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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

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?

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.