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From:   KP Singh <kpsingh@chromium.org>
Date:   Wed, 4 Mar 2020 16:13:03 +0100
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     linux-security-module@vger.kernel.org,
        open list <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Paul Turner <pjt@google.com>, Jann Horn <jannh@google.com>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>
Subject: Re: [PATCH bpf-next v2 3/7] bpf: Introduce BPF_MODIFY_RETURN
Message-ID: <20200304151303.GC9984@chromium.org>
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 <20200304015528.29661-4-kpsingh@chromium.org>
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On 03-M?r 21:08, Andrii Nakryiko wrote:
> On Tue, Mar 3, 2020 at 5:56 PM 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")
> > int BPF_PROG(func_name, int a, int b, int ret)
> > {
> >         // 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    | 130 ++++++++++++++++++++++++++++++---
> >  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, 130 insertions(+), 13 deletions(-)
> >
> 
> This looks good, but I'll Alexei check all the assembly generation
> logic, not too big of an expert on that.
> 
> [...]
> 
> 
> >  static int emit_fallback_jump(u8 **pprog)
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 98ec10b23dbb..3cfdc216a2f4 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -473,6 +473,7 @@ void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start);
> >
> >  enum bpf_tramp_prog_type {
> >         BPF_TRAMP_FENTRY,
> > +       BPF_TRAMP_MODIFY_RETURN,
> 
> This is probably bad idea to re-number BPF_TRAMP_FEXIT for no good
> reason. E.g., if there are some drgn scripts that do some internal
> state printing, this is major inconvenience, while really providing no
> benefit in itself. Consider putting it right before BPF_TRAMP_MAX.

Makes sense, I somehow initially (incorrectly) assumed that the order
represented the order of execution. But the only real demarcation
is the BPF_TRAMP_MAX. Updated it for v3.

- KP

> 
> >         BPF_TRAMP_FEXIT,
> >         BPF_TRAMP_MAX,
> >         BPF_TRAMP_REPLACE, /* more than MAX */
> 
> [...]