Re: [PATCH bpf-next v3 06/10] bpf: Bump MAX_BPF_STACK size to 768 bytes

From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf@vger.kernel.org, "Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Andrii Nakryiko" <andrii@kernel.org>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>,
	netdev@vger.kernel.org
Subject: Re: [PATCH bpf-next v3 06/10] bpf: Bump MAX_BPF_STACK size to 768 bytes
Date: Wed, 15 Sep 2021 23:27:22 +0530	[thread overview]
Message-ID: <20210915175722.viwvzg4ilpumqxid@apollo.localdomain> (raw)
In-Reply-To: <20210915163353.ysltf6edghj75koq@ast-mbp.dhcp.thefacebook.com>

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@gmail.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!

--
Kartikeya