selftests/bpf: Test kfuncs returning arena pointers by value

Cover the by-value struct returns a kfunc may now make: two arena
pointers filling R0:R2, and an arena pointer beside a scalar. Two further
cases drop the tag from one member of a struct and one arm of a union,
and stay rejected naming that member, so what decides is the tag rather
than the member being a pointer. The existing cases for a struct and a
nested struct carrying a plain pointer stay rejected as well.

These cases call the kfuncs from C, so the compiler lowers the by-value
return itself, and a struct or union only lands in R0:R2 with the LLVM 23
BPF ABI. An older clang, and gcc, return it through a hidden pointer in
R1 instead, which shifts the arguments along and fails verification. The
file is therefore guarded on LLVM 23, falling back to a dummy test.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260829061610.1699950-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -1,11 +1,53 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
 #include <test_progs.h>
+#include <bpf/btf.h>
 #include "aggregate_ret_func.skel.h"
 #include "aggregate_ret_kfunc.skel.h"
+#include "aggregate_ret_kfunc_arena.skel.h"
+
+static bool testmod_has_arena_tagged_member(void)
+{
+	struct btf *vmlinux_btf, *module_btf = NULL;
+	const struct btf_type *t;
+	bool tagged = false;
+	__s32 id;
+
+	vmlinux_btf = btf__load_vmlinux_btf();
+	if (!vmlinux_btf)
+		return false;
+
+	module_btf = btf__load_module_btf("bpf_testmod", vmlinux_btf);
+	if (!module_btf)
+		goto out;
+
+	/* prog_test_ret_arena::a is 'void __arena_tag *': PTR -> TYPE_TAG -> void */
+	id = btf__find_by_name_kind(module_btf, "prog_test_ret_arena", BTF_KIND_STRUCT);
+	if (id <= 0)
+		goto out;
+
+	t = btf__type_by_id(module_btf, btf_members(btf__type_by_id(module_btf, id))[0].type);
+	if (!t || !btf_is_ptr(t))
+		goto out;
+
+	t = btf__type_by_id(module_btf, t->type);
+	tagged = t && btf_is_type_tag(t) &&
+		 !strcmp(btf__name_by_offset(module_btf, t->name_off), "arena");
+
+out:
+	btf__free(module_btf);
+	btf__free(vmlinux_btf);
+
+	return tagged;
+}
 
 void test_aggregate_ret(void)
 {
 	RUN_TESTS(aggregate_ret_func);
 	RUN_TESTS(aggregate_ret_kfunc);
+
+	if (testmod_has_arena_tagged_member())
+		RUN_TESTS(aggregate_ret_kfunc_arena);
+	else
+		test__skip();
 }

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

@@ -0,0 +1,121 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf_arena_common.h>
+#include "bpf_misc.h"
+#include "../test_kmods/bpf_testmod_kfunc.h"
+
+#if defined(__clang_major__) && __clang_major__ >= 23
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARENA);
+	__uint(map_flags, BPF_F_MMAPABLE);
+	__uint(max_entries, 2);
+} arena SEC(".maps");
+
+SEC("syscall")
+__arch_x86_64 __arch_arm64
+__load_if_JITed()
+__success __retval(0)
+int aggregate_ret_kfunc_arena(void *ctx)
+{
+	u32 volatile __arena *page = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	u32 volatile __arena *a, *b;
+	struct prog_test_ret_arena r;
+
+	if (!page)
+		return 1;
+
+	/* Both halves come back in R0:R2, pointing at page and page + 4. */
+	r = bpf_kfunc_call_test_ret_arena((u64)page);
+	if (!r.a || !r.b)
+		return 2;
+
+	a = (u32 __arena *)r.a;
+	b = (u32 __arena *)r.b;
+	*a = 1;
+	*b = 2;
+	if (*a != 1)
+		return 3;
+	if (*b != 2)
+		return 4;
+
+	page[0] = 7;
+	if (*a != 7)
+		return 5;
+	page[1] = 9;
+	if (*b != 9)
+		return 6;
+
+	return 0;
+}
+
+SEC("syscall")
+__arch_x86_64 __arch_arm64
+__load_if_JITed()
+__success __retval(0)
+int aggregate_ret_kfunc_arena_mixed(void *ctx)
+{
+	u32 __arena *page = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	struct prog_test_ret_arena_mixed r;
+	u32 volatile __arena *p;
+
+	if (!page)
+		return 1;
+
+	/* An arena pointer in R0 beside a scalar in R2. */
+	r = bpf_kfunc_call_test_ret_arena_mixed((u64)page);
+	if (!r.p)
+		return 2;
+	if (r.tag != 0xbeef)
+		return 3;
+
+	p = (u32 __arena *)r.p;
+	*p = 3;
+	if (*p != 3)
+		return 4;
+
+	return 0;
+}
+
+SEC("syscall")
+__arch_x86_64 __arch_arm64
+__failure __msg("is not composed of scalars or arena pointers")
+__msg("member 'b' has type PTR")
+int aggregate_ret_kfunc_arena_untagged_fail(void *ctx)
+{
+	struct prog_test_ret_arena_untagged r;
+
+	r = bpf_kfunc_call_test_ret_arena_untagged(0);
+
+	return r.a == r.b;
+}
+
+SEC("syscall")
+__arch_x86_64 __arch_arm64
+__failure __msg("is not composed of scalars or arena pointers")
+__msg("member 'b' has type PTR")
+int aggregate_ret_kfunc_arena_union_fail(void *ctx)
+{
+	union prog_test_ret_arena_union r;
+
+	r = bpf_kfunc_call_test_ret_arena_union(0);
+
+	return r.a == r.b;
+}
+
+#else
+
+SEC("socket")
+__description("aggregate_ret_kfunc_arena: needs LLVM 23, dummy test")
+__skip("needs LLVM 23")
+__success
+int dummy_test(void)
+{
+	return 0;
+}
+
+#endif
+
+char _license[] SEC("license") = "GPL";

