Re: [PATCH bpf-next] libbpf: Load btf_vmlinux only once per object.

[KP Singh]

Thanks for taking a look. Sending out a v2 with the fixes.

On 17-Jan 11:02, Andrii Nakryiko wrote:
> On Fri, Jan 17, 2020 at 8:58 AM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
> >
> > From: KP Singh <kpsingh@xxxxxxxxxx>
> >
> > As more programs (TRACING, STRUCT_OPS, and upcoming LSM) use vmlinux
> > BTF information, loading the BTF vmlinux information for every program
> > in an object is sub-optimal. The fix was originally proposed in:
> >
> >    https://lore.kernel.org/bpf/CAEf4BzZodr3LKJuM7QwD38BiEH02Cc1UbtnGpVkCJ00Mf+V_Qg@xxxxxxxxxxxxxx/
> >
> > The btf_vmlinux is populated in the object if any of the programs in
> > the object requires it just before the programs are loaded and freed
> > after the programs finish loading.
> >
> > Reported-by: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> > Reviewed-by: Brendan Jackman <jackmanb@xxxxxxxxxxxx>
> > Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> > ---
> 
> Thanks for the clean up! Few issues, but overall I like this.

Thanks!

> 
> >  tools/lib/bpf/libbpf.c | 148 +++++++++++++++++++++++++++--------------
> >  1 file changed, 97 insertions(+), 51 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 3afaca9bce1d..db0e93882a3b 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -385,6 +385,10 @@ struct bpf_object {
> 
> [...]
> 
> > @@ -2364,6 +2357,38 @@ static int bpf_object__finalize_btf(struct bpf_object *obj)
> >         return 0;
> >  }
> >
> > +static inline bool libbpf_prog_needs_vmlinux_btf(struct bpf_program *prog)
> > +{
> 
> I suspect that at some point this approach won't be flexible enough,
> but it simplifies error handling right now, so I'm ok with it.

Acknowledged.

> 
> > +       if (prog->type == BPF_PROG_TYPE_STRUCT_OPS)
> > +               return true;
> > +
> > +       /* BPF_PROG_TYPE_TRACING programs which do not attach to other programs
> > +        * also need vmlinux BTF
> > +        */
> > +       if (prog->type == BPF_PROG_TYPE_TRACING && !prog->attach_prog_fd)
> > +               return true;
> > +
> > +       return false;
> > +}
> > +
> > +static int bpf_object__load_vmlinux_btf(struct bpf_object *obj)
> > +{
> > +       struct bpf_program *prog;
> > +
> > +       bpf_object__for_each_program(prog, obj) {
> > +               if (libbpf_prog_needs_vmlinux_btf(prog)) {
> > +                       obj->btf_vmlinux = libbpf_find_kernel_btf();
> > +                       if (IS_ERR(obj->btf_vmlinux)) {
> > +                               pr_warn("vmlinux BTF is not found\n");
> 
> please, emit error code as well
> 
> also, clear out btf_vmlinux, otherwise your code will attempt to free
> invalid pointer later on

Done, also changed it so that the error code does not get clobbered.
 
> > +                               return -EINVAL;
> > +                       }
> > +                       return 0;
> > +               }
> > +       }
> > +
> > +       return 0;
> > +}
> 
> [...]
> 
> > @@ -5280,10 +5301,17 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
> >         err = err ? : bpf_object__resolve_externs(obj, obj->kconfig);
> >         err = err ? : bpf_object__sanitize_and_load_btf(obj);
> >         err = err ? : bpf_object__sanitize_maps(obj);
> > +       err = err ? : bpf_object__load_vmlinux_btf(obj);
> >         err = err ? : bpf_object__init_kern_struct_ops_maps(obj);
> >         err = err ? : bpf_object__create_maps(obj);
> >         err = err ? : bpf_object__relocate(obj, attr->target_btf_path);
> >         err = err ? : bpf_object__load_progs(obj, attr->log_level);
> > +
> > +       if (obj->btf_vmlinux) {
> 
> you can skip this check, btf__free(NULL) is handled properly as noop

Done. Skipped.

