Re: [RFC PATCH bpf-next 2/2] selftests/bpf: Test __ksym externs with BTF

[Andrii Nakryiko]

On Wed, Jul 15, 2020 at 2:46 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
>
> Extend ksyms.c selftest to make sure BTF enables direct loads of ksyms.
>
> Note that test is done against the kernel btf extended with kernel VARs.
>
> Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
> ---
>  tools/testing/selftests/bpf/prog_tests/ksyms.c |  2 ++
>  tools/testing/selftests/bpf/progs/test_ksyms.c | 14 ++++++++++++++
>  2 files changed, 16 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms.c b/tools/testing/selftests/bpf/prog_tests/ksyms.c
> index e3d6777226a8..0e7f3bc3b0ae 100644
> --- a/tools/testing/selftests/bpf/prog_tests/ksyms.c
> +++ b/tools/testing/selftests/bpf/prog_tests/ksyms.c
> @@ -65,6 +65,8 @@ void test_ksyms(void)
>               "got %llu, exp %llu\n", data->out__btf_size, btf_size);
>         CHECK(data->out__per_cpu_start != 0, "__per_cpu_start",
>               "got %llu, exp %llu\n", data->out__per_cpu_start, (__u64)0);
> +       CHECK(data->out__rq_cpu != 0, "rq_cpu",
> +             "got %llu, exp %llu\n", data->out__rq_cpu, (__u64)0);
>
>  cleanup:
>         test_ksyms__destroy(skel);
> diff --git a/tools/testing/selftests/bpf/progs/test_ksyms.c b/tools/testing/selftests/bpf/progs/test_ksyms.c
> index 6c9cbb5a3bdf..e777603757e5 100644
> --- a/tools/testing/selftests/bpf/progs/test_ksyms.c
> +++ b/tools/testing/selftests/bpf/progs/test_ksyms.c
> @@ -1,6 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0
>  /* Copyright (c) 2019 Facebook */
>
> +#include "vmlinux.h"
>  #include <stdbool.h>
>  #include <linux/bpf.h>
>  #include <bpf/bpf_helpers.h>
> @@ -9,11 +10,13 @@ __u64 out__bpf_link_fops = -1;
>  __u64 out__bpf_link_fops1 = -1;
>  __u64 out__btf_size = -1;
>  __u64 out__per_cpu_start = -1;
> +__u64 out__rq_cpu = -1;
>
>  extern const void bpf_link_fops __ksym;
>  extern const void __start_BTF __ksym;
>  extern const void __stop_BTF __ksym;
>  extern const void __per_cpu_start __ksym;
> +extern const void runqueues __ksym;

This should ideally look like a real global variable extern:

extern const struct rq runqueues __ksym;


But that's the case for non-per-cpu variables. You didn't seem to
address per-CPU variables in this patch set. How did you intend to
handle that? We should look at a possible BPF helper to access such
variables as well and how the verifier will prevent direct memory
accesses for such variables.

We should have some BPF helper that accepts per-CPU PTR_TO_BTF_ID, and
returns PTR_TO_BTF_ID, but adjusted to desired CPU. And verifier
ideally would allow direct memory access on that resulting
PTR_TO_BTF_ID, but not on per-CPU one. Not sure yet how this should
look like, but the verifier probably needs to know that variable
itself is per-cpu, no?

>  /* non-existing symbol, weak, default to zero */
>  extern const void bpf_link_fops1 __ksym __weak;
>
> @@ -29,4 +32,15 @@ int handler(const void *ctx)
>         return 0;
>  }
>
> +SEC("tp_btf/sys_enter")
> +int handler_tp_btf(const void *ctx)
> +{
> +       const struct rq *rq = &runqueues;
> +
> +       /* rq now points to the runqueue of cpu 0. */
> +       out__rq_cpu = rq->cpu;
> +
> +       return 0;
> +}
> +
>  char _license[] SEC("license") = "GPL";
> --
> 2.27.0.389.gc38d7665816-goog
>