netdev - Re: [PATCH v4 bpf-next 1/5] bpf: add in-kernel split BTF support

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Date:   Tue, 10 Nov 2020 10:31:25 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v4 bpf-next 1/5] bpf: add in-kernel split BTF support

On Tue, Nov 10, 2020 at 9:50 AM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Nov 9, 2020, at 5:19 PM, Andrii Nakryiko <andrii@...nel.org> wrote:
> >
> > Adjust in-kernel BTF implementation to support a split BTF mode of operation.
> > Changes are mostly mirroring libbpf split BTF changes, with the exception of
> > start_id being 0 for in-kernel implementation due to simpler read-only mode.
> >
> > Otherwise, for split BTF logic, most of the logic of jumping to base BTF,
> > where necessary, is encapsulated in few helper functions. Type numbering and
> > string offset in a split BTF are logically continuing where base BTF ends, so
> > most of the high-level logic is kept without changes.
> >
> > Type verification and size resolution is only doing an added resolution of new
> > split BTF types and relies on already cached size and type resolution results
> > in the base BTF.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> > ---
> > kernel/bpf/btf.c | 171 +++++++++++++++++++++++++++++++++--------------
> > 1 file changed, 119 insertions(+), 52 deletions(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 6324de8c59f7..727c1c27053f 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -203,12 +203,17 @@ struct btf {
> >       const char *strings;
> >       void *nohdr_data;
> >       struct btf_header hdr;
> > -     u32 nr_types;
> > +     u32 nr_types; /* includes VOID for base BTF */
> >       u32 types_size;
> >       u32 data_size;
> >       refcount_t refcnt;
> >       u32 id;
> >       struct rcu_head rcu;
> > +
> > +     /* split BTF support */
> > +     struct btf *base_btf;
> > +     u32 start_id; /* first type ID in this BTF (0 for base BTF) */
> > +     u32 start_str_off; /* first string offset (0 for base BTF) */
> > };
> >
> > enum verifier_phase {
> > @@ -449,14 +454,27 @@ static bool btf_type_is_datasec(const struct btf_type *t)
> >       return BTF_INFO_KIND(t->info) == BTF_KIND_DATASEC;
> > }
> >
> > +static u32 btf_nr_types_total(const struct btf *btf)
> > +{
> > +     u32 total = 0;
> > +
> > +     while (btf) {
> > +             total += btf->nr_types;
> > +             btf = btf->base_btf;
> > +     }
> > +
> > +     return total;
> > +}
> > +
> > s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind)
> > {
> >       const struct btf_type *t;
> >       const char *tname;
> > -     u32 i;
> > +     u32 i, total;
> >
> > -     for (i = 1; i <= btf->nr_types; i++) {
> > -             t = btf->types[i];
> > +     total = btf_nr_types_total(btf);
> > +     for (i = 1; i < total; i++) {
> > +             t = btf_type_by_id(btf, i);
> >               if (BTF_INFO_KIND(t->info) != kind)
> >                       continue;
> >
> > @@ -599,8 +617,14 @@ static const struct btf_kind_operations *btf_type_ops(const struct btf_type *t)
> >
> > static bool btf_name_offset_valid(const struct btf *btf, u32 offset)
> > {
> > -     return BTF_STR_OFFSET_VALID(offset) &&
> > -             offset < btf->hdr.str_len;
> > +     if (!BTF_STR_OFFSET_VALID(offset))
> > +             return false;
> > +
> > +     while (offset < btf->start_str_off)
> > +             btf = btf->base_btf;
>
> Do we need "if (!btf) return false;" in the while loop? (and some other loops below)

No, because for base btf start_str_off and start_type_id are always
zero, so loop condition is always false.

>
> [...]
>