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

[KP Singh <kpsingh@chromium.org>]

On 23-Dec 22:36, Andrii Nakryiko wrote:
> On Fri, Dec 20, 2019 at 7:43 AM KP Singh <kpsingh@chromium.org> wrote:
> >
> > From: KP Singh <kpsingh@google.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@google.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(-)
> >
> 
> [...]