On Tue, Apr 27, 2021 at 09:36:54AM -0700, Andrii Nakryiko wrote:
> On Mon, Apr 26, 2021 at 7:53 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Mon, Apr 26, 2021 at 10:14:45AM -0700, Andrii Nakryiko wrote:
> > > On Thu, Apr 22, 2021 at 5:27 PM Alexei Starovoitov
> > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > >
> > > > From: Alexei Starovoitov <ast@xxxxxxxxxx>
> > > >
> > > > Add support for FD_IDX make libbpf prefer that approach to loading programs.
> > > >
> > > > Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> > > > ---
> > > > tools/lib/bpf/bpf.c | 1 +
> > > > tools/lib/bpf/libbpf.c | 70 +++++++++++++++++++++++++++++----
> > > > tools/lib/bpf/libbpf_internal.h | 1 +
> > > > 3 files changed, 65 insertions(+), 7 deletions(-)
> > > >
> > >
>
> [...]
>
> > > > for (i = 0; i < obj->nr_programs; i++) {
> > > > prog = &obj->programs[i];
> > > > if (prog_is_subprog(obj, prog))
> > > > @@ -7256,10 +7308,14 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level)
> > > > continue;
> > > > }
> > > > prog->log_level |= log_level;
> > > > + prog->fd_array = fd_array;
> > >
> > > you are not freeing this memory on success, as far as I can see.
> >
> > hmm. there is free on success below.
>
> right, my bad, I somehow understood as if it was only for error case
>
> >
> > > And
> > > given multiple programs are sharing fd_array, it's a bit problematic
> > > for prog to have fd_array. This is per-object properly, so let's add
> > > it at bpf_object level and clean it up on bpf_object__close()? And by
> > > assigning to obj->fd_array at malloc() site, you won't need to do all
> > > the error-handling free()s below.
> >
> > hmm. that sounds worse.
> > why add another 8 byte to bpf_object that won't be used
> > until this last step of bpf_object__load_progs.
> > And only for the duration of this loading.
> > It's cheaper to have this alloc here with two free()s below.
>
> So if you care about extra 8 bytes, then it's even more efficient to
> have just one obj->fd_array rather than N prog->fd_array, no?
I think it's layer breaking when bpf_program__load()->load_program()
has to reach out to prog->obj to do its work.
The layers are already a mess due to:
&prog->obj->maps[prog->obj->rodata_map_idx]
I wanted to avoid making it uglier.
> And it's
> also not very clean that prog->fd_array will have a dangling pointer
> to deallocated memory after bpf_object__load_progs().
prog->reloc_desc is free and zeroed after __relocate.
prog->insns are freed and _not_ zereod after __load_progs.
so prog->fd_array won't be the first such pointer.
I can add zeroing, of course.
>
> But that brings the entire question of why use fd_array at all here?
> Commit description doesn't explain why libbpf has to use fd_array and
> why it should be preferred. What are the advantages justifying added
> complexity and extra memory allocation/clean up? It also reduces test
> coverage of the "old ways" that offer the same capabilities. I think
> this should be part of the commit description, if we agree that
> fd_array has to be used outside of the auto-generated loader program.
I can add a knob to it to use it during loader gen for the loader gen
and for the runner of the loader prog.
I think it will add more complexity.
The bpf CI runs on older kernels, so the test coverage of "old ways"
is not reduced regardless.
>From the kernel pov BPF_PSEUDO_MAP_FD vs BPF_PSEUDO_MAP_IDX there is
no advantage.
>From the libbpf side patch 9 looked trivial enough _not_ do it conditionally,
but whatever. I don't mind more 'if'-s.
