Re: [RFC bpf-next 1/3] bpf: add module support to btf display helpers

[Yonghong Song]

On 11/14/20 8:04 AM, Alexei Starovoitov wrote:
> On Fri, Nov 13, 2020 at 10:59 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> > On Fri, Nov 13, 2020 at 10:11 AM Alan Maguire <alan.maguire@xxxxxxxxxx> wrote:
> > > bpf_snprintf_btf and bpf_seq_printf_btf use a "struct btf_ptr *"
> > > argument that specifies type information about the type to
> > > be displayed.  Augment this information to include a module
> > > name, allowing such display to support module types.
> > >
> > > Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> > > ---
> > >   include/linux/btf.h            |  8 ++++++++
> > >   include/uapi/linux/bpf.h       |  5 ++++-
> > >   kernel/bpf/btf.c               | 18 ++++++++++++++++++
> > >   kernel/trace/bpf_trace.c       | 42 ++++++++++++++++++++++++++++++++----------
> > >   tools/include/uapi/linux/bpf.h |  5 ++++-
> > >   5 files changed, 66 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > > index 2bf6418..d55ca00 100644
> > > --- a/include/linux/btf.h
> > > +++ b/include/linux/btf.h
> > > @@ -209,6 +209,14 @@ static inline const struct btf_var_secinfo *btf_type_var_secinfo(
> > >   const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id);
> > >   const char *btf_name_by_offset(const struct btf *btf, u32 offset);
> > >   struct btf *btf_parse_vmlinux(void);
> > > +#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> > > +struct btf *bpf_get_btf_module(const char *name);
> > > +#else
> > > +static inline struct btf *bpf_get_btf_module(const char *name)
> > > +{
> > > +       return ERR_PTR(-ENOTSUPP);
> > > +}
> > > +#endif
> > >   struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog);
> > >   #else
> > >   static inline const struct btf_type *btf_type_by_id(const struct btf *btf,
> > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > index 162999b..26978be 100644
> > > --- a/include/uapi/linux/bpf.h
> > > +++ b/include/uapi/linux/bpf.h
> > > @@ -3636,7 +3636,8 @@ struct bpf_stack_build_id {
> > >    *             the pointer data is carried out to avoid kernel crashes during
> > >    *             operation.  Smaller types can use string space on the stack;
> > >    *             larger programs can use map data to store the string
> > > - *             representation.
> > > + *             representation.  Module-specific data structures can be
> > > + *             displayed if the module name is supplied.
> > >    *
> > >    *             The string can be subsequently shared with userspace via
> > >    *             bpf_perf_event_output() or ring buffer interfaces.
> > > @@ -5076,11 +5077,13 @@ struct bpf_sk_lookup {
> > >    * potentially to specify additional details about the BTF pointer
> > >    * (rather than its mode of display) - is included for future use.
> > >    * Display flags - BTF_F_* - are passed to bpf_snprintf_btf separately.
> > > + * A module name can be specified for module-specific data.
> > >    */
> > >   struct btf_ptr {
> > >          void *ptr;
> > >          __u32 type_id;
> > >          __u32 flags;            /* BTF ptr flags; unused at present. */
> > > +       const char *module;     /* optional module name. */
> >
> > I think module name is a wrong API here, similarly how type name was
> > wrong API for specifying the type (and thus we use type_id here).
> > Using the module's BTF ID seems like a more suitable interface. That's
> > what I'm going to use for all kinds of existing BPF APIs that expect
> > BTF type to attach BPF programs.
> >
> > Right now, we use only type_id and implicitly know that it's in
> > vmlinux BTF. With module BTFs, we now need a pair of BTF object ID +
> > BTF type ID to uniquely identify the type. vmlinux BTF now can be
> > specified in two different ways: either leaving BTF object ID as zero
> > (for simplicity and backwards compatibility) or specifying it's actual
> > BTF obj ID (which pretty much always should be 1, btw). This feels
> > like a natural extension, WDYT?
> >
> > And similar to type_id, no one should expect users to specify these
> > IDs by hand, Clang built-in and libbpf should work together to figure
> > this out for the kernel to use.
> >
> > BTW, with module names there is an extra problem for end users. Some
> > types could be either built-in or built as a module (e.g., XFS data
> > structures). Why would we require BPF users to care which is the case
> > on any given host?
>
> +1.
> As much as possible libbpf should try to hide the difference between
> type in a module vs type in the vmlinux, since that difference most of the
> time is irrelevant from bpf prog pov.

I just crafted a llvm patch where for __builtin_btf_type_id(), a 64bit 
value is returned instead of a 32bit value. libbpf can use the lower
32bit as the btf_type_id and upper 32bit as the kernel module btf id.

   https://reviews.llvm.org/D91489

feel free to experiment with it to see whether it helps.