netdev - Re: [bpf PATCH] samples/bpf: fix broken xdp_rxq

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Date:   Wed, 4 Dec 2019 17:54:56 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf <bpf@...r.kernel.org>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Network Development <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        "Daniel T. Lee" <danieltimlee@...il.com>
Subject: Re: [bpf PATCH] samples/bpf: fix broken xdp_rxq_info due to map order assumptions

On Mon, Dec 2, 2019 at 4:37 AM Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>
> In the days of using bpf_load.c the order in which the 'maps' sections
> were defines in BPF side (*_kern.c) file, were used by userspace side
> to identify the map via using the map order as an index. In effect the
> order-index is created based on the order the maps sections are stored
> in the ELF-object file, by the LLVM compiler.
>
> This have also carried over in libbpf via API bpf_map__next(NULL, obj)
> to extract maps in the order libbpf parsed the ELF-object file.
>
> When BTF based maps were introduced a new section type ".maps" were
> created. I found that the LLVM compiler doesn't create the ".maps"
> sections in the order they are defined in the C-file. The order in the
> ELF file is based on the order the map pointer is referenced in the code.
>
> This combination of changes lead to xdp_rxq_info mixing up the map
> file-descriptors in userspace, resulting in very broken behaviour, but
> without warning the user.
>
> This patch fix issue by instead using bpf_object__find_map_by_name()
> to find maps via their names. (Note, this is the ELF name, which can
> be longer than the name the kernel retains).
>
> Fixes: be5bca44aa6b ("samples: bpf: convert some XDP samples from bpf_load to libbpf")
> Fixes: 451d1dc886b5 ("samples: bpf: update map definition to new syntax BTF-defined map")
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>

Applied. Thanks