Re: [PATCH bpf-next 1/3] bpf, x64: Replace some stack_size usage with offset variables

[Jiri Olsa <jolsa@redhat.com>]

On Mon, Dec 06, 2021 at 01:41:15PM -0800, Andrii Nakryiko wrote:
> On Sat, Dec 4, 2021 at 6:07 AM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > As suggested by Andrii, adding variables for registers and ip
> > address offsets, which makes the code more clear, rather than
> > abusing single stack_size variable for everything.
> >
> > Also describing the stack layout in the comment.
> >
> > There is no function change.
> >
> > Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  arch/x86/net/bpf_jit_comp.c | 42 ++++++++++++++++++++++++-------------
> >  1 file changed, 28 insertions(+), 14 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index 1d7b0c69b644..b106e80e8d9c 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -1941,7 +1941,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> >                                 void *orig_call)
> >  {
> >         int ret, i, nr_args = m->nr_args;
> > -       int stack_size = nr_args * 8;
> > +       int regs_off, ip_off, stack_size = nr_args * 8;
> >         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];
> > @@ -1956,14 +1956,33 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> >         if (!is_valid_bpf_tramp_flags(flags))
> >                 return -EINVAL;
> >
> > +       /* Generated trampoline stack layout:
> > +        *
> > +        * RBP + 8         [ return address  ]
> > +        * RBP + 0         [ RBP             ]
> > +        *
> > +        * RBP - 8         [ return value    ]  BPF_TRAMP_F_CALL_ORIG or
> > +        *                                      BPF_TRAMP_F_RET_FENTRY_RET flags
> > +        *
> > +        *                 [ reg_argN        ]  always
> > +        *                 [ ...             ]
> > +        * RBP - regs_off  [ reg_arg1        ]
> > +        *
> 
> I think it's also worth mentioning that context passed into
> fentry/fexit programs are pointing here (makes it a bit easier to
> track those ctx[-1] and ctx[-2] in the next patch.

ok, jirka

> 
> 
> > +        * RBP - ip_off    [ traced function ]  BPF_TRAMP_F_IP_ARG flag
> > +        */
> > +
> >         /* room for return value of orig_call or fentry prog */
> >         save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
> >         if (save_ret)
> >                 stack_size += 8;
> >
> > +       regs_off = stack_size;
> > +
> >         if (flags & BPF_TRAMP_F_IP_ARG)
> >                 stack_size += 8; /* room for IP address argument */
> >
> > +       ip_off = stack_size;
> > +
> 
> [...]
>