netdev - Re: [PATCH bpf-next v3 1/3] libbpf: add helpers for preparing netlink attributes

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Date:   Tue, 20 Apr 2021 23:37:48 -0700
From:   Yonghong Song <yhs@...com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>, <bpf@...r.kernel.org>
CC:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 1/3] libbpf: add helpers for preparing netlink
 attributes



On 4/20/21 12:37 PM, Kumar Kartikeya Dwivedi wrote:
> This change introduces a few helpers to wrap open coded attribute
> preparation in netlink.c.
> 
> Every nested attribute's closure must happen using the helper
> nlattr_end_nested, which sets its length properly. NLA_F_NESTED is
> enforeced using nlattr_begin_nested helper. Other simple attributes

typo: enforced

> can be added directly.
> 
> The maxsz parameter corresponds to the size of the request structure
> which is being filled in, so for instance with req being:
> 
> struct {
> 	struct nlmsghdr nh;
> 	struct tcmsg t;
> 	char buf[4096];
> } req;
> 
> Then, maxsz should be sizeof(req).
> 
> This change also converts the open coded attribute preparation with the
> helpers. Note that the only failure the internal call to nlattr_add
> could result in the nested helper would be -EMSGSIZE, hence that is what
> we return to our caller.
> 
> Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>   tools/lib/bpf/netlink.c | 37 ++++++++++++++-----------------
>   tools/lib/bpf/nlattr.h  | 48 +++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 64 insertions(+), 21 deletions(-)
> 
[...]