Subject: [RFC 0/3] bpf: Add d_path helper

From: Jiri Olsa <[email-masked]>
Date: Wed, 1 Apr 2020 13:09:04 +0200

hi,
adding d_path helper to return full path for 'path' object.

I originally added and used 'file_path' helper, which did the same,
but used 'struct file' object. Then realized that file_path is just
a wrapper for d_path, so we'd cover more calling sites if we add
d_path helper and allowed resolving BTF object within another object,
so we could call d_path also with file pointer, like:

  bpf_d_path(&file->f_path, buf, size);

This feature is mainly to be able to add dpath (filepath originally)
function to bpftrace, which seems to work nicely now, like:

  # bpftrace -e 'kretfunc:fget { printf("%s\n", dpath(args->ret->f_path));  }' 

I'm not completely sure this is all safe and bullet proof and there's
no other way to do this, hence RFC post.

I'd be happy also with file_path function, but I thought it'd be
a shame not to try to add d_path with the verifier change.
I'm open to any suggestions ;-)

thanks,
jirka

---
Jiri Olsa (3):
      bpf: Add support to check if BTF object is nested in another object
      bpf: Add d_path helper
      selftests/bpf: Add test for d_path helper

 include/linux/bpf.h                             |   3 ++
 include/uapi/linux/bpf.h                        |  14 ++++++-
 kernel/bpf/btf.c                                |  69 +++++++++++++++++++++++++++++++++
 kernel/bpf/verifier.c                           |  18 ++++++++-
 kernel/trace/bpf_trace.c                        |  31 +++++++++++++++
 scripts/bpf_helpers_doc.py                      |   2 +
 tools/include/uapi/linux/bpf.h                  |  14 ++++++-
 tools/testing/selftests/bpf/prog_tests/d_path.c | 196 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 tools/testing/selftests/bpf/progs/test_d_path.c |  71 ++++++++++++++++++++++++++++++++++
 9 files changed, 414 insertions(+), 4 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/d_path.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_d_path.c