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

@@ -981,6 +981,34 @@ __bpf_kfunc struct prog_test_ret_ptr bpf_kfunc_call_test_ret_ptr(u64 tag)
 	return r;
 }
 
+__bpf_kfunc struct prog_test_ret_arena bpf_kfunc_call_test_ret_arena(u64 addr)
+{
+	struct prog_test_ret_arena r = { .a = (void *)addr, .b = (void *)(addr + 4) };
+
+	return r;
+}
+
+__bpf_kfunc struct prog_test_ret_arena_mixed bpf_kfunc_call_test_ret_arena_mixed(u64 addr)
+{
+	struct prog_test_ret_arena_mixed r = { .p = (void *)addr, .tag = 0xbeef };
+
+	return r;
+}
+
+__bpf_kfunc struct prog_test_ret_arena_untagged bpf_kfunc_call_test_ret_arena_untagged(u64 addr)
+{
+	struct prog_test_ret_arena_untagged r = { .a = (void *)addr, .b = NULL };
+
+	return r;
+}
+
+__bpf_kfunc union prog_test_ret_arena_union bpf_kfunc_call_test_ret_arena_union(u64 addr)
+{
+	union prog_test_ret_arena_union r = { .a = (void *)addr };
+
+	return r;
+}
+
 __bpf_kfunc struct prog_test_ret_nested bpf_kfunc_call_test_ret_nested(u64 tag)
 {
 	struct prog_test_ret_nested r = { .in = { .p = NULL }, .tag = tag };
@@ -1574,6 +1602,10 @@ BTF_ID_FLAGS(func, bpf_kfunc_call_test_i128)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_pair)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_fastcall, KF_FASTCALL)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_ptr)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_arena)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_arena_mixed)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_arena_untagged)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_arena_union)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_nested)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_ptr_arr)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_ret_arr_struct)

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

@@ -26,6 +26,12 @@ struct prog_test_ref_kfunc {
 };
 #endif
 
+#if __has_attribute(btf_type_tag)
+#define __arena_tag __attribute__((btf_type_tag("arena")))
+#else
+#define __arena_tag
+#endif
+
 struct bpf_iter_testmod_seq;
 
 struct prog_test_pass1 {
@@ -70,6 +76,26 @@ struct prog_test_ret_ptr {	/* 16 bytes: contains a pointer */
 	__u64 tag;
 };
 
+struct prog_test_ret_arena {	/* 16 bytes: two arena pointers */
+	void __arena_tag *a;
+	void __arena_tag *b;
+};
+
+struct prog_test_ret_arena_mixed {	/* 16 bytes: an arena pointer and a scalar */
+	void __arena_tag *p;
+	__u64 tag;
+};
+
+struct prog_test_ret_arena_untagged {	/* 16 bytes: 'b' lacks the arena tag */
+	void __arena_tag *a;
+	void *b;
+};
+
+union prog_test_ret_arena_union {	/* 8 bytes: 'b' lacks the arena tag */
+	void __arena_tag *a;
+	void *b;
+};
+
 struct prog_test_ret_nested {	/* 16 bytes: the pointer hides one level down */
 	struct {
 		void *p;
@@ -200,6 +226,10 @@ struct prog_test_ret_nested bpf_kfunc_call_test_ret_nested(__u64 tag) __ksym;
 struct prog_test_ret_ptr_arr bpf_kfunc_call_test_ret_ptr_arr(void) __ksym;
 struct prog_test_ret_arr_struct bpf_kfunc_call_test_ret_arr_struct(void) __ksym;
 struct prog_test_ret_arr2d bpf_kfunc_call_test_ret_arr2d(void) __ksym;
+struct prog_test_ret_arena bpf_kfunc_call_test_ret_arena(__u64 addr) __ksym;
+struct prog_test_ret_arena_mixed bpf_kfunc_call_test_ret_arena_mixed(__u64 addr) __ksym;
+struct prog_test_ret_arena_untagged bpf_kfunc_call_test_ret_arena_untagged(__u64 addr) __ksym;
+union prog_test_ret_arena_union bpf_kfunc_call_test_ret_arena_union(__u64 addr) __ksym;
 struct prog_test_ret_deep bpf_kfunc_call_test_ret_deep(__u64 v) __ksym;
 struct prog_test_ret_big bpf_kfunc_call_test_ret_big(void) __ksym;
 __u64 bpf_kfunc_call_stack_arg(__u64 a, __u64 b, __u64 c, __u64 d,