Re: [PATCH bpf-next v2 00/11] bpf: add support for new btf kind BTF_KIND_TAG

[Yonghong Song <yhs@fb.com>]

On 9/13/21 9:40 AM, Jose E. Marchesi wrote:
> 
>> cc Jose E. Marchesi
>>
>> Hi, Jose, just let you know that the BTF format for BTF_KIND_TAG is
>> changed since v1 as the new format can simplify kernel/libbpf
>> implementation. Thanks!
> 
> Noted.  Thanks for the update.

Hi, Jose,

This is just another update on btf_tag development.
Now, btf_tag is divided into btf_decl_tag and btf_type_tag
for tagging declarations and types as clang compiler prefers
not to mix them with each other. All compiler works in llvm
has done and you can check upstream llvm-project "main" branch
for implementation.

The patch set below (under review)
    https://lore.kernel.org/bpf/20211209173537.1525283-1-yhs@fb.com/
actually tried to use btf_type_tag for linux kernel __user
annotation so bpf verifier can use it.

Another question from Omar (open source drgn maintainer)
 
https://developers.facebook.com/blog/post/2021/12/09/drgn-how-linux-kernel-team-meta-debugs-kernel-scale/
mentioned that btf_tag information will also help drgn since it
can then especially distinguish between __percpu pointer from
other pointers. Currently drgn is using dwarf, clang compiled
kernel puts btf_tag information in dwarf. Based on our earlier
discussion, gcc intends to generate btf tags for BTF only. Maybe
we could discuss to also generate for dwarf? Do we need a flag?

Please let me know if you have any questions.
Happy to help in whatever way to get gcc also implementing btf tag
support.

Thanks!

Yonghong

> 
>>
>> On 9/13/21 8:51 AM, Yonghong Song wrote:
>>> LLVM14 added support for a new C attribute ([1])
>>>     __attribute__((btf_tag("arbitrary_str")))
>>> This attribute will be emitted to dwarf ([2]) and pahole
>>> will convert it to BTF. Or for bpf target, this
>>> attribute will be emitted to BTF directly ([3], [4]).
>>> The attribute is intended to provide additional
>>> information for
>>>     - struct/union type or struct/union member
>>>     - static/global variables
>>>     - static/global function or function parameter.
>>> This new attribute can be used to add attributes
>>> to kernel codes, e.g., pre- or post- conditions,
>>> allow/deny info, or any other info in which only
>>> the kernel is interested. Such attributes will
>>> be processed by clang frontend and emitted to
>>> dwarf, converting to BTF by pahole. Ultimiately
>>> the verifier can use these information for
>>> verification purpose.
>>> The new attribute can also be used for bpf
>>> programs, e.g., tagging with __user attributes
>>> for function parameters, specifying global
>>> function preconditions, etc. Such information
>>> may help verifier to detect user program
>>> bugs.
>>> After this series, pahole dwarf->btf converter
>>> will be enhanced to support new llvm tag
>>> for btf_tag attribute. With pahole support,
>>> we will then try to add a few real use case,
>>> e.g., __user/__rcu tagging, allow/deny list,
>>> some kernel function precondition, etc,
>>> in the kernel.
>>> In the rest of the series, Patches 1-2 had
>>> kernel support. Patches 3-4 added
>>> libbpf support. Patch 5 added bpftool
>>> support. Patches 6-10 added various selftests.
>>> Patch 11 added documentation for the new kind.
>>>     [1] https://reviews.llvm.org/D106614
>>>     [2] https://reviews.llvm.org/D106621
>>>     [3] https://reviews.llvm.org/D106622
>>>     [4] https://reviews.llvm.org/D109560
>>> Changelog:
>>>     v1 -> v2:
>>>       - BTF ELF format changed in llvm ([4] above),
>>>         so cross-board change to use the new format.
>>>       - Clarified in commit message that BTF_KIND_TAG
>>>         is not emitted by bpftool btf dump format c.
>>>       - Fix various comments from Andrii.
>>> Yonghong Song (11):
>>>     btf: change BTF_KIND_* macros to enums
>>>     bpf: support for new btf kind BTF_KIND_TAG
>>>     libbpf: rename btf_{hash,equal}_int to btf_{hash,equal}_int_tag
>>>     libbpf: add support for BTF_KIND_TAG
>>>     bpftool: add support for BTF_KIND_TAG
>>>     selftests/bpf: test libbpf API function btf__add_tag()
>>>     selftests/bpf: change NAME_NTH/IS_NAME_NTH for BTF_KIND_TAG format
>>>     selftests/bpf: add BTF_KIND_TAG unit tests
>>>     selftests/bpf: test BTF_KIND_TAG for deduplication
>>>     selftests/bpf: add a test with a bpf program with btf_tag attributes
>>>     docs/bpf: add documentation for BTF_KIND_TAG
>>>    Documentation/bpf/btf.rst                     |  27 +-
>>>    include/uapi/linux/btf.h                      |  52 +--
>>>    kernel/bpf/btf.c                              | 120 +++++++
>>>    tools/bpf/bpftool/btf.c                       |  12 +
>>>    tools/include/uapi/linux/btf.h                |  52 +--
>>>    tools/lib/bpf/btf.c                           |  85 ++++-
>>>    tools/lib/bpf/btf.h                           |  15 +
>>>    tools/lib/bpf/btf_dump.c                      |   3 +
>>>    tools/lib/bpf/libbpf.c                        |  31 +-
>>>    tools/lib/bpf/libbpf.map                      |   5 +
>>>    tools/lib/bpf/libbpf_internal.h               |   2 +
>>>    tools/testing/selftests/bpf/btf_helpers.c     |   7 +-
>>>    tools/testing/selftests/bpf/prog_tests/btf.c  | 318 ++++++++++++++++--
>>>    .../selftests/bpf/prog_tests/btf_tag.c        |  14 +
>>>    .../selftests/bpf/prog_tests/btf_write.c      |  21 ++
>>>    tools/testing/selftests/bpf/progs/tag.c       |  39 +++
>>>    tools/testing/selftests/bpf/test_btf.h        |   3 +
>>>    17 files changed, 736 insertions(+), 70 deletions(-)
>>>    create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_tag.c
>>>    create mode 100644 tools/testing/selftests/bpf/progs/tag.c
>>>