netdev - Re: [PATCH bpf-next v3 06/10] bpf: Bump MAX_BPF

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Date:   Wed, 15 Sep 2021 19:56:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 06/10] bpf: Bump MAX_BPF_STACK size to 768 bytes

On Wed, Sep 15, 2021 at 10:57 AM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> On Wed, Sep 15, 2021 at 10:03:53PM IST, Alexei Starovoitov wrote:
> > On Wed, Sep 15, 2021 at 10:39:39AM +0530, Kumar Kartikeya Dwivedi wrote:
> > > Increase the maximum stack size accessible to BPF program to 768 bytes.
> > > This is done so that gen_loader can use 94 additional fds for kfunc BTFs
> > > that it passes in to fd_array from the remaining space available for the
> > > loader_stack struct to expand.
> > >
> > > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > > ---
> > >  include/linux/filter.h | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/filter.h b/include/linux/filter.h
> > > index 4a93c12543ee..b214189ece62 100644
> > > --- a/include/linux/filter.h
> > > +++ b/include/linux/filter.h
> > > @@ -82,8 +82,8 @@ struct ctl_table_header;
> > >   */
> > >  #define BPF_SYM_ELF_TYPE   't'
> > >
> > > -/* BPF program can access up to 512 bytes of stack space. */
> > > -#define MAX_BPF_STACK      512
> > > +/* BPF program can access up to 768 bytes of stack space. */
> > > +#define MAX_BPF_STACK      768
> >
> > Yikes.
> > I guess you meant as RFC, right? You didn't really propose
> > to increase prog stack size just for that, right?
> >
>
> Yes, and right, it's ugly :/.
>
> > In the later patch:
> > +/* MAX_BPF_STACK is 768 bytes, so (64 + 32 + 94 (MAX_KFUNC_DESCS) + 2) * 4 */
> >  #define MAX_USED_MAPS 64
> >  #define MAX_USED_PROGS 32
> >
> > @@ -31,6 +33,8 @@ struct loader_stack {
> >         __u32 btf_fd;
> >         __u32 map_fd[MAX_USED_MAPS];
> >         __u32 prog_fd[MAX_USED_PROGS];
> > +       /* Update insn->off store when reordering kfunc_btf_fd */
> > +       __u32 kfunc_btf_fd[MAX_KFUNC_DESCS];
> >         __u32 inner_map_fd;
> > };
> >
> > There are few other ways to do that.
> > For example:
> > A: rename map_fd[] into fds[] and store both map and btf FDs in there.
> > B: move map and btf FDs into data instead of stack.
>
> Both are great suggestions, I thought about A but not B, but it will be better
> (even though it requires more changes, we can do full 256 BTF fds using B).
> Thanks!

btw fd_array doesn't have to have valid FDs in all slots
when passed into the prog_load command.
If bpf prog doesn't index into them they can have garbage.
Just mentioning in case that simplifies the implementation.
I suspect packing btf_fds right after map_fds with no gaps will not be
hard to do, but in case it's somehow too painful there could be a gap.