From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: "Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: bpf <bpf@vger.kernel.org>, Jiri Benc <jbenc@redhat.com>
Subject: Re: Reason for libbpf rejecting SECTION symbols in 'maps' section
Date: Fri, 24 Sep 2021 16:52:57 -0700 [thread overview]
Message-ID: <CAEf4BzZ5HXrhhpbZ573Hh2yjwxFf3Gu-WekafYZqCBhVgQ=zRg@mail.gmail.com> (raw)
In-Reply-To: <87wnn5yl4p.fsf@toke.dk>
On Fri, Sep 24, 2021 at 9:49 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Hi Andrii
>
> We ran into an issue with binutils[0] mangling BPF object files, which
> makes libbpf sad. Specifically, binutils will create SECTION symbols for
> every section in .symtab, which trips this check in
> bpf_object__init_user_maps():
>
> if (GELF_ST_TYPE(sym.st_info) == STT_SECTION
> || GELF_ST_BIND(sym.st_info) == STB_LOCAL) {
> pr_warn("map '%s' (legacy): static maps are not supported\n", map_name);
> return -ENOTSUP;
> }
>
> Given the error message I can understand why it's checking for
> STB_LOCAL, but why is the check for STT_SECTION there? And is there any
> reason why libbpf couldn't just skip the SECTION symbols instead of
> bugging out?
Static functions are often referenced through STT_SECTION symbol +
some offset. I don't remember by now if I encountered cases where
static variables can be referenced through section symbol + offset, I
suspect I did, which is why I added this check.
But thinking about this now, we should just ignore the STT_SECTION
symbol. If Clang really referenced map through STT_SECTION symbol,
we'll later won't find a corresponding bpf_map instance for a
corresponding relocation.
So I think it's fine to drop the STT_SECTION.
>
> Hope you can help shed some light on the history here.
>
> -Toke
>
>
> [0] This happens because rpmbuild has a script that automatically that
> runs 'strip' on every object file in an rpm; and so when we package up
> the kernel selftests, we end up with mangled object files. Newer
> versions of binutils don't do this, but the one on RHEL does.
>
next prev parent reply other threads:[~2021-09-24 23:53 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-24 16:49 Reason for libbpf rejecting SECTION symbols in 'maps' section Toke Høiland-Jørgensen
2021-09-24 23:52 ` Andrii Nakryiko [this message]
2021-09-25 9:19 ` Toke Høiland-Jørgensen
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