selftests/bpf: Add a test for bpf_cgroup_from_id lookup in non-root cgns

Make sure that we only switch the cgroup namespace and enter a new
cgroup in a child process separate from test_progs, to not mess up the
environment for subsequent tests.

To remove this cgroup, we need to wait for the child to exit, and then
rmdir its cgroup. If the read call fails, or waitpid succeeds, we know
the child exited (read call would fail when the last pipe end is closed,
otherwise waitpid waits until exit(2) is called). We then invoke a newly
introduced remove_cgroup_pid() helper, that identifies cgroup path using
the passed in pid of the now dead child, instead of using the current
process pid (getpid()).

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20250915032618.1551762-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 71 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c b/tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c
index adda85f97058..4b42fbc96efc 100644
--- a/tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c
+++ b/tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c
@@ -4,6 +4,8 @@
 #define _GNU_SOURCE
 #include <cgroup_helpers.h>
 #include <test_progs.h>
+#include <sched.h>
+#include <sys/wait.h>
 
 #include "cgrp_kfunc_failure.skel.h"
 #include "cgrp_kfunc_success.skel.h"
@@ -87,6 +89,72 @@ static const char * const success_tests[] = {
 	"test_cgrp_from_id",
 };
 
+static void test_cgrp_from_id_ns(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct cgrp_kfunc_success *skel;
+	struct bpf_program *prog;
+	int pid, pipe_fd[2];
+
+	skel = open_load_cgrp_kfunc_skel();
+	if (!ASSERT_OK_PTR(skel, "open_load_skel"))
+		return;
+
+	if (!ASSERT_OK(skel->bss->err, "pre_mkdir_err"))
+		goto cleanup;
+
+	prog = skel->progs.test_cgrp_from_id_ns;
+
+	if (!ASSERT_OK(pipe(pipe_fd), "pipe"))
+		goto cleanup;
+
+	pid = fork();
+	if (!ASSERT_GE(pid, 0, "fork result")) {
+		close(pipe_fd[0]);
+		close(pipe_fd[1]);
+		goto cleanup;
+	}
+
+	if (pid == 0) {
+		int ret = 0;
+
+		close(pipe_fd[0]);
+
+		if (!ASSERT_GE(cgroup_setup_and_join("cgrp_from_id_ns"), 0, "join cgroup"))
+			exit(1);
+
+		if (!ASSERT_OK(unshare(CLONE_NEWCGROUP), "unshare cgns"))
+			exit(1);
+
+		ret = bpf_prog_test_run_opts(bpf_program__fd(prog), &opts);
+		if (!ASSERT_OK(ret, "test run ret"))
+			exit(1);
+
+		if (!ASSERT_OK(opts.retval, "test run retval"))
+			exit(1);
+
+		if (!ASSERT_EQ(write(pipe_fd[1], &ret, sizeof(ret)), sizeof(ret), "write pipe"))
+			exit(1);
+
+		exit(0);
+	} else {
+		int res;
+
+		close(pipe_fd[1]);
+
+		ASSERT_EQ(read(pipe_fd[0], &res, sizeof(res)), sizeof(res), "read res");
+		ASSERT_EQ(waitpid(pid, NULL, 0), pid, "wait on child");
+
+		remove_cgroup_pid("cgrp_from_id_ns", pid);
+
+		ASSERT_OK(res, "result from run");
+	}
+
+	close(pipe_fd[0]);
+cleanup:
+	cgrp_kfunc_success__destroy(skel);
+}
+
 void test_cgrp_kfunc(void)
 {
 	int i, err;
@@ -102,6 +170,9 @@ void test_cgrp_kfunc(void)
 		run_success_test(success_tests[i]);
 	}
 
+	if (test__start_subtest("test_cgrp_from_id_ns"))
+		test_cgrp_from_id_ns();
+
 	RUN_TESTS(cgrp_kfunc_failure);
 
 cleanup: