netdev - Re: [PATCH bpf-next 1/2] libbpf: Add probe for large INSN limit

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Date:   Mon, 6 Jan 2020 23:52:17 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Michal Rostecki <mrostecki@...e.de>, bpf@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/2] libbpf: Add probe for large INSN limit

On 12/27/19 12:06 PM, Michal Rostecki wrote:
> Introduce a new probe which checks whether kernel has large maximum
> program size (1M) which was increased in commit c04c0d2b968a ("bpf:
> increase complexity limit and maximum program size").
> 
> Based on the similar check in Cilium[0], authored by Daniel Borkmann.
> 
> [0] https://github.com/cilium/cilium/commit/657d0f585afd26232cfa5d4e70b6f64d2ea91596
> 
> Co-authored-by: Daniel Borkmann <daniel@...earbox.net>
> Signed-off-by: Michal Rostecki <mrostecki@...e.de>
> ---
>   tools/lib/bpf/libbpf.h        |  1 +
>   tools/lib/bpf/libbpf.map      |  1 +
>   tools/lib/bpf/libbpf_probes.c | 23 +++++++++++++++++++++++
>   3 files changed, 25 insertions(+)
> 
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index fe592ef48f1b..26bf539f1b3c 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -521,6 +521,7 @@ LIBBPF_API bool bpf_probe_prog_type(enum bpf_prog_type prog_type,
>   LIBBPF_API bool bpf_probe_map_type(enum bpf_map_type map_type, __u32 ifindex);
>   LIBBPF_API bool bpf_probe_helper(enum bpf_func_id id,
>   				 enum bpf_prog_type prog_type, __u32 ifindex);
> +LIBBPF_API bool bpf_probe_large_insn_limit(__u32 ifindex);
>   
>   /*
>    * Get bpf_prog_info in continuous memory
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index e9713a574243..b300d74c921a 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -219,6 +219,7 @@ LIBBPF_0.0.7 {
>   		bpf_object__detach_skeleton;
>   		bpf_object__load_skeleton;
>   		bpf_object__open_skeleton;
> +		bpf_probe_large_insn_limit;
>   		bpf_prog_attach_xattr;
>   		bpf_program__attach;
>   		bpf_program__name;
> diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
> index a9eb8b322671..925f95106a52 100644
> --- a/tools/lib/bpf/libbpf_probes.c
> +++ b/tools/lib/bpf/libbpf_probes.c
> @@ -17,6 +17,8 @@
>   #include "libbpf.h"
>   #include "libbpf_internal.h"
>   
> +#define INSN_REPEAT 4128
> +
>   static bool grep(const char *buffer, const char *pattern)
>   {
>   	return !!strstr(buffer, pattern);
> @@ -321,3 +323,24 @@ bool bpf_probe_helper(enum bpf_func_id id, enum bpf_prog_type prog_type,
>   
>   	return res;
>   }
> +
> +/*
> + * Probe for availability of kernel commit (5.3):
> + *
> + * c04c0d2b968a ("bpf: increase complexity limit and maximum program size")
> + */
> +bool bpf_probe_large_insn_limit(__u32 ifindex)
> +{
> +	struct bpf_insn insns[INSN_REPEAT + 1];
> +	int i;

Looks good, but lets test for 'BPF_MAXINSNS + 1' number of total insns, less
arbitrary than 4128.

> +	for (i = 0; i < INSN_REPEAT; i++)
> +		insns[i] = BPF_MOV64_IMM(BPF_REG_0, 1);
> +	insns[INSN_REPEAT] = BPF_EXIT_INSN();
> +
> +	errno = 0;
> +	probe_load(BPF_PROG_TYPE_SCHED_CLS, insns, ARRAY_SIZE(insns), NULL, 0,
> +		   ifindex);
> +
> +	return errno != E2BIG && errno != EINVAL;
> +}
>