Message-ID: <CAEf4BzY4jVKN=3CdaLU1WOekGbT915dweNx0R4KMrW8U7E20cw@mail.gmail.com>
Date:   Fri, 23 Jul 2021 08:54:32 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Quentin Monnet <quentin@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH bpf-next v2 2/5] libbpf: rename btf__get_from_id() as btf__load_from_kernel_by_id()

On Fri, Jul 23, 2021 at 2:31 AM Quentin Monnet <quentin@...valent.com> wrote:
>
> 2021-07-22 17:39 UTC-0700 ~ Andrii Nakryiko <andrii.nakryiko@...il.com>
> > On Wed, Jul 21, 2021 at 8:38 AM Quentin Monnet <quentin@...valent.com> wrote:
> >>
> >> Rename function btf__get_from_id() as btf__load_from_kernel_by_id() to
> >> better indicate what the function does. Change the new function so that,
> >> instead of requiring a pointer to the pointer to update and returning
> >> with an error code, it takes a single argument (the id of the BTF
> >> object) and returns the corresponding pointer. This is more in line with
> >> the existing constructors.
> >>
> >> The other tools calling the deprecated btf__get_from_id() function will
> >> be updated in a future commit.
> >>
> >> References:
> >>
> >> - https://github.com/libbpf/libbpf/issues/278
> >> - https://github.com/libbpf/libbpf/wiki/Libbpf:-the-road-to-v1.0#btfh-apis
> >>
> >> v2:
> >> - Instead of a simple renaming, change the new function to make it
> >>   return the pointer to the btf struct.
> >> - API v0.5.0 instead of v0.6.0.
> >
> > We generally keep such version changes to cover letters. It keeps each
> > individual commit clean and collects full history in the cover letter
> > which becomes a body of merge commit when the whole patch set is
> > applied. For next revision please consolidate the history in the cover
> > letter. Thanks!
>
> OK will do.
> I've seen other folks detailing the changes on individual patches, and
> done so in the past, although it's true the current trend is to have it
> in the cover letter (and I understand the motivation).
>
> >
> >>
> >> Signed-off-by: Quentin Monnet <quentin@...valent.com>
> >> Acked-by: John Fastabend <john.fastabend@...il.com>
> >> ---
> >>  tools/lib/bpf/btf.c      | 25 +++++++++++++++++--------
> >>  tools/lib/bpf/btf.h      |  1 +
> >>  tools/lib/bpf/libbpf.c   |  5 +++--
> >>  tools/lib/bpf/libbpf.map |  1 +
> >>  4 files changed, 22 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> >> index 7e0de560490e..6654bdee7ad7 100644
> >> --- a/tools/lib/bpf/btf.c
> >> +++ b/tools/lib/bpf/btf.c
> >> @@ -1383,21 +1383,30 @@ struct btf *btf_get_from_fd(int btf_fd, struct btf *base_btf)
> >>         return btf;
> >>  }
> >>
> >> +struct btf *btf__load_from_kernel_by_id(__u32 id)
> >> +{
> >> +       struct btf *btf;
> >> +       int btf_fd;
> >> +
> >> +       btf_fd = bpf_btf_get_fd_by_id(id);
> >> +       if (btf_fd < 0)
> >> +               return ERR_PTR(-errno);
> >
> > please use libbpf_err_ptr() for consistency, see
> > bpf_object__open_mem() for an example
>
> I can do that, but I'll need to uncouple btf__get_from_id() from the new
> function. If it calls btf__load_from_kernel_by_id() and
> LIBBPF_STRICT_CLEAN_PTRS is set, it would change its return value.

No it won't, if libbpf_get_error() is used right after the API call.
With CLEAN_PTRS the result pointer is NULL but actual error is passed
through errno. libbpf_get_error() knows about this and extracts error
from errno if passed NULL pointer. With returning ERR_PTR(-errno) from
btf__load_from_kernel_by_id() you are breaking CLEAN_PTRS guarantees.

>
> >
> >> +
> >> +       btf = btf_get_from_fd(btf_fd, NULL);
> >> +       close(btf_fd);
> >> +
> >> +       return libbpf_ptr(btf);
> >> +}
> >> +
> >>  int btf__get_from_id(__u32 id, struct btf **btf)
> >>  {
> >>         struct btf *res;
> >> -       int err, btf_fd;
> >> +       int err;
> >>
> >>         *btf = NULL;
> >> -       btf_fd = bpf_btf_get_fd_by_id(id);
> >> -       if (btf_fd < 0)
> >> -               return libbpf_err(-errno);
> >> -
> >> -       res = btf_get_from_fd(btf_fd, NULL);
> >> +       res = btf__load_from_kernel_by_id(id);
> >>         err = libbpf_get_error(res);
> >>
> >> -       close(btf_fd);
> >> -
> >>         if (err)
> >>                 return libbpf_err(err);
> >>
> >> diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
> >> index fd8a21d936ef..3db9446bc133 100644
> >> --- a/tools/lib/bpf/btf.h
> >> +++ b/tools/lib/bpf/btf.h
> >> @@ -68,6 +68,7 @@ LIBBPF_API const void *btf__get_raw_data(const struct btf *btf, __u32 *size);
> >>  LIBBPF_API const char *btf__name_by_offset(const struct btf *btf, __u32 offset);
> >>  LIBBPF_API const char *btf__str_by_offset(const struct btf *btf, __u32 offset);
> >>  LIBBPF_API int btf__get_from_id(__u32 id, struct btf **btf);
> >> +LIBBPF_API struct btf *btf__load_from_kernel_by_id(__u32 id);
> >
> > let's move this definition to after btf__parse() to keep all
> > "constructors" together (we can move btf__get_from_id() there for
> > completeness as well, I suppose).
>
> I thought about that but wasn't sure, OK will do.

Ok, thanks.

>
> >
> >>  LIBBPF_API int btf__get_map_kv_tids(const struct btf *btf, const char *map_name,
> >>                                     __u32 expected_key_size,
> >>                                     __u32 expected_value_size,
> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> >> index 242e97892043..eff005b1eba1 100644
> >> --- a/tools/lib/bpf/libbpf.c
> >> +++ b/tools/lib/bpf/libbpf.c
> >> @@ -9576,8 +9576,8 @@ static int libbpf_find_prog_btf_id(const char *name, __u32 attach_prog_fd)
> >>  {
> >>         struct bpf_prog_info_linear *info_linear;
> >>         struct bpf_prog_info *info;
> >> -       struct btf *btf = NULL;
> >>         int err = -EINVAL;
> >> +       struct btf *btf;
> >>
> >>         info_linear = bpf_program__get_prog_info_linear(attach_prog_fd, 0);
> >>         err = libbpf_get_error(info_linear);
> >> @@ -9591,7 +9591,8 @@ static int libbpf_find_prog_btf_id(const char *name, __u32 attach_prog_fd)
> >>                 pr_warn("The target program doesn't have BTF\n");
> >>                 goto out;
> >>         }
> >> -       if (btf__get_from_id(info->btf_id, &btf)) {
> >> +       btf = btf__load_from_kernel_by_id(info->btf_id);
> >> +       if (libbpf_get_error(btf)) {
> >
> > there seems to be a bug in existing code and you are keeping it. On
> > error err will be 0. Let's fix it. Same for above if (!info->btf_id),
> > please fix that as well while you are at it.
>
> Oh right, I saw that err was initialised at -EINVAL and did not notice
> it was changed for the info_linear. I'll address it.

cool, thanks

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