From: Ian Rogers <irogers@google.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: "Alexei Starovoitov" <ast@kernel.org>,
"Daniel Borkmann" <daniel@iogearbox.net>,
"Martin KaFai Lau" <kafai@fb.com>,
"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
"Andrii Nakryiko" <andriin@fb.com>,
"John Fastabend" <john.fastabend@gmail.com>,
"KP Singh" <kpsingh@chromium.org>,
"Quentin Monnet" <quentin@isovalent.com>,
"Jakub Kicinski" <kuba@kernel.org>,
"Jiri Olsa" <jolsa@kernel.org>,
"Toke Høiland-Jørgensen" <toke@redhat.com>,
Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
"open list" <linux-kernel@vger.kernel.org>,
"Stanislav Fomichev" <sdf@google.com>
Subject: Re: [RFC PATCH] bpftool btf: Add prefix option to dump C
Date: Fri, 31 Jul 2020 18:47:36 -0700 [thread overview]
Message-ID: <CAP-5=fXMUWFs6YtQVuxjenCrOmKtKYCqZE3YofwdR=ArDYSwbQ@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzaBYaFJ3eUinS9nHeykJ0xEbZpwLts33ZDp1PT=bkyjww@mail.gmail.com>
On Tue, Jul 21, 2020 at 11:58 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Tue, Jul 21, 2020 at 10:44 PM Ian Rogers <irogers@google.com> 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@google.com>
> > ---
> > .../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
>
> [...]
next prev parent reply other threads:[~2020-08-01 1:47 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-22 5:43 [RFC PATCH] bpftool btf: Add prefix option to dump C Ian Rogers
2020-07-22 6:57 ` Andrii Nakryiko
2020-08-01 1:47 ` Ian Rogers [this message]
2020-08-01 3:47 ` Andrii Nakryiko
2020-08-06 17:58 ` Andrii Nakryiko
2020-08-06 19:42 ` Ian Rogers
2020-08-07 0:29 ` Andrii Nakryiko
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