linux-kernel - Re: linux-next: build warning after merge of the bpf-next tree

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Message-ID: <6c03d212-775c-cddb-b0d0-d7b00571694b@ghiti.fr>
Date:   Wed, 15 Jan 2020 15:48:18 -0500
From:   Alexandre Ghiti <alexandre@...ti.fr>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Zong Li <zong.li@...ive.com>
Cc:     Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: linux-next: build warning after merge of the bpf-next tree

On 1/14/20 6:23 AM, Alexei Starovoitov wrote:
> On Sun, Jan 12, 2020 at 8:33 PM Zong Li<zong.li@...ive.com>  wrote:
>> I'm not quite familiar with btf, so I have no idea why there are two
>> weak symbols be added in 8580ac9404f6 ("bpf: Process in-kernel BTF")
> I can explain what these weak symbols are for, but that won't change
> the fact that compiler or linker are buggy. The weak symbols should work
> in all cases and compiler should pick correct relocation.
> In this case it sounds that compiler picked relative relocation and failed
> to reach zero from that address.

Sorry for the response delay: I now agree that there is nothing weird 
about those
relocations. All compiler/linker I took a look at (arm64, ppc64 and 
riscv64) correctly
emit an absolute relocation to the address 0 in case of a weak 
unresolved symbol,
so there's no buggy compiler/linker.

And regarding ppc warning, the kernel being compiled as -pie, the 
scripts looks
for absolute relocations which it considers as "bad", except for one 
that is known
to be weak and that is ignored: I have just sent a patch to fix this 
script so that weak
undefined symbol relocations are not considered as bad.

Thanks,

Alex