selftests/bpf: Test returning referenced kptr from struct_ops programs

Test struct_ops programs returning referenced kptr. When the return type
of a struct_ops operator is pointer to struct, the verifier should
only allow programs that return a scalar NULL or a non-local kptr with the
correct type in its unmodified form.

Signed-off-by: Amery Hung <amery.hung@bytedance.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20250217190640.1748177-6-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/test_struct_ops_kptr_return.c

@@ -0,0 +1,16 @@
+#include <test_progs.h>
+
+#include "struct_ops_kptr_return.skel.h"
+#include "struct_ops_kptr_return_fail__wrong_type.skel.h"
+#include "struct_ops_kptr_return_fail__invalid_scalar.skel.h"
+#include "struct_ops_kptr_return_fail__nonzero_offset.skel.h"
+#include "struct_ops_kptr_return_fail__local_kptr.skel.h"
+
+void test_struct_ops_kptr_return(void)
+{
+	RUN_TESTS(struct_ops_kptr_return);
+	RUN_TESTS(struct_ops_kptr_return_fail__wrong_type);
+	RUN_TESTS(struct_ops_kptr_return_fail__invalid_scalar);
+	RUN_TESTS(struct_ops_kptr_return_fail__nonzero_offset);
+	RUN_TESTS(struct_ops_kptr_return_fail__local_kptr);
+}

tools/testing/selftests/bpf/progs/struct_ops_kptr_return.c

@@ -0,0 +1,30 @@
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "../test_kmods/bpf_testmod.h"
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+void bpf_task_release(struct task_struct *p) __ksym;
+
+/* This test struct_ops BPF programs returning referenced kptr. The verifier should
+ * allow a referenced kptr or a NULL pointer to be returned. A referenced kptr to task
+ * here is acquried automatically as the task argument is tagged with "__ref".
+ */
+SEC("struct_ops/test_return_ref_kptr")
+struct task_struct *BPF_PROG(kptr_return, int dummy,
+			     struct task_struct *task, struct cgroup *cgrp)
+{
+	if (dummy % 2) {
+		bpf_task_release(task);
+		return NULL;
+	}
+	return task;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops testmod_kptr_return = {
+	.test_return_ref_kptr = (void *)kptr_return,
+};
+
+

tools/testing/selftests/bpf/progs/struct_ops_kptr_return_fail__invalid_scalar.c

@@ -0,0 +1,26 @@
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "../test_kmods/bpf_testmod.h"
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct cgroup *bpf_cgroup_acquire(struct cgroup *p) __ksym;
+void bpf_task_release(struct task_struct *p) __ksym;
+
+/* This test struct_ops BPF programs returning referenced kptr. The verifier should
+ * reject programs returning a non-zero scalar value.
+ */
+SEC("struct_ops/test_return_ref_kptr")
+__failure __msg("At program exit the register R0 has smin=1 smax=1 should have been in [0, 0]")
+struct task_struct *BPF_PROG(kptr_return_fail__invalid_scalar, int dummy,
+			     struct task_struct *task, struct cgroup *cgrp)
+{
+	bpf_task_release(task);
+	return (struct task_struct *)1;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops testmod_kptr_return = {
+	.test_return_ref_kptr = (void *)kptr_return_fail__invalid_scalar,
+};

tools/testing/selftests/bpf/progs/struct_ops_kptr_return_fail__local_kptr.c

@@ -0,0 +1,34 @@
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "../test_kmods/bpf_testmod.h"
+#include "bpf_experimental.h"
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct cgroup *bpf_cgroup_acquire(struct cgroup *p) __ksym;
+void bpf_task_release(struct task_struct *p) __ksym;
+
+/* This test struct_ops BPF programs returning referenced kptr. The verifier should
+ * reject programs returning a local kptr.
+ */
+SEC("struct_ops/test_return_ref_kptr")
+__failure __msg("At program exit the register R0 is not a known value (ptr_or_null_)")
+struct task_struct *BPF_PROG(kptr_return_fail__local_kptr, int dummy,
+			     struct task_struct *task, struct cgroup *cgrp)
+{
+	struct task_struct *t;
+
+	bpf_task_release(task);
+
+	t = bpf_obj_new(typeof(*task));
+	if (!t)
+		return NULL;
+
+	return t;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops testmod_kptr_return = {
+	.test_return_ref_kptr = (void *)kptr_return_fail__local_kptr,
+};

