Re: [PATCH bpf-next 1/2] bpf: btf: Support optional flags for BTF_SET8 sets

[Alexei Starovoitov]

On Thu, Jan 4, 2024 at 3:23 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Wed, Jan 03, 2024 at 04:31:55PM -0700, Daniel Xu wrote:
> > This commit adds support for optional flags on BTF_SET8s.
> > struct btf_id_set8 already supported 32 bits worth of flags, but was
> > only used for alignment purposes before.
> >
> > We now use these bits to encode flags. The next commit will tag all
> > kfunc sets with a flag so that pahole can recognize which
> > BTF_ID_FLAGS(func, ..) are actual kfuncs.
> >
> > Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> > ---
> >  include/linux/btf_ids.h | 14 +++++++++-----
> >  1 file changed, 9 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> > index a9cb10b0e2e9..88f914579fa1 100644
> > --- a/include/linux/btf_ids.h
> > +++ b/include/linux/btf_ids.h
> > @@ -183,17 +183,21 @@ extern struct btf_id_set name;
> >   * .word (1 << 3) | (1 << 1) | (1 << 2)
> >   *
> >   */
> > -#define __BTF_SET8_START(name, scope)                        \
> > +#define ___BTF_SET8_START(name, scope, flags)                \
> >  asm(                                                 \
> >  ".pushsection " BTF_IDS_SECTION ",\"a\";       \n"   \
> >  "." #scope " __BTF_ID__set8__" #name ";        \n"   \
> >  "__BTF_ID__set8__" #name ":;                   \n"   \
> > -".zero 8                                       \n"   \
> > +".zero 4                                       \n"   \
> > +".long " #flags                               "\n"   \
> >  ".popsection;                                  \n");
> >
> > -#define BTF_SET8_START(name)                         \
> > +#define __BTF_SET8_START(name, scope, flags, ...)    \
> > +___BTF_SET8_START(name, scope, flags)
> > +
> > +#define BTF_SET8_START(name, ...)                    \
> >  __BTF_ID_LIST(name, local)                           \
> > -__BTF_SET8_START(name, local)
> > +__BTF_SET8_START(name, local, ##__VA_ARGS__, 0)
>
> I think it'd better to use something like:
>
>   BTF_SET8_KFUNCS_START(fsverity_set_ids)
>
> instead of:
>
>   BTF_SET8_START(fsverity_set_ids, BTF_SET8_KFUNC)
>
> and to keep current BTF_SET8_START without flags argument, like:
>
>   #define BTF_SET8_START(name) \
>     __BTF_SET8_START(... , 0, ...
>
>   #define BTF_SET8_KFUNCS_START(name) \
>     __BTF_SET8_START(... , BTF_SET8_KFUNC, ...

I was about to suggest the same :)

We can drop SET8 part as well, since it's implementation detail.
Just BTF_KFUNCS_START and pair it with BTF_KFUNCS_END
that will be the same as BTF_SET8_END.
Until we need to do something else with these macros.

>
> also I'd rename BTF_SET8_KFUNC to BTF_SET8_KFUNCS (with S)
>
> do you have the pahole changes somewhere? would be great to
> see all the related changes and try the whole thing

+1
without corresponding pahole changes it's not clear whether
it actually helps.