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From: Yonghong Song <>
To: <>
Cc: Alexei Starovoitov <>,
	Andrii Nakryiko <>,
	Daniel Borkmann <>,
	John Fastabend <>, <>,
	Kumar Kartikeya Dwivedi <>,
	Martin KaFai Lau <>
Subject: [PATCH bpf-next v4 0/4] bpf: Implement two type cast kfuncs
Date: Sun, 20 Nov 2022 11:54:21 -0800	[thread overview]
Message-ID: <> (raw)

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
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
  . 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.


  v3 -> v4:
    - remove unnecessary bpf_ctx_convert.t error checking
    - add and use meta.ret_btf_id instead of meta.arg_constant.value for
    - 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
      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


             reply	other threads:[~2022-11-20 19:54 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-20 19:54 Yonghong Song [this message]
2022-11-20 19:54 ` [PATCH bpf-next v4 1/4] bpf: Add support for kfunc set with common btf_ids Yonghong Song
2022-11-20 19:54 ` [PATCH bpf-next v4 2/4] bpf: Add a kfunc to type cast from bpf uapi ctx to kernel ctx Yonghong Song
2022-11-20 19:54 ` [PATCH bpf-next v4 3/4] bpf: Add a kfunc for generic type cast Yonghong Song
2022-11-20 20:16   ` Alexei Starovoitov
2022-11-20 20:49     ` Kumar Kartikeya Dwivedi
2022-11-20 22:34       ` Alexei Starovoitov
2022-11-20 23:32         ` Alexei Starovoitov
2022-11-20 23:47           ` Alexei Starovoitov
2022-11-20 19:54 ` [PATCH bpf-next v4 4/4] bpf: Add type cast unit tests Yonghong Song
2022-11-21  0:00 ` [PATCH bpf-next v4 0/4] bpf: Implement two type cast kfuncs patchwork-bot+netdevbpf
2022-11-21  2:10 ` John Fastabend
2022-11-21 17:29   ` Yonghong Song
2022-11-22  1:48     ` John Fastabend
2022-11-22  4:52       ` Alexei Starovoitov
2022-11-23  3:18         ` John Fastabend
2022-11-23 20:46           ` Alexei Starovoitov
2022-11-29 16:30             ` Alan Maguire

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