Re: [RFC bpf-next 3/3] selftests/bpf: verify module-specific types can be shown via bpf_snprintf_btf

On Fri, Nov 13, 2020 at 10:11 AM Alan Maguire [off-list ref] wrote:

Verify that specifying a module name in "struct btf_ptr *" along
with a type id of a module-specific type will succeed.

veth_stats_rx() is chosen because its function signature consists
of a module-specific type "struct veth_stats" and a kernel-specific
one "struct net_device".

Signed-off-by: Alan Maguire <redacted>
---
 .../selftests/bpf/prog_tests/snprintf_btf_mod.c    | 96 ++++++++++++++++++++++
 tools/testing/selftests/bpf/progs/btf_ptr.h        |  1 +
 tools/testing/selftests/bpf/progs/veth_stats_rx.c  | 73 ++++++++++++++++
 3 files changed, 170 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/snprintf_btf_mod.c
 create mode 100644 tools/testing/selftests/bpf/progs/veth_stats_rx.c

[...]

+       err = veth_stats_rx__load(skel);
+       if (CHECK(err, "skel_load", "failed to load skeleton: %d\n", err))
+               goto cleanup;
+
+       bss = skel->bss;
+
+       bss->veth_stats_btf_id = btf__find_by_name(veth_btf, "veth_stats");

This is really awkward that this needs to be done from user-space.
Libbpf will be able to do this regardless of whether the type is in
vmlinux or kernel module. See my comments on patch #1.

+
+       if (CHECK(bss->veth_stats_btf_id <= 0, "find 'struct veth_stats'",
+                 "could not find 'struct veth_stats' in veth BTF: %d",
+                 bss->veth_stats_btf_id))
+               goto cleanup;
+

[...]

+       btf_ids[0] = veth_stats_btf_id;
+       ptrs[0] = (void *)PT_REGS_PARM1_CORE(ctx);
+#if __has_builtin(__builtin_btf_type_id)

nit: there are a bunch of selftests that just assume we have this
built-in, so I don't think you need to guard it with #if here.

+       btf_ids[1] = bpf_core_type_id_kernel(struct net_device);
+       ptrs[1] = (void *)PT_REGS_PARM2_CORE(ctx);
+#endif

[...]