[RFC PATCH bpf-next 0/3] bpf: Implement bpf_get_kern_btf_id() kfunc

[Yonghong Song <yhs@fb.com>]

Currenty, a non-tracing bpf program typically has a single 'context' argument
with predefined uapi struct type. Following these uapi struct, user is able
to access other fields defined in uapi header. Inside the kernel, the
user-seen 'context' argument is replaced with 'kernel context' (or 'kcontext'
in short) which can access more information than what uapi header provides.
To access other info not in uapi header, people typically do two things:
  (1). extend uapi to access more fields rooted from 'context'.
  (2). use bpf_probe_read_kernl() helper to read particular field based on
    kcontext.
Using (1) needs uapi change and using (2) makes code more complex since
direct memory access is not allowed.

There are already a few instances trying to access more information from
kcontext:
  (1). trying to access some fields from perf_event kcontext.
  (2). trying to access some fields from xdp kcontext.

This patch set tried to allow direct memory access for kcontext fields
by introducing bpf_get_kern_btf_id() kfunc.

Martin mentioned a use case like type casting below:
  #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))
basically a 'unsigned char *" casted to 'struct skb_shared_info *'. This patch
set tries to support such a use case as well with bpf_get_kern_btf_id().

For the patch series, Patch 1 is a preparation patch. Patch 2 did some
but incomplete implementation. Please see details in Patch 2 for what
is missing. The goal of this RFC patch is to seek comments about whether
such a kfunc is helpful and what scope (e.g., type casting like skb_shinfo(SKB)
can be implemented with this helper) it should cover. Patch 3 has two
simple tests.

  [1] https://lore.kernel.org/bpf/ad15b398-9069-4a0e-48cb-4bb651ec3088@meta.com/
  [2] https://lore.kernel.org/bpf/20221109215242.1279993-1-john.fastabend@gmail.com/

Yonghong Song (3):
  bpf: Add support for kfunc set with generic btf_ids
  bpf: Implement bpf_get_kern_btf_id() kfunc
  bpf: Add bpf_get_kern_btf_id() tests

 include/linux/bpf.h                           |  2 +
 kernel/bpf/btf.c                              | 75 ++++++++++++++++-
 kernel/bpf/helpers.c                          | 18 ++++-
 kernel/bpf/verifier.c                         |  8 +-
 .../bpf/prog_tests/get_kern_btf_id.c          | 81 +++++++++++++++++++
 .../selftests/bpf/progs/get_kern_btf_id.c     | 44 ++++++++++
 6 files changed, 225 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/get_kern_btf_id.c
 create mode 100644 tools/testing/selftests/bpf/progs/get_kern_btf_id.c

-- 
2.30.2