Re: [PATCH bpf-next 1/2] bpf: Pass the same orig_call value to trampoline functions

[Martin KaFai Lau <martin.lau@linux.dev>]

On 5/12/25 1:57 PM, Ilya Leoshkevich wrote:
> There is currently some confusion in the s390x JIT regarding whether
> orig_call can be NULL and what that means. Originally the NULL value
> was used to distinguish the struct_ops case, but this was superseded by
> BPF_TRAMP_F_INDIRECT (see commit 0c970ed2f87c ("s390/bpf: Fix indirect
> trampoline generation").
> 
> The remaining reason to have this check is that NULL can actually be
> passed to the arch_bpf_trampoline_size() call - but not to the
> respective arch_prepare_bpf_trampoline()! call - by
> bpf_struct_ops_prepare_trampoline().
> 
> Remove this asymmetry by passing stub_func to both functions, so that
> JITs may rely on orig_call never being NULL.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>   kernel/bpf/bpf_struct_ops.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
> index db13ee70d94d..96113633e391 100644
> --- a/kernel/bpf/bpf_struct_ops.c
> +++ b/kernel/bpf/bpf_struct_ops.c
> @@ -601,7 +601,7 @@ int bpf_struct_ops_prepare_trampoline(struct bpf_tramp_links *tlinks,
>   	if (model->ret_size > 0)
>   		flags |= BPF_TRAMP_F_RET_FENTRY_RET;
>   
> -	size = arch_bpf_trampoline_size(model, flags, tlinks, NULL);
> +	size = arch_bpf_trampoline_size(model, flags, tlinks, stub_func);

The change looks ok but not sure why it is needed.

I can see why stub_func is needed to generate the final image in 
arch_prepare_bpf_trampoline() in x86. The "arch_bpf_trampoline_size()" here is 
generating a temporary image, so NULL or not doesn't seem to matter.

Does the s390 jit need to use the actual stub_func address somewhere in the 
temporary and/or final image?