[PATCH bpf-next v4 0/4] bpf: Implement two type cast kfuncs

[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 'kctx'
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
    kctx.
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
kctx:
  . trying to access some fields from perf_event kctx ([1]).
  . trying to access some fields from xdp kctx ([2]).

This patch set tried to allow direct memory access for kctx fields
by introducing bpf_cast_to_kern_ctx() 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_rdonly_cast().

For the patch series, Patch 1 added support for a kfunc available to all
prog types. Patch 2 added bpf_cast_to_kern_ctx() kfunc. Patch 3 added
bpf_rdonly_cast() kfunc. Patch 4 added a few positive and negative 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/

Changelog:
  v3 -> v4:
    - remove unnecessary bpf_ctx_convert.t error checking
    - add and use meta.ret_btf_id instead of meta.arg_constant.value for
      bpf_cast_to_kern_ctx().
    - add PTR_TRUSTED to the return PTR_TO_BTF_ID type for bpf_cast_to_kern_ctx().
  v2 -> v3:
    - rebase on top of bpf-next (for merging conflicts)
    - add the selftest to s390x deny list
  rfcv1 -> v2:
    - break original one kfunc into two.
    - add missing error checks and error logs.
    - adapt to the new conventions in
      https://lore.kernel.org/all/20221118015614.2013203-1-memxor@gmail.com/
      for example, with __ign and __k suffix.
    - added support in fixup_kfunc_call() to replace kfunc calls with a single mov.

Yonghong Song (4):
  bpf: Add support for kfunc set with common btf_ids
  bpf: Add a kfunc to type cast from bpf uapi ctx to kernel ctx
  bpf: Add a kfunc for generic type cast
  bpf: Add type cast unit tests

 include/linux/btf.h                           |   5 +
 kernel/bpf/btf.c                              |  28 +++++
 kernel/bpf/helpers.c                          |  24 +++-
 kernel/bpf/verifier.c                         |  46 ++++++-
 tools/testing/selftests/bpf/DENYLIST.s390x    |   1 +
 .../selftests/bpf/prog_tests/type_cast.c      | 114 ++++++++++++++++++
 tools/testing/selftests/bpf/progs/type_cast.c |  83 +++++++++++++
 7 files changed, 299 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/type_cast.c
 create mode 100644 tools/testing/selftests/bpf/progs/type_cast.c

-- 
2.30.2