On Mon, 2022-05-09 at 14:04 -0700, Alexei Starovoitov wrote:
> 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.
I think you are right.
