Re: [RFC PATCH] bpftool btf: Add prefix option to dump C

[Ian Rogers]

On Tue, Jul 21, 2020 at 11:58 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Jul 21, 2020 at 10:44 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> >
> > When bpftool dumps types and enum members into a header file for
> > inclusion the names match those in the original source. If the same
> > header file needs to be included in the original source and the bpf
> > program, the names of structs, unions, typedefs and enum members will
> > have naming collisions.
>
> vmlinux.h is not really intended to be used from user-space, because
> it's incompatible with pretty much any other header that declares any
> type. Ideally we should make this better, but that might require some
> compiler support. We've been discussing with Yonghong extending Clang
> with a compile-time check for whether some type is defined or not,
> which would allow to guard every type and only declare it
> conditionally, if it's missing. But that's just an idea at this point.

Thanks Andrii! We're not looking at user-space code but the BPF code.
The prefix idea comes from a way to solve this problem in C++ with
namespaces:

namespace vmlinux {
#include "vmlinux.h"
}

As the BPF programs are C code then the prefix acts like the
namespace. It seems strange to need to extend the language.

> Regardless, vmlinux.h is also very much Clang-specific, and shouldn't
> work well with GCC. Could you elaborate on the specifics of the use
> case you have in mind? That could help me see what might be the right
> solution. Thanks!

So the use-case is similar to btf_iter.h:
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/testing/selftests/bpf/progs/bpf_iter.h
To avoid collisions with somewhat cleaner macro or not games.

Prompted by your concern I was looking into changing bpf_iter.h to use
a prefix to show what the difference would be like. I also think that
there may be issues with our kernel and tool set up that may mean that
the prefix is unnecessary, if I fix something else. Anyway, to give an
example I needed to build the selftests but this is failing for me.
What I see is:

$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git
$ cd bpf-next
$ make defconfig
$ cat >>.config <<EOF
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_INFO_BTF=y
EOF
$ make -j all
$ mkdir /tmp/selftests
$ make O=/tmp/selftests/ TARGETS=bpf kselftest
...
  CLANG    /tmp/selftests//kselftest/bpf/tools/build/bpftool/profiler.bpf.o
skeleton/profiler.bpf.c:18:21: error: invalid application of 'sizeof'
to an incomplete type 'struct bpf_perf_event_value'
        __uint(value_size, sizeof(struct bpf_perf_event_value));
                           ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Checking with bpftool the vmlinux lacks struct bpf_perf_event_value
but as this is unconditionally defined in bpf.h this seems wrong. Do
you have any suggestions and getting a working build?

> > To avoid these collisions an approach is to redeclare the header file
> > types and enum members, which leads to duplication and possible
> > inconsistencies. Another approach is to use preprocessor macros
> > to rename conflicting names, but this can be cumbersome if there are
> > many conflicts.
> >
> > This patch adds a prefix option for the dumped names. Use of this option
> > can avoid name conflicts and compile time errors.
> >
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  .../bpf/bpftool/Documentation/bpftool-btf.rst |  7 ++++++-
> >  tools/bpf/bpftool/btf.c                       | 18 ++++++++++++++---
> >  tools/lib/bpf/btf.h                           |  1 +
> >  tools/lib/bpf/btf_dump.c                      | 20 +++++++++++++------
> >  4 files changed, 36 insertions(+), 10 deletions(-)
> >
>
> [...]
>
> > diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> > index 491c7b41ffdc..fea4baab00bd 100644
> > --- a/tools/lib/bpf/btf.h
> > +++ b/tools/lib/bpf/btf.h
> > @@ -117,6 +117,7 @@ struct btf_dump;
> >
> >  struct btf_dump_opts {
> >         void *ctx;
> > +       const char *name_prefix;
> >  };
>
> BTW, we can't do that, this breaks ABI. btf_dump_opts were added
> before we understood the problem of backward/forward  compatibility of
> libbpf APIs, unfortunately.

This could be fixed by adding a "new" API for the parameter, which
would be unfortunate compared to just amending the existing API. There
may be solutions that are less duplicative.

Thanks,
Ian

> >
> >  typedef void (*btf_dump_printf_fn_t)(void *ctx, const char *fmt, va_list args);
> > diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> > index e1c344504cae..baf2b4d82e1e 100644
> > --- a/tools/lib/bpf/btf_dump.c
> > +++ b/tools/lib/bpf/btf_dump.c
>
> [...]