netdev - Re: [PATCH bpf-next 05/10] libbpf: add BPF static linker APIs

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Date:   Wed, 10 Mar 2021 19:29:52 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 05/10] libbpf: add BPF static linker APIs

On Wed, Mar 10, 2021 at 6:34 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Mar 09, 2021 at 08:04:26PM -0800, Andrii Nakryiko wrote:
> > +
> > +     struct btf *strtab_btf; /* we use struct btf to manage strings */
> ...
> > +     str_off = btf__add_str(linker->strtab_btf, sec->sec_name);
> > +     sec->shdr->sh_name = str_off;
>
> That bit took me an hour to grok.
> That single line comment above is far far from obvious.

Heh, I guess I've been working with BTF, ELF and pahole for too long
to notice that it's so non-obvious. pahole wraps `struct btf` in a
similar fashion for deduplicated string management.

> What the logic is relying on is that string section in BTF format
> has the same zero terminated set of strings as ELF's .strtab section.
> There is no BTF anywhere here in this 'strtab_btf'.
> The naming choice made it double hard.

Right. strtab_strs would probably be a slightly better choice.

> My understanding that you're using that instead of renaming btf_add_mem()
> into something generic to rely on string hashmap for string dedup?

It's not about renaming btf_add_mem(). btf_add_mem() just implements
memory re-allocation (with exponential increase). But here we want to
not add a new string if it's already present. So it's much more
complicated logic than btf_add_mem().

>
> The commit log in patch 2 that introduces btf_raw_strs() sort of talks about
> this code puzzle, but I would never guessed that's what you meant based
> on patch 2 alone.
>
> Did you consider some renaming/generalizing of string management to
> avoid btf__add_str() through out the patch 5?
> The "btf_" prefix makes things challenging to read.
> Especially when patch 6 is using btf__add_str() to add to real BTF.

Right. I guess we can extract the "set of strings" data structure out
of `struct btf` into libbpf-internal data structure. Then use it from
struct btf and separately (and directly) from struct bpf_linker. I'll
see what that would involve in terms of refactoring.

>
> Mainly pointing it out for others who might be looking at the patches.

That's a good point, I should have probably at least mentioned that
bit more explicitly.