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

From: Jiri Olsa <[email-masked]>
Date: Tue, 8 Jun 2021 22:58:32 +0200

On Tue, Jun 08, 2021 at 11:49:31AM -0700, Andrii Nakryiko wrote:
> On Sat, Jun 5, 2021 at 4:12 AM Jiri Olsa <[email-masked]> 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 <[email-masked]>
> > ---
> >  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?

jirka

> 
> >                 if (flags & BPF_TRAMP_F_ORIG_STACK) {
> >                         emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
> > @@ -2052,7 +2062,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> >                 }
> >
> >         if (flags & BPF_TRAMP_F_RESTORE_REGS)
> > -               restore_regs(m, &prog, nr_args, stack_size);
> > +               restore_regs(m, &prog, nr_args, stack_size - ip_arg);
> >
> >         /* This needs to be done regardless. If there were fmod_ret programs,
> >          * the return value is only updated on the stack and still needs to be
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 16fc600503fb..6cbf3c81c650 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -559,6 +559,11 @@ struct btf_func_model {
> >   */
> >  #define BPF_TRAMP_F_ORIG_STACK         BIT(3)
> >
> > +/* First argument is IP address of the caller. Makes sense for fentry/fexit
> > + * programs only.
> > + */
> > +#define BPF_TRAMP_F_IP_ARG             BIT(4)
> > +
> >  /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
> >   * bytes on x86.  Pick a number to fit into BPF_IMAGE_SIZE / 2
> >   */
> > --
> > 2.31.1
> >
>