Re: [PATCH v2] bpf/scripts: Generate GCC compatible helpers

[Andrii Nakryiko]

CC Quentin as well

On Mon, Jul 11, 2022 at 5:11 PM James Hilliard
<james.hilliard1@xxxxxxxxx> wrote:
>
> On Mon, Jul 11, 2022 at 5:36 PM Yonghong Song <yhs@xxxxxx> wrote:
> >
> >
> >
> > On 7/6/22 10:28 AM, James Hilliard wrote:
> > > The current bpf_helper_defs.h helpers are llvm specific and don't work
> > > correctly with gcc.
> > >
> > > GCC appears to required kernel helper funcs to have the following
> > > attribute set: __attribute__((kernel_helper(NUM)))
> > >
> > > Generate gcc compatible headers based on the format in bpf-helpers.h.
> > >
> > > This adds conditional blocks for GCC while leaving clang codepaths
> > > unchanged, for example:
> > >       #if __GNUC__ && !__clang__
> > >       void *bpf_map_lookup_elem(void *map, const void *key) __attribute__((kernel_helper(1)));
> > >       #else
> > >       static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;
> > >       #endif
> >
> > It does look like that gcc kernel_helper attribute is better than
> > '(void *) 1' style. The original clang uses '(void *) 1' style is
> > just for simplicity.
>
> Isn't the original style going to be needed for backwards compatibility with
> older clang versions for a while?

I'm curious, is there any added benefit to having this special
kernel_helper attribute vs what we did in Clang for a long time? Did
GCC do it just to be different and require workarounds like this or
there was some technical benefit to this?

This duplication of definitions with #if for each one looks really
awful, IMO. I'd rather have a macro invocation like below (or
something along those lines) for each helper:

BPF_HELPER_DEF(2, void *, bpf_map_update_elem, void *map, const void
*key, const void *value, __u64 flags);

And then define BPF_HELPER_DEF() once based on whether it's Clang or GCC.

>
> >
> > Do you mind to help implement similar attribute in clang so we
> > don't need "#if" here?
>
> That's well outside my area of expertise unfortunately.
>
> >
> > >
> > >       #if __GNUC__ && !__clang__
> > >       long bpf_map_update_elem(void *map, const void *key, const void *value, __u64 flags) __attribute__((kernel_helper(2)));
> > >       #else
> > >       static long (*bpf_map_update_elem)(void *map, const void *key, const void *value, __u64 flags) = (void *) 2;
> > >       #endif
> > >
> > > See:
> > > https://github.com/gcc-mirror/gcc/blob/releases/gcc-12.1.0/gcc/config/bpf/bpf-helpers.h#L24-L27
> > >
> > > This fixes the following build error:
> > > error: indirect call in function, which are not supported by eBPF
> > >
> > > Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
> > > ---
> > > Changes v1 -> v2:
> > >    - more details in commit log
> > > ---
> > >   scripts/bpf_doc.py | 43 ++++++++++++++++++++++++++-----------------
> > >   1 file changed, 26 insertions(+), 17 deletions(-)
> > >
> > [...]