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From:   KP Singh <kpsingh@chromium.org>
Date:   Mon, 23 Mar 2020 20:44:30 +0100
To:     Yonghong Song <yhs@fb.com>
Cc:     linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        Brendan Jackman <jackmanb@google.com>,
        Florent Revest <revest@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Paul Turner <pjt@google.com>, Jann Horn <jannh@google.com>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and
 execution
Message-ID: <20200323194430.GA18787@chromium.org>
References: <20200323164415.12943-1-kpsingh@chromium.org>
 <20200323164415.12943-5-kpsingh@chromium.org>
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On 23-M?r 12:16, Yonghong Song wrote:
> 
> 
> On 3/23/20 9:44 AM, KP Singh wrote:
> > From: KP Singh <kpsingh@google.com>
> > 
> > JITed BPF programs are dynamically attached to the LSM hooks
> > using BPF trampolines. The trampoline prologue generates code to handle
> > conversion of the signature of the hook to the appropriate BPF context.
> > 
> > The allocated trampoline programs are attached to the nop functions
> > initialized as LSM hooks.
> > 
> > BPF_PROG_TYPE_LSM programs must have a GPL compatible license and
> > and need CAP_SYS_ADMIN (required for loading eBPF programs).
> > 
> > Upon attachment:
> > 
> > * A BPF fexit trampoline is used for LSM hooks with a void return type.
> > * A BPF fmod_ret trampoline is used for LSM hooks which return an
> >    int. The attached programs can override the return value of the
> >    bpf LSM hook to indicate a MAC Policy decision.
> > 
> > Signed-off-by: KP Singh <kpsingh@google.com>
> > Reviewed-by: Brendan Jackman <jackmanb@google.com>
> > Reviewed-by: Florent Revest <revest@google.com>
> > ---
> >   include/linux/bpf.h     |  4 ++++
> >   include/linux/bpf_lsm.h | 11 +++++++++++
> >   kernel/bpf/bpf_lsm.c    | 29 +++++++++++++++++++++++++++++
> >   kernel/bpf/btf.c        |  9 ++++++++-
> >   kernel/bpf/syscall.c    | 26 ++++++++++++++++++++++----
> >   kernel/bpf/trampoline.c | 17 +++++++++++++----
> >   kernel/bpf/verifier.c   | 19 +++++++++++++++----
> >   7 files changed, 102 insertions(+), 13 deletions(-)
> > 
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index af81ec7b783c..adf2e5a6de4b 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -433,6 +433,10 @@ struct btf_func_model {
> >    * programs only. Should not be used with normal calls and indirect calls.
> >    */
> >   #define BPF_TRAMP_F_SKIP_FRAME		BIT(2)
> > +/* Override the return value of the original function. This flag only makes
> > + * sense for fexit trampolines.
> > + */
> > +#define BPF_TRAMP_F_OVERRIDE_RETURN     BIT(3)
> 
> Whether the return value is overridable is determined by hook return type as
> below. Do we still need this flag?

Apologies, this is a relic and should not have been there, will send a
new revision with this removed.

- KP

> 
> >   /* 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
> [...]