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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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?

> +       default:
> +               return false;
> +       }
> +
> +       return false;
> +}
> +

[...]