netdev - Re: [net-next PATCH V1] samples/bpf: bpf_load.c detect and abort if ELF maps section size is wrong

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Date:   Fri, 28 Apr 2017 20:35:21 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     kafai@...com, netdev@...r.kernel.org, eric@...it.org,
        Daniel Borkmann <borkmann@...earbox.net>
Subject: Re: [net-next PATCH V1] samples/bpf: bpf_load.c detect and abort if
 ELF maps section size is wrong

On Fri, Apr 28, 2017 at 04:25:04PM +0200, Jesper Dangaard Brouer wrote:
> The struct bpf_map_def was extended in commit fb30d4b71214 ("bpf: Add tests
> for map-in-map") with member unsigned int inner_map_idx.  This changed the size
> of the maps section in the generated ELF _kern.o files.
> 
> Unfortunately the loader in bpf_load.c does not detect or handle this.  Thus,
> older _kern.o files became incompatible, and caused hard-to-debug errors
> where the syscall validation rejected BPF_MAP_CREATE request.
> 
> This patch only detect the situation and aborts load_bpf_file(). It also
> add code comments warning people that read this loader for inspiration
> for these pitfalls.
> 
> Fixes: fb30d4b71214 ("bpf: Add tests for map-in-map")
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>

Thanks!
Acked-by: Alexei Starovoitov <ast@...nel.org>

> Is it worth to implement proper backward-compat loading of older ELF objects
> with this bpf-loader?

probably yes, since it looks like a bunch of code in samples/bpf/ still
depend on it and some features are missing in tools/lib/bpf,
so unless we actively work on improving libbpf.a 
we won't be able to get rid of this 'sample' loader for some time.