netdev - RE: [PATCH bpf] bpf: force .BTF section start to zero when dumping from vmlinux

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Date:   Wed, 27 Nov 2019 20:37:07 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        Stanislav Fomichev <sdf@...gle.com>,
        Andrii Nakryiko <andriin@...com>
Subject: RE: [PATCH bpf] bpf: force .BTF section start to zero when dumping
 from vmlinux

Stanislav Fomichev wrote:
> While trying to figure out why fentry_fexit selftest doesn't pass for me
> (old pahole, broken BTF), I found out that my latest patch can break vmlinux
> .BTF generation. objcopy preserves section start when doing --only-section,
> so there is a chance (depending on where pahole inserts .BTF section) to
> have leading empty zeroes. Let's explicitly force section offset to zero.
> 
> Before:
> $ objcopy --set-section-flags .BTF=alloc -O binary \
> 	--only-section=.BTF vmlinux .btf.vmlinux.bin
> $ xxd .btf.vmlinux.bin | head -n1
> 00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
> 
> After:
> $ objcopy --change-section-address .BTF=0 \
> 	--set-section-flags .BTF=alloc -O binary \
> 	--only-section=.BTF vmlinux .btf.vmlinux.bin
> $ xxd .btf.vmlinux.bin | head -n1
> 00000000: 9feb 0100 1800 0000 0000 0000 80e1 1c00  ................
>           ^BTF magic
> 
> As part of this change, I'm also dropping '2>/dev/null' from objcopy
> invocation to be able to catch possible other issues (objcopy doesn't
> produce any warnings for me anymore, it did before with --dump-section).

Agree dropping /dev/null seems like a good choice. Otherwise seems reasonable
to me.

Acked-by: John Fastabend <john.fastabend@...il.com>