tools/testing/selftests/bpf/progs/struct_ops_kptr_return_fail__nonzero_offset.c

@@ -0,0 +1,25 @@
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "../test_kmods/bpf_testmod.h"
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct cgroup *bpf_cgroup_acquire(struct cgroup *p) __ksym;
+void bpf_task_release(struct task_struct *p) __ksym;
+
+/* This test struct_ops BPF programs returning referenced kptr. The verifier should
+ * reject programs returning a modified referenced kptr.
+ */
+SEC("struct_ops/test_return_ref_kptr")
+__failure __msg("dereference of modified trusted_ptr_ ptr R0 off={{[0-9]+}} disallowed")
+struct task_struct *BPF_PROG(kptr_return_fail__nonzero_offset, int dummy,
+			     struct task_struct *task, struct cgroup *cgrp)
+{
+	return (struct task_struct *)&task->jobctl;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops testmod_kptr_return = {
+	.test_return_ref_kptr = (void *)kptr_return_fail__nonzero_offset,
+};

tools/testing/selftests/bpf/progs/struct_ops_kptr_return_fail__wrong_type.c

@@ -0,0 +1,30 @@
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "../test_kmods/bpf_testmod.h"
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct cgroup *bpf_cgroup_acquire(struct cgroup *p) __ksym;
+void bpf_task_release(struct task_struct *p) __ksym;
+
+/* This test struct_ops BPF programs returning referenced kptr. The verifier should
+ * reject programs returning a referenced kptr of the wrong type.
+ */
+SEC("struct_ops/test_return_ref_kptr")
+__failure __msg("At program exit the register R0 is not a known value (ptr_or_null_)")
+struct task_struct *BPF_PROG(kptr_return_fail__wrong_type, int dummy,
+			     struct task_struct *task, struct cgroup *cgrp)
+{
+	struct task_struct *ret;
+
+	ret = (struct task_struct *)bpf_cgroup_acquire(cgrp);
+	bpf_task_release(task);
+
+	return ret;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops testmod_kptr_return = {
+	.test_return_ref_kptr = (void *)kptr_return_fail__wrong_type,
+};

tools/testing/selftests/bpf/test_kmods/bpf_testmod.c

@@ -1182,11 +1182,19 @@ static int bpf_testmod_ops__test_refcounted(int dummy,
 	return 0;
 }
 
+static struct task_struct *
+bpf_testmod_ops__test_return_ref_kptr(int dummy, struct task_struct *task__ref,
+				      struct cgroup *cgrp)
+{
+	return NULL;
+}
+
 static struct bpf_testmod_ops __bpf_testmod_ops = {
 	.test_1 = bpf_testmod_test_1,
 	.test_2 = bpf_testmod_test_2,
 	.test_maybe_null = bpf_testmod_ops__test_maybe_null,
 	.test_refcounted = bpf_testmod_ops__test_refcounted,
+	.test_return_ref_kptr = bpf_testmod_ops__test_return_ref_kptr,
 };
 
 struct bpf_struct_ops bpf_bpf_testmod_ops = {

tools/testing/selftests/bpf/test_kmods/bpf_testmod.h

@@ -6,6 +6,7 @@
 #include <linux/types.h>
 
 struct task_struct;
+struct cgroup;
 
 struct bpf_testmod_test_read_ctx {
 	char *buf;
@@ -38,6 +39,9 @@ struct bpf_testmod_ops {
 	int (*unsupported_ops)(void);
 	/* Used to test ref_acquired arguments. */
 	int (*test_refcounted)(int dummy, struct task_struct *task);
+	/* Used to test returning referenced kptr. */
+	struct task_struct *(*test_return_ref_kptr)(int dummy, struct task_struct *task,
+						    struct cgroup *cgrp);
 
 	/* The following fields are used to test shadow copies. */
 	char onebyte;