From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
Martin KaFai Lau <kafai@fb.com>, Jiri Olsa <jolsa@kernel.org>,
dwarves@vger.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 09:34:30 -0300 [thread overview]
Message-ID: <YIgE1hAaa3Hzwni8@kernel.org> (raw)
In-Reply-To: <YIf3rHTLqW7yZxFJ@krava>
Em Tue, Apr 27, 2021 at 01:38:20PM +0200, Jiri Olsa escreveu:
> 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.
> > 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
You mean encoding it in BTF, in 'struct btf_type'? Seems important to
have it, there are free bits there:
/* Max # of type identifier */
#define BTF_MAX_TYPE 0x000fffff
/* Max offset into the string section */
#define BTF_MAX_NAME_OFFSET 0x00ffffff
/* Max # of struct/union/enum members or func args */
#define BTF_MAX_VLEN 0xffff
struct btf_type {
__u32 name_off;
/* "info" bits arrangement
* bits 0-15: vlen (e.g. # of struct's members)
* bits 16-23: unused
* bits 24-27: kind (e.g. int, ptr, array...etc)
* bits 28-30: unused
* bit 31: kind_flag, currently used by
* struct, union and fwd
*/
__u32 info;
/* "size" is used by INT, ENUM, STRUCT, UNION and DATASEC.
* "size" tells the size of the type it is describing.
*
* "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
* FUNC, FUNC_PROTO and VAR.
* "type" is a type_id referring to another type.
*/
union {
__u32 size;
__u32 type;
};
};
And tools that expect to trace a function can get that information from
the BTF info instead of getting some failure when trying to trace those
functions, right?
- Arnaldo
next prev parent reply other threads:[~2021-04-27 12:34 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-23 21:37 [PATCH dwarves] btf: Generate btf for functions in the .BTF_ids section Martin KaFai Lau
2021-04-26 23:26 ` Andrii Nakryiko
2021-04-27 11:38 ` Jiri Olsa
2021-04-27 12:34 ` Arnaldo Carvalho de Melo [this message]
2021-04-27 12:43 ` Jiri Olsa
2021-04-27 20:38 ` Andrii Nakryiko
2021-04-28 13:10 ` Arnaldo Carvalho de Melo
2021-04-28 19:45 ` Andrii Nakryiko
2021-04-28 22:05 ` Arnaldo Carvalho de Melo
2021-05-01 0:16 ` Martin KaFai Lau
2021-05-01 22:23 ` Jiri Olsa
2021-04-27 11:42 ` Jiri Olsa
2021-04-27 18:20 ` Martin KaFai Lau
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=YIgE1hAaa3Hzwni8@kernel.org \
--to=acme@kernel.org \
--cc=andrii.nakryiko@gmail.com \
--cc=andrii@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=dwarves@vger.kernel.org \
--cc=jolsa@kernel.org \
--cc=jolsa@redhat.com \
--cc=kafai@fb.com \
--cc=kernel-team@fb.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).