Subject: Re: debug annotations for bpf progs. Was: [PATCH bpf-next 1/3] bpf: preserve command of the process that loaded the program

From: Stanislav Fomichev <[email-masked]>
Date: Tue, 15 Oct 2019 15:26:12 -0700

On 10/15, Alexei Starovoitov wrote:
> On Fri, Oct 11, 2019 at 5:38 PM Stanislav Fomichev <[email-masked]> wrote:
> >
> > On 10/11, Alexei Starovoitov wrote:
> > > On Fri, Oct 11, 2019 at 9:21 AM Stanislav Fomichev <[email-masked]> wrote:
> > > >
> > > > Even though we have the pointer to user_struct and can recover
> > > > uid of the user who has created the program, it usually contains
> > > > 0 (root) which is not very informative. Let's store the comm of the
> > > > calling process and export it via bpf_prog_info. This should help
> > > > answer the question "which process loaded this particular program".
> > > >
> > > > Signed-off-by: Stanislav Fomichev <[email-masked]>
> > > > ---
> > > >  include/linux/bpf.h      | 1 +
> > > >  include/uapi/linux/bpf.h | 2 ++
> > > >  kernel/bpf/syscall.c     | 4 ++++
> > > >  3 files changed, 7 insertions(+)
> > > >
> > > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > > index 5b9d22338606..b03ea396afe5 100644
> > > > --- a/include/linux/bpf.h
> > > > +++ b/include/linux/bpf.h
> > > > @@ -421,6 +421,7 @@ struct bpf_prog_aux {
> > > >                 struct work_struct work;
> > > >                 struct rcu_head rcu;
> > > >         };
> > > > +       char created_by_comm[BPF_CREATED_COMM_LEN];
> > > >  };
> > > >
> > > >  struct bpf_array {
> > > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > > index a65c3b0c6935..4e883ecbba1e 100644
> > > > --- a/include/uapi/linux/bpf.h
> > > > +++ b/include/uapi/linux/bpf.h
> > > > @@ -326,6 +326,7 @@ enum bpf_attach_type {
> > > >  #define BPF_F_NUMA_NODE                (1U << 2)
> > > >
> > > >  #define BPF_OBJ_NAME_LEN 16U
> > > > +#define BPF_CREATED_COMM_LEN   16U
> > >
> > > Nack.
> > > 16 bytes is going to be useless.
> > > We found it the hard way with prog_name.
> > > If you want to embed additional debug information
> > > please use BTF for that.
> > BTF was my natural choice initially, but then I saw created_by_uid and
> > thought created_by_comm might have a chance :-)
> >
> > To clarify, by BTF you mean creating some unused global variable
> > and use its name as the debugging info? Or there is some better way?
> 
> I was thinking about adding new section to .btf.ext with this extra data,
> but global variable is a better idea indeed.
> We'd need to standardize such variables names, so that
> bpftool can parse and print it while doing 'bpftool prog show'.
> We see more and more cases where services use more than
> one program in single .c file to accomplish their goals.
> Tying such debug info (like 'created_by_comm') to each program
> individually isn't quite right.
> In that sense global variables are better, since they cover the
> whole .c file.
> Beyond 'created_by_comm' there are others things that people
> will likely want to know.
> Like which version of llvm was used to compile this .o file.
> Which unix user name compiled it.
> The name of service/daemon that will be using this .o
> and so on.
> May be some standard prefix to such global variables will do?
> Like "bpftool prog show" can scan global data for
> "__annotate_#name" and print both name and string contents ?
> For folks who regularly ssh into servers to debug bpf progs
> that will help a lot.
> May be some annotations llvm can automatically add to .o.
> Thoughts?
We started some proof-of-concept prototyping yesterday; the idea, roughly:
* build system generates build_info.h header which contains:
  char __attribute__((section("aux_timestamp"))) *__aux_<build timestamp> = "";
  char __attribute__((section("aux_version"))) *__aux_<version> = "";
  ...
* clang has -include flag which includes this auto-generated file,
  so we don't rely on users including it
* 'bpftool show btf | grep aux_' can be used for low-level debugging

It's not pretty, but it gets the job done. I agree that having some sort of
convention is nice to make it more usable. If we can agree on a pre-defined
section (aux in my case) so that bpftool can take "variable" from
aux_<variable> section name and print "value" from __aux_<value>,
that would be nice.

One thing I still have no idea how to implement with this scheme
is some alternative to created_by_comm. There is no easy way to
add some BTF at runtime (load) time. Ideas?

> "__annotate_#name" and print both name and string contents ?
As Andrii just pointed out, this requires knowing where to look
for the obj files to print the contents of the vars :-(