linux-kernel - Re: [PATCH bpf-next v1 09/13] bpf: lsm: Add a helper function bpf_lsm_event

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Message-ID: <20191230151135.GC70684@google.com>
Date:   Mon, 30 Dec 2019 16:11:35 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     KP Singh <kpsingh@...omium.org>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, linux-security-module@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Garnier <thgarnie@...omium.org>,
        Michael Halcrow <mhalcrow@...gle.com>,
        Paul Turner <pjt@...gle.com>,
        Brendan Gregg <brendan.d.gregg@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Christian Brauner <christian@...uner.io>,
        Mickaël Salaün <mic@...ikod.net>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Quentin Monnet <quentin.monnet@...ronome.com>,
        Andrey Ignatov <rdna@...com>, Joe Stringer <joe@...d.net.nz>
Subject: Re: [PATCH bpf-next v1 09/13] bpf: lsm: Add a helper function
 bpf_lsm_event_output

On 23-Dec 22:36, Andrii Nakryiko wrote:
> On Fri, Dec 20, 2019 at 7:43 AM KP Singh <kpsingh@...omium.org> wrote:
> >
> > From: KP Singh <kpsingh@...gle.com>
> >
> > This helper is similar to bpf_perf_event_output except that
> > it does need a ctx argument which is more usable in the
> > BTF based LSM programs where the context is converted to
> > the signature of the attacthed BTF type.
> >
> > An example usage of this function would be:
> >
> > struct {
> >          __uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
> >          __uint(key_size, sizeof(int));
> >          __uint(value_size, sizeof(u32));
> > } perf_map SEC(".maps");
> >
> > BPF_TRACE_1(bpf_prog1, "lsm/bprm_check_security,
> >             struct linux_binprm *, bprm)
> > {
> >         char buf[BUF_SIZE];
> >         int len;
> >         u64 flags = BPF_F_CURRENT_CPU;
> >
> >         /* some logic that fills up buf with len data */
> >         len = fill_up_buf(buf);
> >         if (len < 0)
> >                 return len;
> >         if (len > BU)
> >                 return 0;
> >
> >         bpf_lsm_event_output(&perf_map, flags, buf, len);
> 
> This seems to be generally useful and not LSM-specific, so maybe name
> it more generically as bpf_event_output instead?

Agreed, I am happy to rename this.

> 
> I'm also curious why we needed both bpf_perf_event_output and
> bpf_perf_event_output_raw_tp, if it could be done as simply as you did
> it here. What's different between those three and why your
> bpf_lsm_event_output doesn't need pt_regs passed into them?

That's because my implementation uses the following function from
bpf_trace.c:

u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
		     void *ctx, u64 ctx_size, bpf_ctx_copy_t ctx_copy)

This does not require a pt_regs argument and handles fetching them
internally:

	regs = this_cpu_ptr(&bpf_pt_regs.regs[nest_level - 1]);

	perf_fetch_caller_regs(regs);
	perf_sample_data_init(sd, 0, 0);
	sd->raw = &raw;

	ret = __bpf_perf_event_output(regs, map, flags, sd);

- KP

> 
> >         return 0;
> > }
> >
> > Signed-off-by: KP Singh <kpsingh@...gle.com>
> > ---
> >  include/uapi/linux/bpf.h       | 10 +++++++++-
> >  kernel/bpf/verifier.c          |  1 +
> >  security/bpf/ops.c             | 21 +++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h | 10 +++++++++-
> >  4 files changed, 40 insertions(+), 2 deletions(-)
> >
> 
> [...]