Re: [PATCH bpf-next 3/7] bpf: Introduce BPF_MODIFY_RETURN

[KP Singh <kpsingh@chromium.org>]

On 03-Mär 14:37, Andrii Nakryiko wrote:
> On Tue, Mar 3, 2020 at 6:12 AM KP Singh <kpsingh@chromium.org> wrote:
> >
> > From: KP Singh <kpsingh@google.com>
> >
> > When multiple programs are attached, each program receives the return
> > value from the previous program on the stack and the last program
> > provides the return value to the attached function.
> >
> > The fmod_ret bpf programs are run after the fentry programs and before
> > the fexit programs. The original function is only called if all the
> > fmod_ret programs return 0 to avoid any unintended side-effects. The
> > success value, i.e. 0 is not currently configurable but can be made so
> > where user-space can specify it at load time.
> >
> > For example:
> >
> > int func_to_be_attached(int a, int b)
> > {  <--- do_fentry
> >
> > do_fmod_ret:
> >    <update ret by calling fmod_ret>
> >    if (ret != 0)
> >         goto do_fexit;
> >
> > original_function:
> >
> >     <side_effects_happen_here>
> >
> > }  <--- do_fexit
> >
> > The fmod_ret program attached to this function can be defined as:
> >
> > SEC("fmod_ret/func_to_be_attached")
> > BPF_PROG(func_name, int a, int b, int ret)
> 
> same as on cover letter, return type is missing

Fixed. Thanks!

> 
> > {
> >         // This will skip the original function logic.
> >         return 1;
> > }
> >
> > The first fmod_ret program is passed 0 in its return argument.
> >
> > Signed-off-by: KP Singh <kpsingh@google.com>
> > ---
> >  arch/x86/net/bpf_jit_comp.c    | 96 ++++++++++++++++++++++++++++++++--
> >  include/linux/bpf.h            |  1 +
> >  include/uapi/linux/bpf.h       |  1 +
> >  kernel/bpf/btf.c               |  3 +-
> >  kernel/bpf/syscall.c           |  1 +
> >  kernel/bpf/trampoline.c        |  5 +-
> >  kernel/bpf/verifier.c          |  1 +
> >  tools/include/uapi/linux/bpf.h |  1 +
> >  8 files changed, 103 insertions(+), 6 deletions(-)
> >
> 
> [...]
> 
> >
> > +       if (fmod_ret->nr_progs) {
> > +               branches = kcalloc(fmod_ret->nr_progs, sizeof(u8 *),
> > +                                  GFP_KERNEL);
> > +               if (!branches)
> > +                       return -ENOMEM;
> > +               if (invoke_bpf_mod_ret(m, &prog, fmod_ret, stack_size,
> > +                                      branches))
> 
> branches leaks here

Good catch, sloppy work here by me.

> 
> > +                       return -EINVAL;
> > +       }
> > +
> >         if (flags & BPF_TRAMP_F_CALL_ORIG) {
> > -               if (fentry->nr_progs)
> > +               if (fentry->nr_progs || fmod_ret->nr_progs)
> >                         restore_regs(m, &prog, nr_args, stack_size);
> >
> >                 /* call original function */
> > @@ -1573,6 +1649,14 @@ int arch_prepare_bpf_trampoline(void *image, void *image_end,
> 
> there is early return one line above here, you need to free branches
> in that case to not leak memory
> 
> So I guess it's better to do goto cleanup approach at this point?

yeah, agreed, updated to doing a cleanup at the end.

- KP

> 
> >                 emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
> >         }
> >
> > +       if (fmod_ret->nr_progs) {
> > +               align16_branch_target(&prog);
> > +               for (i = 0; i < fmod_ret->nr_progs; i++)
> > +                       emit_cond_near_jump(&branches[i], prog, branches[i],
> > +                                           X86_JNE);
> > +               kfree(branches);
> > +       }
> > +
> 
> [...]