Re: [PATCH bpf-next v5 09/11] selftests/bpf: Add C tests for 16-byte returns in R0:R2

> diff --git a/tools/testing/selftests/bpf/progs/aggregate_ret_struct_c.c b/tools/testing/selftests/bpf/progs/aggregate_ret_struct_c.c
> new file mode 100644
> index 000000000000..83b1a3751988
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/aggregate_ret_struct_c.c

[ ... ]

> +struct pair {
> +	__u64 hi;	/* R0 */
> +	__u64 lo;	/* R2 */
> +};
> +
> +union upair {
> +	__u64 halves[2];
> +	struct {
> +		__u64 lo;	/* R0 */
> +		__u64 hi;	/* R2 */
> +	} parts;
> +};

This isn't a bug, but struct pair puts hi in R0 while union upair's
parts puts lo in R0 - was the inverted naming in the union deliberate,
or would matching the struct's order (or first/second) make the R0:R2
mapping easier to follow?

> +
> +static __noinline struct pair make_pair(__u64 a, __u64 b)
> +{
> +	struct pair p = { .hi = a + b, .lo = a - b };
> +
> +	return p;
> +}
> +
> +SEC("tc")
> +__load_if_JITed()
> +__success __retval(0)
> +int aggregate_ret_struct_c_test(struct __sk_buff *skb)
> +{
> +	__u64 a = skb->len ^ MIX_A;
> +	__u64 b = skb->len ^ MIX_B;
> +	struct pair p;
> +
> +	p = make_pair(a, b);
> +	if (p.hi != a + b)
> +		return 1;
> +	if (p.lo != a - b)
> +		return 2;
> +
> +	return 0;
> +}

[ ... ]

> +static __noinline union upair make_upair(__u64 a, __u64 b)
> +{
> +	union upair p;
> +
> +	p.halves[0] = a + b;
> +	p.halves[1] = a - b;
> +	return p;
> +}
> +
> +SEC("tc")
> +__load_if_JITed()
> +__success __retval(0)
> +int aggregate_ret_union_c_test(struct __sk_buff *skb)
> +{
> +	__u64 a = skb->len ^ MIX_A;
> +	__u64 b = skb->len ^ MIX_B;
> +	union upair p;
> +
> +	p = make_upair(a, b);
> +	if (p.parts.lo != a + b)
> +		return 1;
> +	if (p.parts.hi != a - b)
> +		return 2;
> +
> +	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/31740414277