.h>
 
@@ -264,6 +265,43 @@ TEST_F(child, no_strange_EBADF)
 	EXPECT_EQ(errno, ESRCH);
 }
 
+TEST(pidfd_self)
+{
+	int memfd = sys_memfd_create("test_self", 0);
+	long page_size = sysconf(_SC_PAGESIZE);
+	int newfd;
+	char *ptr;
+
+	ASSERT_GE(memfd, 0);
+	ASSERT_EQ(ftruncate(memfd, page_size), 0);
+
+	/*
+	 * Map so we can assert that the duplicated fd references the same
+	 * memory.
+	 */
+	ptr = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
+		   MAP_SHARED, memfd, 0);
+	ASSERT_NE(ptr, MAP_FAILED);
+	ptr[0] = 'x';
+	ASSERT_EQ(munmap(ptr, page_size), 0);
+
+	/* Now get a duplicate of our memfd. */
+	newfd = sys_pidfd_getfd(PIDFD_SELF, memfd, 0);
+	ASSERT_GE(newfd, 0);
+	ASSERT_NE(memfd, newfd);
+
+	/* Now map duplicate fd and make sure it references the same memory. */
+	ptr = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
+		   MAP_SHARED, newfd, 0);
+	ASSERT_NE(ptr, MAP_FAILED);
+	ASSERT_EQ(ptr[0], 'x');
+	ASSERT_EQ(munmap(ptr, page_size), 0);
+
+	/* Cleanup. */
+	close(memfd);
+	close(newfd);
+}
+
 #if __NR_pidfd_getfd == -1
 int main(void)
 {
diff --git a/tools/testing/selftests/pidfd/pidfd_setns_test.c b/tools/testing/selftests/pidfd/pidfd_setns_test.c
index 7c2a4349170a..8e1510744aaa 100644
--- a/tools/testing/selftests/pidfd/pidfd_setns_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_setns_test.c
@@ -752,4 +752,10 @@ TEST(setns_einval)
 	close(fd);
 }
 
+TEST(setns_pidfd_self_disallowed)
+{
+	ASSERT_EQ(setns(PIDFD_SELF, 0), -1);
+	EXPECT_EQ(errno, EBADF);
+}
+
 TEST_HARNESS_MAIN
diff --git a/tools/testing/selftests/pidfd/pidfd_test.c b/tools/testing/selftests/pidfd/pidfd_test.c
index 9faa686f90e4..18802b657352 100644
--- a/tools/testing/selftests/pidfd/pidfd_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_test.c
@@ -85,6 +85,19 @@ static int test_pidfd_send_signal_simple_success(void)
 				   test_name);
 
 	signal_received = 0;
+
+	/* Now try the same thing only using PIDFD_SELF. */
+	ret = sys_pidfd_send_signal(PIDFD_SELF, SIGUSR1, NULL, 0);
+	if (ret < 0)
+		ksft_exit_fail_msg("%s test: Failed to send PIDFD_SELF signal\n",
+				   test_name);
+
+	if (signal_received != 1)
+		ksft_exit_fail_msg("%s test: Failed to receive PIDFD_SELF signal\n",
+				   test_name);
+
+	signal_received = 0;
+
 	ksft_test_result_pass("%s test: Sent signal\n", test_name);
 	return 0;
 }
-- 
2.46.2