From: Kui-Feng Lee <kuifeng@fb.com>
To: "alexei.starovoitov@gmail.com" <alexei.starovoitov@gmail.com>
Cc: "daniel@iogearbox.net" <daniel@iogearbox.net>,
"ast@kernel.org" <ast@kernel.org>,
"andrii@kernel.org" <andrii@kernel.org>,
"bpf@vger.kernel.org" <bpf@vger.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/4] bpf, x86: Create bpf_trace_run_ctx on the caller thread's stack
Date: Mon, 21 Mar 2022 19:00:38 +0000 [thread overview]
Message-ID: <a3270d0687f90ee9a87ec65b6617da4cbf4e3ca5.camel@fb.com> (raw)
In-Reply-To: <CAADnVQJTyjDM-5Mo_R+B0_gj6tZH5zfP9k1dD48h=Nrc7p8rWA@mail.gmail.com>
On Sun, 2022-03-20 at 13:08 -0700, Alexei Starovoitov wrote:
> On Sun, Mar 20, 2022 at 2:31 AM Kui-Feng Lee <kuifeng@fb.com> 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@fb.com>
> > > > ---
> > > > 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?
I use the following document to figure out opcodes.
https://ref.x86asm.net/coder64.html#modrm_byte_32_64
It lists available addressing modes and codes.
By the way, I found I had missed SIB byte, that provides extra
features. It seems working for this case. I will try it.
>
> 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
next prev parent reply other threads:[~2022-03-21 19:00 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-16 0:42 [PATCH bpf-next v2 0/4] Attach a cookie to a tracing program Kui-Feng Lee
2022-03-16 0:42 ` [PATCH bpf-next v2 1/4] bpf, x86: Generate trampolines from bpf_links Kui-Feng Lee
2022-03-16 0:42 ` [PATCH bpf-next v2 2/4] bpf, x86: Create bpf_trace_run_ctx on the caller thread's stack Kui-Feng Lee
2022-03-18 19:09 ` Alexei Starovoitov
2022-03-20 9:31 ` Kui-Feng Lee
2022-03-20 20:08 ` Alexei Starovoitov
2022-03-21 19:00 ` Kui-Feng Lee [this message]
2022-03-21 23:04 ` Andrii Nakryiko
2022-03-21 23:25 ` Alexei Starovoitov
2022-03-21 23:38 ` Andrii Nakryiko
2022-03-21 23:08 ` Andrii Nakryiko
2022-03-22 15:30 ` Kui-Feng Lee
2022-03-22 21:08 ` Andrii Nakryiko
2022-03-16 0:42 ` [PATCH bpf-next v2 3/4] bpf, x86: Support BPF cookie for fentry/fexit/fmod_ret Kui-Feng Lee
2022-03-18 19:13 ` Alexei Starovoitov
2022-03-21 23:24 ` Andrii Nakryiko
2022-03-21 23:37 ` Andrii Nakryiko
2022-04-12 16:50 ` Kui-Feng Lee
2022-03-22 1:15 ` Alexei Starovoitov
2022-03-22 4:32 ` Andrii Nakryiko
2022-04-06 5:35 ` Kui-Feng Lee
2022-04-06 17:00 ` Andrii Nakryiko
2022-03-21 23:18 ` Andrii Nakryiko
2022-03-22 16:08 ` Kui-Feng Lee
2022-03-22 21:06 ` Andrii Nakryiko
2022-04-06 22:44 ` Kui-Feng Lee
2022-03-16 0:42 ` [PATCH bpf-next v2 4/4] selftest/bpf: The test cses of " Kui-Feng Lee
2022-03-18 19:21 ` Alexei Starovoitov
2022-03-20 8:43 ` Kui-Feng Lee
2022-03-21 23:29 ` Andrii Nakryiko
2022-03-21 23:36 ` Andrii Nakryiko
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