Re: [RFC bpf-next 00/12] Use uapi kernel headers with vmlinux.h

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2022-10-28 at 11:56 -0700, Yonghong Song wrote:
> > > [...]
> > 
> > Ok, could we change the problem to detecting if some type is defined.
> > Would it be possible to have something like
> > 
> > #if !__is_type_defined(struct abc)
> > struct abc {
> > };
> > #endif
> > 
> > I think we talked about this and there were problems with this
> > approach, but I don't remember details and how insurmountable the
> > problem is. Having a way to check whether some type is defined would
> > be very useful even outside of -target bpf parlance, though, so maybe
> > it's the problem worth attacking?
> 
> Yes, we discussed this before. This will need to add additional work
> in preprocessor. I just made a discussion topic in llvm discourse
> 
> https://discourse.llvm.org/t/add-a-type-checking-macro-is-type-defined-type/66268
> 
> Let us see whether we can get some upstream agreement or not.

I did a small investigation of this feature.

The main pre-requirement is construction of the symbol table during
source code pre-processing, which implies necessity to parse the
source code at the same time. It is technically possible in clang, as
lexing, pre-processing and AST construction happens at the same time
when in compilation mode.

The prototype is available here [1], it includes:
- Change in the pre-processor that adds an optional callback
  "IsTypeDefinedFn" & necessary parsing of __is_type_defined
  construct.
- Change in Sema module (responsible for parsing/AST & symbol table)
  that installs the appropriate "IsTypeDefinedFn" in the pre-processor
  instance.

However, this prototype builds a backward dependency between
pre-processor and semantic analysis. There are currently no such
dependencies in the clang code base.

This makes it impossible to do pre-processing and compilation
separately, e.g. consider the following example:

$ cat test.c

  struct foo { int x; };
  
  #if __is_type_defined(foo)
    const int x = 1;
  #else
    const int x = 2;
  #endif
  
$ clang -cc1 -ast-print test.c -o -

  struct foo {
      int x;
  };
  const int x = 1;

$ clang -E test.c -o -

  # ... some line directives ...
  struct foo { int x; };
  const int x = 2;

Note that __is_type_defined is computed to different value in the
first and second calls. This is so because semantic analysis (AST,
symbol table) is not done for -E.

It also breaks that C11 standard which clearly separates
pre-processing and semantic analysis phases, see [2] 5.1.1.2.

So, my conclusion is as follows: this is technically possible in clang
but has no chance to reach llvm upstream.

Thanks,
Eduard

[1] https://github.com/llvm/llvm-project/compare/main...eddyz87:llvm-project:is-type-defined-experiment
[2] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf


> 
> > 
> > > 
> > > > 
> > > > BTW, I suggest splitting libbpf btf_dedup and btf_dump changes into a
> > > > separate series and sending them as non-RFC sooner. Those improvements
> > > > are independent of all the header guards stuff, let's get them landed
> > > > sooner.
> > > > 
> > > > > After some discussion with Alexei and Yonghong I'd like to request
> > > > > your comments regarding a somewhat brittle and partial solution to
> > > > > this issue that relies on adding `#ifndef FOO_H ... #endif` guards in
> > > > > the generated `vmlinux.h`.
> > > > > 
> > > > 
> > > > [...]
> > > > 
> > > > > Eduard Zingerman (12):
> > > > >     libbpf: Deduplicate unambigous standalone forward declarations
> > > > >     selftests/bpf: Tests for standalone forward BTF declarations
> > > > >       deduplication
> > > > >     libbpf: Support for BTF_DECL_TAG dump in C format
> > > > >     selftests/bpf: Tests for BTF_DECL_TAG dump in C format
> > > > >     libbpf: Header guards for selected data structures in vmlinux.h
> > > > >     selftests/bpf: Tests for header guards printing in BTF dump
> > > > >     bpftool: Enable header guards generation
> > > > >     kbuild: Script to infer header guard values for uapi headers
> > > > >     kbuild: Header guards for types from include/uapi/*.h in kernel BTF
> > > > >     selftests/bpf: Script to verify uapi headers usage with vmlinux.h
> > > > >     selftests/bpf: Known good uapi headers for test_uapi_headers.py
> > > > >     selftests/bpf: script for infer_header_guards.pl testing
> > > > > 
> > > > >    scripts/infer_header_guards.pl                | 191 +++++
> > > > >    scripts/link-vmlinux.sh                       |  13 +-
> > > > >    tools/bpf/bpftool/btf.c                       |   4 +-
> > > > >    tools/lib/bpf/btf.c                           | 178 ++++-
> > > > >    tools/lib/bpf/btf.h                           |   7 +-
> > > > >    tools/lib/bpf/btf_dump.c                      | 232 +++++-
> > > > >    .../selftests/bpf/good_uapi_headers.txt       | 677 ++++++++++++++++++
> > > > >    tools/testing/selftests/bpf/prog_tests/btf.c  | 152 ++++
> > > > >    .../selftests/bpf/prog_tests/btf_dump.c       |  11 +-
> > > > >    .../bpf/progs/btf_dump_test_case_decl_tag.c   |  39 +
> > > > >    .../progs/btf_dump_test_case_header_guards.c  |  94 +++
> > > > >    .../bpf/test_uapi_header_guards_infer.sh      |  33 +
> > > > >    .../selftests/bpf/test_uapi_headers.py        | 197 +++++
> > > > >    13 files changed, 1816 insertions(+), 12 deletions(-)
> > > > >    create mode 100755 scripts/infer_header_guards.pl
> > > > >    create mode 100644 tools/testing/selftests/bpf/good_uapi_headers.txt
> > > > >    create mode 100644 tools/testing/selftests/bpf/progs/btf_dump_test_case_decl_tag.c
> > > > >    create mode 100644 tools/testing/selftests/bpf/progs/btf_dump_test_case_header_guards.c
> > > > >    create mode 100755 tools/testing/selftests/bpf/test_uapi_header_guards_infer.sh
> > > > >    create mode 100755 tools/testing/selftests/bpf/test_uapi_headers.py
> > > > > 
> > > > > --
> > > > > 2.34.1
> > > > >