Re: [PATCH bpf-next 2/3] bpf: Allow using bpf_sk_storage in FENTRY/FEXIT/RAW_TP

[Martin KaFai Lau <kafai@fb.com>]

On Fri, Nov 06, 2020 at 05:14:14PM -0800, Andrii Nakryiko wrote:
> On Fri, Nov 6, 2020 at 2:08 PM Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > This patch enables the FENTRY/FEXIT/RAW_TP tracing program to use
> > the bpf_sk_storage_(get|delete) helper, so those tracing programs
> > can access the sk's bpf_local_storage and the later selftest
> > will show some examples.
> >
> > The bpf_sk_storage is currently used in bpf-tcp-cc, tc,
> > cg sockops...etc which is running either in softirq or
> > task context.
> >
> > This patch adds bpf_sk_storage_get_tracing_proto and
> > bpf_sk_storage_delete_tracing_proto.  They will check
> > in runtime that the helpers can only be called when serving
> > softirq or running in a task context.  That should enable
> > most common tracing use cases on sk.
> >
> > During the load time, the new tracing_allowed() function
> > will ensure the tracing prog using the bpf_sk_storage_(get|delete)
> > helper is not tracing any *sk_storage*() function itself.
> > The sk is passed as "void *" when calling into bpf_local_storage.
> >
> > Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> > ---
> >  include/net/bpf_sk_storage.h |  2 +
> >  kernel/trace/bpf_trace.c     |  5 +++
> >  net/core/bpf_sk_storage.c    | 73 ++++++++++++++++++++++++++++++++++++
> >  3 files changed, 80 insertions(+)
> >
> 
> [...]
> 
> > +       switch (prog->expected_attach_type) {
> > +       case BPF_TRACE_RAW_TP:
> > +               /* bpf_sk_storage has no trace point */
> > +               return true;
> > +       case BPF_TRACE_FENTRY:
> > +       case BPF_TRACE_FEXIT:
> > +               btf_vmlinux = bpf_get_btf_vmlinux();
> > +               btf_id = prog->aux->attach_btf_id;
> > +               t = btf_type_by_id(btf_vmlinux, btf_id);
> > +               tname = btf_name_by_offset(btf_vmlinux, t->name_off);
> > +               return !strstr(tname, "sk_storage");
> 
> I'm always feeling uneasy about substring checks... Also, KP just
> fixed the issue with string-based checks for LSM. Can we use a
> BTF_ID_SET of blacklisted functions instead?
KP one is different.  It accidentally whitelist-ed more than it should.

It is a blacklist here.  It is actually cleaner and safer to blacklist
all functions with "sk_storage" and too pessimistic is fine here.

> 
> > +       default:
> > +               return false;
> > +       }
> > +
> > +       return false;
> > +}
> > +
> 
> [...]