netdev - Re: [PATCH 10/19] bpf: Allow to store caller's ip as argument

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Date:   Tue, 8 Jun 2021 23:11:18 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, Daniel Xu <dxu@...uu.xyz>,
        Viktor Malik <vmalik@...hat.com>
Subject: Re: [PATCH 10/19] bpf: Allow to store caller's ip as argument

On Tue, Jun 08, 2021 at 02:02:56PM -0700, Andrii Nakryiko wrote:
> On Tue, Jun 8, 2021 at 1:58 PM Jiri Olsa <jolsa@...hat.com> wrote:
> >
> > On Tue, Jun 08, 2021 at 11:49:31AM -0700, Andrii Nakryiko wrote:
> > > On Sat, Jun 5, 2021 at 4:12 AM Jiri Olsa <jolsa@...nel.org> wrote:
> > > >
> > > > When we will have multiple functions attached to trampoline
> > > > we need to propagate the function's address to the bpf program.
> > > >
> > > > Adding new BPF_TRAMP_F_IP_ARG flag to arch_prepare_bpf_trampoline
> > > > function that will store origin caller's address before function's
> > > > arguments.
> > > >
> > > > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > > > ---
> > > >  arch/x86/net/bpf_jit_comp.c | 18 ++++++++++++++----
> > > >  include/linux/bpf.h         |  5 +++++
> > > >  2 files changed, 19 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > > > index b77e6bd78354..d2425c18272a 100644
> > > > --- a/arch/x86/net/bpf_jit_comp.c
> > > > +++ b/arch/x86/net/bpf_jit_comp.c
> > > > @@ -1951,7 +1951,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > > >                                 void *orig_call)
> > > >  {
> > > >         int ret, i, cnt = 0, nr_args = m->nr_args;
> > > > -       int stack_size = nr_args * 8;
> > > > +       int stack_size = nr_args * 8, ip_arg = 0;
> > > >         struct bpf_tramp_progs *fentry = &tprogs[BPF_TRAMP_FENTRY];
> > > >         struct bpf_tramp_progs *fexit = &tprogs[BPF_TRAMP_FEXIT];
> > > >         struct bpf_tramp_progs *fmod_ret = &tprogs[BPF_TRAMP_MODIFY_RETURN];
> > > > @@ -1975,6 +1975,9 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > > >                  */
> > > >                 orig_call += X86_PATCH_SIZE;
> > > >
> > > > +       if (flags & BPF_TRAMP_F_IP_ARG)
> > > > +               stack_size += 8;
> > > > +
> > >
> > > nit: move it a bit up where we adjust stack_size for BPF_TRAMP_F_CALL_ORIG flag?
> >
> > ok
> >
> > >
> > > >         prog = image;
> > > >
> > > >         EMIT1(0x55);             /* push rbp */
> > > > @@ -1982,7 +1985,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > > >         EMIT4(0x48, 0x83, 0xEC, stack_size); /* sub rsp, stack_size */
> > > >         EMIT1(0x53);             /* push rbx */
> > > >
> > > > -       save_regs(m, &prog, nr_args, stack_size);
> > > > +       if (flags & BPF_TRAMP_F_IP_ARG) {
> > > > +               emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
> > > > +               EMIT4(0x48, 0x83, 0xe8, X86_PATCH_SIZE); /* sub $X86_PATCH_SIZE,%rax*/
> > > > +               emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -stack_size);
> > > > +               ip_arg = 8;
> > > > +       }
> > >
> > > why not pass flags into save_regs and let it handle this case without
> > > this extra ip_arg adjustment?
> > >
> > > > +
> > > > +       save_regs(m, &prog, nr_args, stack_size - ip_arg);
> > > >
> > > >         if (flags & BPF_TRAMP_F_CALL_ORIG) {
> > > >                 /* arg1: mov rdi, im */
> > > > @@ -2011,7 +2021,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> > > >         }
> > > >
> > > >         if (flags & BPF_TRAMP_F_CALL_ORIG) {
> > > > -               restore_regs(m, &prog, nr_args, stack_size);
> > > > +               restore_regs(m, &prog, nr_args, stack_size - ip_arg);
> > > >
> > >
> > > similarly (and symmetrically), pass flags into restore_regs() to
> > > handle that ip_arg transparently?
> >
> > so you mean something like:
> >
> >         if (flags & BPF_TRAMP_F_IP_ARG)
> >                 stack_size -= 8;
> >
> > in both save_regs and restore_regs function, right?
> 
> yes, but for save_regs it will do more (emit_ldx and stuff)

so the whole stuff then, ok

jirka