Re: [PATCH dwarves] btf: Generate btf for functions in the .BTF_ids section

From: Jiri Olsa <jolsa@redhat.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Martin KaFai Lau <kafai@fb.com>, Jiri Olsa <jolsa@kernel.org>,
	dwarves@vger.kernel.org,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	bpf <bpf@vger.kernel.org>, Kernel Team <kernel-team@fb.com>,
	Andrii Nakryiko <andrii@kernel.org>
Subject: Re: [PATCH dwarves] btf: Generate btf for functions in the .BTF_ids section
Date: Tue, 27 Apr 2021 13:38:20 +0200	[thread overview]
Message-ID: <YIf3rHTLqW7yZxFJ@krava> (raw)
In-Reply-To: <CAEf4BzY16ziMkOMdNGNjQOmiACF3E5nFn2LhtUUQbo-y-AP7Tg@mail.gmail.com>

On Mon, Apr 26, 2021 at 04:26:11PM -0700, Andrii Nakryiko wrote:
> On Fri, Apr 23, 2021 at 2:37 PM Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > BTF is currently generated for functions that are in ftrace list
> > or extern.
> >
> > A recent use case also needs BTF generated for functions included in
> > allowlist.  In particular, the kernel
> > commit e78aea8b2170 ("bpf: tcp: Put some tcp cong functions in allowlist for bpf-tcp-cc")
> > allows bpf program to directly call a few tcp cc kernel functions.  Those
> > functions are specified under an ELF section .BTF_ids.  The symbols
> > in this ELF section is like __BTF_ID__func__<kernel_func>__[digit]+.
> > For example, __BTF_ID__func__cubictcp_init__1.  Those kernel
> > functions are currently allowed only if CONFIG_DYNAMIC_FTRACE is
> > set to ensure they are in the ftrace list but this kconfig dependency
> > is unnecessary.
> >
> > pahole can generate BTF for those kernel functions if it knows they
> > are in the allowlist.  This patch is to capture those symbols
> > in the .BTF_ids section and generate BTF for them.
> >
> > Cc: Andrii Nakryiko <andrii@kernel.org>
> > Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> > ---
> 
> I wonder if we just record all functions how bad that would be. Jiri,
> do you remember from the time you were experimenting with static
> functions how much more functions we'd be recording if we didn't do
> ftrace filtering?

hum, I can't find that.. but should be just matter of removing
that is_ftrace_func check

if we decided to do that, maybe we could add some bit indicating
that the function is traceable? it would save us check with
available_filter_functions file

jirka

> 
> >  btf_encoder.c | 136 +++++++++++++++++++++++++++++++++++++++++++++++---
> >  libbtf.c      |  10 ++++
> >  libbtf.h      |   2 +
> >  3 files changed, 142 insertions(+), 6 deletions(-)
> >
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index 80e896961d4e..48c183915ddd 100644
> > --- a/btf_encoder.c
> > +++ b/btf_encoder.c
> > @@ -106,6 +106,121 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
> >         return 0;
> >  }
> >
> > +#define BTF_ID_FUNC_PREFIX "__BTF_ID__func__"
> > +#define BTF_ID_FUNC_PREFIX_LEN (sizeof(BTF_ID_FUNC_PREFIX) - 1)
> > +
> > +static char **listed_functions;
> > +static int listed_functions_alloc;
> > +static int listed_functions_cnt;
> 
> maybe just use struct btf as a container of strings, which is what you
> need here? You can do btf__add_str() and btf__find_str(), which are
> both fast and memory-efficient, and you won't have to manage all the
> memory and do sorting, etc, etc.
> 
> [...]
> 