> 
> > +               btf__free(obj->btf_vmlinux);
> > +               obj->btf_vmlinux = NULL;
> > +       }
> > +
> >         if (err)
> >                 goto out;
> 
> [...]
> 
> > +
> > +static inline int __find_vmlinux_btf_id(struct btf *btf, const char *name,
> > +                                       enum bpf_attach_type attach_type)
> > +{
> > +       int err;
> > +
> > +       if (attach_type == BPF_TRACE_RAW_TP)
> > +               err = find_btf_by_prefix_kind(btf, BTF_TRACE_PREFIX, name,
> > +                                             BTF_KIND_TYPEDEF);
> > +       else
> > +               err = btf__find_by_name_kind(btf, name, BTF_KIND_FUNC);
> > +
> > +       return err;
> > +}
> > +
> >  int libbpf_find_vmlinux_btf_id(const char *name,
> >                                enum bpf_attach_type attach_type)
> >  {
> >         struct btf *btf = libbpf_find_kernel_btf();
> 
> I had complaints previously about doing too much heavy-lifting in
> variable assignment, not sure why this slipped through. Can you please
> split this into variable declaration and separate assignment below?

Done.

> 
> > -       char raw_tp_btf[128] = BTF_PREFIX;
> > -       char *dst = raw_tp_btf + sizeof(BTF_PREFIX) - 1;
> > -       const char *btf_name;
> > -       int err = -EINVAL;
> > -       __u32 kind;
> >
> >         if (IS_ERR(btf)) {
> >                 pr_warn("vmlinux BTF is not found\n");
> >                 return -EINVAL;
> >         }
> >
> > -       if (attach_type == BPF_TRACE_RAW_TP) {
> > -               /* prepend "btf_trace_" prefix per kernel convention */
> > -               strncat(dst, name, sizeof(raw_tp_btf) - sizeof(BTF_PREFIX));
> > -               btf_name = raw_tp_btf;
> > -               kind = BTF_KIND_TYPEDEF;
> > -       } else {
> > -               btf_name = name;
> > -               kind = BTF_KIND_FUNC;
> > -       }
> > -       err = btf__find_by_name_kind(btf, btf_name, kind);
> > -       btf__free(btf);
> > -       return err;
> > +       return __find_vmlinux_btf_id(btf, name, attach_type);
> >  }
> >
> >  static int libbpf_find_prog_btf_id(const char *name, __u32 attach_prog_fd)
> > @@ -6567,10 +6612,11 @@ static int libbpf_find_prog_btf_id(const char *name, __u32 attach_prog_fd)
> >         return err;
> >  }
> >
> > -static int libbpf_find_attach_btf_id(const char *name,
> > -                                    enum bpf_attach_type attach_type,
> > -                                    __u32 attach_prog_fd)
> > +static int libbpf_find_attach_btf_id(struct bpf_program *prog)
> >  {
> > +       enum bpf_attach_type attach_type = prog->expected_attach_type;
> > +       __u32 attach_prog_fd = prog->attach_prog_fd;
> > +       const char *name = prog->section_name;
> >         int i, err;
> >
> >         if (!name)
> > @@ -6585,8 +6631,8 @@ static int libbpf_find_attach_btf_id(const char *name,
> >                         err = libbpf_find_prog_btf_id(name + section_defs[i].len,
> >                                                       attach_prog_fd);
> >                 else
> > -                       err = libbpf_find_vmlinux_btf_id(name + section_defs[i].len,
> > -                                                        attach_type);
> > +                       err = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
> > +                               name + section_defs[i].len, attach_type);
> 
> argument indentation is off here, please fix

Fixed.

- KP
> 
> >                 if (err <= 0)
> >                         pr_warn("%s is not found in vmlinux BTF\n", name);
> >                 return err;
> > --
> > 2.20.1
> >