Add two tests for an arena map's mmap()ed region: arena_split checks
that a partial munmap() that would split the mapping fails with -EINVAL,
and arena_fork checks that a forked child does not inherit the mapping
(mincore() reports the range as unmapped).

Signed-off-by: Ruslan Valiyev <linuxoid@gmail.com>
---
v2:
- Drop the regression-test framing and per-callback details; use
  short one-line comments.
- Split into two separate tests, arena_split (partial unmap) and
  arena_fork.
- Unpack the mincore() child check into explicit statements.

 .../selftests/bpf/prog_tests/arena_fork.c     | 53 +++++++++++++++++++
 .../selftests/bpf/prog_tests/arena_split.c    | 34 ++++++++++++
 2 files changed, 87 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_fork.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/arena_split.c

diff --git a/tools/testing/selftests/bpf/prog_tests/arena_fork.c b/tools/testing/selftests/bpf/prog_tests/arena_fork.c
new file mode 100644
index 0000000000000..bb134e9adb607
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/arena_fork.c
@@ -0,0 +1,53 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <errno.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+
+/* Make sure an arena mapping is not inherited across fork(). */
+
+#define NR_PAGES 3
+
+void test_arena_fork(void)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_MMAPABLE);
+	long ps = sysconf(_SC_PAGESIZE);
+	size_t sz = (size_t)NR_PAGES * ps;
+	void *area;
+	int fd, ret, status;
+	pid_t pid;
+
+	fd = bpf_map_create(BPF_MAP_TYPE_ARENA, "arena_fork", 0, 0, NR_PAGES, &opts);
+	if (!ASSERT_OK_FD(fd, "arena map create"))
+		return;
+
+	area = mmap(NULL, sz, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (!ASSERT_NEQ(area, MAP_FAILED, "mmap arena"))
+		goto close_fd;
+
+	pid = fork();
+	if (pid == 0) {
+		unsigned char vec;
+		int rc;
+
+		/* If the mapping was not inherited the range is unmapped in
+		 * the child, so mincore() fails with ENOMEM. A success means
+		 * the child wrongly inherited the mapping.
+		 */
+		rc = mincore(area, ps, &vec);
+		if (rc == 0)
+			_exit(1);
+		_exit(errno == ENOMEM ? 0 : 2);
+	}
+	if (ASSERT_GE(pid, 0, "fork")) {
+		while ((ret = waitpid(pid, &status, 0)) < 0 && errno == EINTR)
+			;
+		if (ASSERT_EQ(ret, pid, "waitpid"))
+			ASSERT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0,
+				    "child must not inherit arena mapping");
+	}
+
+	munmap(area, sz);
+close_fd:
+	close(fd);
+}
diff --git a/tools/testing/selftests/bpf/prog_tests/arena_split.c b/tools/testing/selftests/bpf/prog_tests/arena_split.c
new file mode 100644
index 0000000000000..8a59cfdb2ef89
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/arena_split.c
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <errno.h>
+#include <sys/mman.h>
+
+/* Make sure an arena mapping cannot be split by a partial munmap(). */
+
+#define NR_PAGES 3
+
+void test_arena_split(void)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, opts, .map_flags = BPF_F_MMAPABLE);
+	long ps = sysconf(_SC_PAGESIZE);
+	size_t sz = (size_t)NR_PAGES * ps;
+	void *area;
+	int fd, ret, err;
+
+	fd = bpf_map_create(BPF_MAP_TYPE_ARENA, "arena_split", 0, 0, NR_PAGES, &opts);
+	if (!ASSERT_OK_FD(fd, "arena map create"))
+		return;
+
+	area = mmap(NULL, sz, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (!ASSERT_NEQ(area, MAP_FAILED, "mmap arena"))
+		goto close_fd;
+
+	ret = munmap((char *)area + ps, ps);
+	err = errno;
+	if (ASSERT_ERR(ret, "split munmap"))
+		ASSERT_EQ(err, EINVAL, "split munmap errno");
+
+	munmap(area, sz);
+close_fd:
+	close(fd);
+}

base-commit: 174914ea551314c52a61713b9c4bde9e42d48073
-- 
2.43.0