Re: [PATCH bpf-next v3 10/11] selftests/bpf: Test kfuncs returning arena pointers by value

[Yonghong Song <yonghong.song@linux.dev>]

On 8/27/26 12:17 AM, bot+bpf-ci@kernel.org wrote:
>> diff --git a/tools/testing/selftests/bpf/progs/aggregate_ret_kfunc_arena.c b/tools/testing/selftests/bpf/progs/aggregate_ret_kfunc_arena.c
>> new file mode 100644
>> index 0000000000000..94c35e1b547cd
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/progs/aggregate_ret_kfunc_arena.c
>> @@ -0,0 +1,129 @@
>> +// 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");
>> +
>> +/*
>> + * A returned member carries the arena type tag but not the address space
>> + * qualifier, so the program casts it into the arena address space itself
>> + * rather than the compiler doing it.
>> + */
>> +#define arena_ptr(p) ((u32 volatile __arena *)(p))
> This isn't a bug, but could the cast be spelled out at each use site, or
> the macro named after the type it produces (e.g. arena_u32p())? The name
> arena_ptr() reads as a generic address-space cast, but it hardcodes 'u32
> volatile', which matters for the load/store widths the test checks. Other
> arena selftest programs write the cast inline with the type visible at the
> use site (for example '(char __arena *)ptr' in progs/arena_mem_usage.c).

Okay, let us inline arena_ptr(p) directly in the code.

>
>> +
>> +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 = arena_ptr(r.a);
>> +	b = arena_ptr(r.b);
>> +	*a = 1;
>> +	*b = 2;
>> +	if (*a != 1)
>> +		return 3;
>> +	if (*b != 2)
>> +		return 4;
>> +
>> +	/* The halves are the first two slots of the page the program allocated. */
>> +	page[0] = 7;
>> +	if (*a != 7)
>> +		return 5;
>> +	page[1] = 9;
>> +	if (*b != 9)
>> +		return 6;
>> +
>> +	return 0;
>> +}
> [ ... ]
>
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/33045482525