netdev - Re: [PATCH bpf-next v3 3/8] libbpf: Add weak ksym support to gen

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Date:   Fri, 15 Oct 2021 06:51:40 +0000
From:   Song Liu <songliubraving@...com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 3/8] libbpf: Add weak ksym support to
 gen_loader



> On Oct 14, 2021, at 1:56 PM, Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
> 
> This extends existing ksym relocation code to also support relocating
> weak ksyms. Care needs to be taken to zero out the src_reg (currently
> BPF_PSEUOD_BTF_ID, always set for gen_loader by bpf_object__relocate_data)
> when the BTF ID lookup fails at runtime.  This is not a problem for
> libbpf as it only sets ext->is_set when BTF ID lookup succeeds (and only
> proceeds in case of failure if ext->is_weak, leading to src_reg
> remaining as 0 for weak unresolved ksym).
> 
> A pattern similar to emit_relo_kfunc_btf is followed of first storing
> the default values and then jumping over actual stores in case of an
> error. For src_reg adjustment, we also need to perform it when copying
> the populated instruction, so depending on if copied insn[0].imm is 0 or
> not, we decide to jump over the adjustment.
> 
> We cannot reach that point unless the ksym was weak and resolved and
> zeroed out, as the emit_check_err will cause us to jump to cleanup
> label, so we do not need to recheck whether the ksym is weak before
> doing the adjustment after copying BTF ID and BTF FD.
> 
> This is consistent with how libbpf relocates weak ksym. Logging
> statements are added to show the relocation result and aid debugging.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>

Acked-by: Song Liu <songliubraving@...com>