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@xxxxxxxxx>
> > ---
> > 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
