netdev - Re: [PATCH v2 bpf-next 07/11] libbpf: add BPF static linker BTF and BTF.ext support

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Message-ID: <CAEf4BzbLtWcgex0+zEfy=6n2783N3nWCX2RkE3Nh1peUHaFXkw@mail.gmail.com>
Date:   Wed, 17 Mar 2021 15:10:06 -0700
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 v2 bpf-next 07/11] libbpf: add BPF static linker BTF and
 BTF.ext support

On Tue, Mar 16, 2021 at 10:25 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Sat, Mar 13, 2021 at 11:35:33AM -0800, Andrii Nakryiko wrote:
> > +             for (j = 0; j < n; j++, src_var++) {
> > +                     void *sec_vars = dst_sec->sec_vars;
> > +
> > +                     sec_vars = libbpf_reallocarray(sec_vars,
> > +                                                    dst_sec->sec_var_cnt + 1,
> > +                                                    sizeof(*dst_sec->sec_vars));
> > +                     if (!sec_vars)
> > +                             return -ENOMEM;
> > +
> > +                     dst_sec->sec_vars = sec_vars;
> > +                     dst_sec->sec_var_cnt++;
> > +
> > +                     dst_var = &dst_sec->sec_vars[dst_sec->sec_var_cnt - 1];
> > +                     dst_var->type = obj->btf_type_map[src_var->type];
> > +                     dst_var->size = src_var->size;
> > +                     dst_var->offset = src_sec->dst_off + src_var->offset;
> > +             }
> > +     }
> > +
> > +     return 0;
> > +}
> > +
> > +static void *add_btf_ext_rec(struct btf_ext_sec_data *ext_data, const void *src_rec)
> > +{
> > +     size_t new_sz = (ext_data->rec_cnt + 1) * ext_data->rec_sz;
> > +     void *tmp;
> > +
> > +     tmp = realloc(ext_data->recs, new_sz);
> > +     if (!tmp)
> > +             return NULL;
> > +
> > +     ext_data->recs = tmp;
> > +     ext_data->rec_cnt++;
> > +
> > +     tmp += new_sz - ext_data->rec_sz;
> > +     memcpy(tmp, src_rec, ext_data->rec_sz);
>
> while reading this and previous patch the cnt vs sz difference was
> constantly throwing me off. Not a big deal, of course.
> Did you consider using _cnt everywhere and use finalize method
> to convert everything to size?
> Like in this function libbpf_reallocarray() instead of realloc() would
> probably be easier to read and more consistent, since btf_ext_sec_data
> is measuring things in _cnt.

will switch this and add_sym() to reallocarray, given both are dealing
with real fixed-size records (not just bytes)

> In the previous patch the section is in _sz which I guess is necessary
> because sections can contain differently sized objects?

yes, it could be records of different sizes (e.g., relocations,
symbols), or just unstructured data (e.g., .data, string table, etc).

>
> btw, strset abstraction is really nice. It made the patches much easier
> to read.

Thanks. Yeah, it simplified existing BTF/btf_dedup logic quite a bit
as well, it was a good suggestion!