Re: [PATCH] bpf: avoid warning for unused register_bpf_struct_ops()

[Martin KaFai Lau]

On 12/4/25 1:42 AM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> The macro originally introduced in commit f6be98d19985 ("bpf, net:
> switch to dynamic registration") causes a warning in the new smc code
> because of the way it evaluates the arguments:
>
> In file included from include/linux/bpf_verifier.h:7,
>                   from net/smc/smc_hs_bpf.c:13:
> net/smc/smc_hs_bpf.c: In function 'bpf_smc_hs_ctrl_init':
> include/linux/bpf.h:2076:50: error: statement with no effect [-Werror=unused-value]
>   2076 | #define register_bpf_struct_ops(st_ops, type) ({ (void *)(st_ops); 0; })
>        |                                                  ^~~~~~~~~~~~~~~~
> net/smc/smc_hs_bpf.c:139:16: note: in expansion of macro 'register_bpf_struct_ops'
>    139 |         return register_bpf_struct_ops(&bpf_smc_hs_ctrl_ops, smc_hs_ctrl);
>        |                ^~~~~~~~~~~~~~~~~~~~~~~
>
> Work around this using an inline function that takes the argument,
> the same way as the normal implementation. Since the second argument to
> register_bpf_struct_ops() is a type rather than an object, this still
> has to be a macro, but it can call a new inline helper internally like
> the normal one does.

Thanks for the patch. This has been fixed in 
"https://lore.kernel.org/bpf/988c61e5fea280872d81b3640f1f34d0619cfbbf.1764843951.git.geert@xxxxxxxxxxxxxx/"; 
to completely remove its usage from smc. The smc usage without 
CONFIG_BPF_JIT was an overlook. This empty register_bpf_struct_ops 
should be removed from the bpf-next tree as a cleanup.