Re: [PATCH v8 bpf-next 07/13] bpf: Add btf_struct_ids_match function

Adding btf_struct_ids_match function to check if given address provided
by BTF object + offset is also address of another nested BTF object.

This allows to pass an argument to helper, which is defined via parent
BTF object + offset, like for bpf_d_path (added in following changes):

  SEC("fentry/filp_close")
  int BPF_PROG(prog_close, struct file *file, void *id)
  {
    ...
    ret = bpf_d_path(&file->f_path, ...

The first bpf_d_path argument is hold by verifier as BTF file object
plus offset of f_path member.

The btf_struct_ids_match function will walk the struct file object and
check if there's nested struct path object on the given offset.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 include/linux/bpf.h   |  2 ++
 kernel/bpf/btf.c      | 29 +++++++++++++++++++++++++++++
 kernel/bpf/verifier.c | 18 ++++++++++++------
 3 files changed, 43 insertions(+), 6 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index bae557ff2da8..c981e258fed3 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1306,6 +1306,8 @@ int btf_struct_access(struct bpf_verifier_log *log,
                      const struct btf_type *t, int off, int size,
                      enum bpf_access_type atype,
                      u32 *next_btf_id);
+bool btf_struct_ids_match(struct bpf_verifier_log *log,
+                         int off, u32 id, u32 mid);
 int btf_resolve_helper_id(struct bpf_verifier_log *log,
                          const struct bpf_func_proto *fn, int);

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 1ab5fd5bf992..562d4453fad3 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -4140,6 +4140,35 @@ int btf_struct_access(struct bpf_verifier_log *log,
        return -EINVAL;
 }

+bool btf_struct_ids_match(struct bpf_verifier_log *log,
+                         int off, u32 id, u32 mid)
+{
+       const struct btf_type *type;
+       u32 nid;
+       int err;
+