On Sat, May 07, 2022 at 08:21:14PM -0700, Kui-Feng Lee wrote:
>
> + /* Prepare struct bpf_tramp_run_ctx.
> + * sub rsp, sizeof(struct bpf_tramp_run_ctx)
> + */
> + EMIT4(0x48, 0x83, 0xEC, sizeof(struct bpf_tramp_run_ctx));
> +
> if (fentry->nr_links)
> if (invoke_bpf(m, &prog, fentry, regs_off,
> flags & BPF_TRAMP_F_RET_FENTRY_RET))
> @@ -2098,6 +2121,11 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> }
>
> if (flags & BPF_TRAMP_F_CALL_ORIG) {
> + /* pop struct bpf_tramp_run_ctx
> + * add rsp, sizeof(struct bpf_tramp_run_ctx)
> + */
> + EMIT4(0x48, 0x83, 0xC4, sizeof(struct bpf_tramp_run_ctx));
> +
> restore_regs(m, &prog, nr_args, regs_off);
>
> /* call original function */
> @@ -2110,6 +2138,11 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> im->ip_after_call = prog;
> memcpy(prog, x86_nops[5], X86_PATCH_SIZE);
> prog += X86_PATCH_SIZE;
> +
> + /* Prepare struct bpf_tramp_run_ctx.
> + * sub rsp, sizeof(struct bpf_tramp_run_ctx)
> + */
> + EMIT4(0x48, 0x83, 0xEC, sizeof(struct bpf_tramp_run_ctx));
> }
>
> if (fmod_ret->nr_links) {
> @@ -2133,6 +2166,11 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> goto cleanup;
> }
>
> + /* pop struct bpf_tramp_run_ctx
> + * add rsp, sizeof(struct bpf_tramp_run_ctx)
> + */
> + EMIT4(0x48, 0x83, 0xC4, sizeof(struct bpf_tramp_run_ctx));
> +
What is the point of all of these additional sub/add rsp ?
It seems unconditionally increasing stack_size by sizeof(struct bpf_tramp_run_ctx)
will achieve the same and above 4 extra insns won't be needed.
