On Sun, Mar 20, 2022 at 2:31 AM Kui-Feng Lee <kuifeng@xxxxxx> wrote:
>
> On Fri, 2022-03-18 at 12:09 -0700, Alexei Starovoitov wrote:
> > On Tue, Mar 15, 2022 at 05:42:29PM -0700, Kui-Feng Lee wrote:
> > > BPF trampolines will create a bpf_trace_run_ctx on their stacks,
> > > and
> > > set/reset the current bpf_run_ctx whenever calling/returning from a
> > > bpf_prog.
> > >
> > > Signed-off-by: Kui-Feng Lee <kuifeng@xxxxxx>
> > > ---
> > > arch/x86/net/bpf_jit_comp.c | 32 ++++++++++++++++++++++++++++++++
> > > include/linux/bpf.h | 12 ++++++++----
> > > kernel/bpf/syscall.c | 4 ++--
> > > kernel/bpf/trampoline.c | 21 +++++++++++++++++----
> > > 4 files changed, 59 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/arch/x86/net/bpf_jit_comp.c
> > > b/arch/x86/net/bpf_jit_comp.c
> > > index 1228e6e6a420..29775a475513 100644
> > > --- a/arch/x86/net/bpf_jit_comp.c
> > > +++ b/arch/x86/net/bpf_jit_comp.c
> > > @@ -1748,10 +1748,33 @@ static int invoke_bpf_prog(const struct
> > > btf_func_model *m, u8 **pprog,
> > > {
> > > u8 *prog = *pprog;
> > > u8 *jmp_insn;
> > > + int ctx_cookie_off = offsetof(struct bpf_trace_run_ctx,
> > > bpf_cookie);
> > > struct bpf_prog *p = l->prog;
> > >
> > > + EMIT1(0x52); /* push rdx */
> >
> > Why save/restore rdx?
>
> >
> > > +
> > > + /* mov rdi, 0 */
> > > + emit_mov_imm64(&prog, BPF_REG_1, 0, 0);
> > > +
> > > + /* Prepare struct bpf_trace_run_ctx.
> > > + * sub rsp, sizeof(struct bpf_trace_run_ctx)
> > > + * mov rax, rsp
> > > + * mov QWORD PTR [rax + ctx_cookie_off], rdi
> > > + */
> >
> > How about the following instead:
> > sub rsp, sizeof(struct bpf_trace_run_ctx)
> > mov qword ptr [rsp + ctx_cookie_off], 0
> > ?
>
> AFAIK, rsp can not be used with the base + displacement addressing
> mode. Although, it can be used with base + index + displacement
> addressing mode.
Where did you find this?
0: 48 c7 44 24 08 00 00 mov QWORD PTR [rsp+0x8],0x0
7: 00 00
> >
> > > + EMIT4(0x48, 0x83, 0xEC, sizeof(struct bpf_trace_run_ctx));
> > > + EMIT3(0x48, 0x89, 0xE0);
> > > + EMIT4(0x48, 0x89, 0x78, ctx_cookie_off);
> > > +
> > > + /* mov rdi, rsp */
> > > + EMIT3(0x48, 0x89, 0xE7);
> > > + /* mov QWORD PTR [rdi + sizeof(struct bpf_trace_run_ctx)],
> > > rax */
> > > + emit_stx(&prog, BPF_DW, BPF_REG_1, BPF_REG_0, sizeof(struct
> > > bpf_trace_run_ctx));
> >
> > why not to do:
> > mov qword ptr[rsp + sizeof(struct bpf_trace_run_ctx)], rsp
> > ?
>
> The same reason as above.
0: 48 89 64 24 08 mov QWORD PTR [rsp+0x8],rsp
