netdev - Re: [RFC][PATCH bpf-next] libbpf: add bpf_object__open

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Message-ID: <20191001184813.23d004d2@carbon>
Date:   Tue, 1 Oct 2019 18:48:13 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     brouer@...hat.com, <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        <ast@...com>, <daniel@...earbox.net>, <toke@...hat.com>,
        <kpsingh@...omium.org>, <andrii.nakryiko@...il.com>,
        <kernel-team@...com>
Subject: Re: [RFC][PATCH bpf-next] libbpf: add bpf_object__open_{file,mem}
 w/ sized opts

On Mon, 30 Sep 2019 09:42:39 -0700
Andrii Nakryiko <andriin@...com> wrote:

> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 2e83a34f8c79..1cf2cf8d80f3 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -47,6 +47,12 @@ do {				\
>  #define pr_info(fmt, ...)	__pr(LIBBPF_INFO, fmt, ##__VA_ARGS__)
>  #define pr_debug(fmt, ...)	__pr(LIBBPF_DEBUG, fmt, ##__VA_ARGS__)
>  
> +#define OPTS_VALID(opts) (!(opts) || (opts)->sz >= sizeof((opts)->sz))

Do be aware that C sizeof() will include the padding the compiler does.
Thus, when extending a struct (e.g. in a newer version) the size
(sizeof) might not actually increase (if compiler padding room exist).

> +#define OPTS_HAS(opts, field) \
> +	((opts) && opts->sz >= offsetofend(typeof(*(opts)), field))
> +#define OPTS_GET(opts, field, fallback_value) \
> +	(OPTS_HAS(opts, field) ? (opts)->field : fallback_value)

I do think, that these two "accessor" defines address the padding issue
I described above.

p.s. I appreciate that you are working on this, and generally like the idea.
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer