netdev - Re: [PATCH v7 bpf-next 7/7] selftests: bpf: add dummy prog for bpf2bpf with tailcall

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Date:   Thu, 3 Sep 2020 12:51:14 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     ast@...nel.org, daniel@...earbox.net, bpf@...r.kernel.org,
        netdev@...r.kernel.org, bjorn.topel@...el.com,
        magnus.karlsson@...el.com
Subject: Re: [PATCH v7 bpf-next 7/7] selftests: bpf: add dummy prog for
 bpf2bpf with tailcall

On Wed, Sep 02, 2020 at 10:08:15PM +0200, Maciej Fijalkowski wrote:
> diff --git a/tools/testing/selftests/bpf/progs/tailcall6.c b/tools/testing/selftests/bpf/progs/tailcall6.c
> new file mode 100644
> index 000000000000..e72ca5869b58
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/tailcall6.c
> @@ -0,0 +1,38 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +
> +struct {
> +	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
> +	__uint(max_entries, 2);
> +	__uint(key_size, sizeof(__u32));
> +	__uint(value_size, sizeof(__u32));
> +} jmp_table SEC(".maps");
> +
> +#define TAIL_FUNC(x) 				\
> +	SEC("classifier/" #x)			\
> +	int bpf_func_##x(struct __sk_buff *skb)	\
> +	{					\
> +		return x;			\
> +	}
> +TAIL_FUNC(0)
> +TAIL_FUNC(1)
> +
> +static __attribute__ ((noinline))
> +int subprog_tail(struct __sk_buff *skb)
> +{
> +	bpf_tail_call(skb, &jmp_table, 0);
> +
> +	return skb->len * 2;
> +}
> +
> +SEC("classifier")
> +int entry(struct __sk_buff *skb)
> +{
> +	bpf_tail_call(skb, &jmp_table, 1);
> +
> +	return subprog_tail(skb);
> +}

Could you add few more tests to exercise the new feature more thoroughly?
Something like tailcall3.c that checks 32 limit, but doing tail_call from subprog.
And another test that consume non-trival amount of stack in each function.
Adding 'volatile char arr[128] = {};' would do the trick.