On Wed, Dec 1, 2021 at 1:32 PM Jiri Olsa [off-list ref] wrote:

On Tue, Nov 30, 2021 at 10:53:58PM -0800, Andrii Nakryiko wrote:

quoted

On Wed, Nov 24, 2021 at 12:41 AM Jiri Olsa [off-list ref] wrote:

quoted

Adding support to create multiple probes within single perf event.
This way we can associate single bpf program with multiple kprobes,
because bpf program gets associated with the perf event.

The perf_event_attr is not extended, current fields for kprobe
attachment are used for multi attachment.

I'm a bit concerned with complicating perf_event_attr further to
support this multi-attach. For BPF, at least, we now have
bpf_perf_link and corresponding BPF_LINK_CREATE command in bpf()
syscall which allows much simpler and cleaner API to do this. Libbpf
will actually pick bpf_link-based attachment if kernel supports it. I
think we should better do bpf_link-based approach from the get go.

Another thing I'd like you to keep in mind and think about is BPF
cookie. Currently kprobe/uprobe/tracepoint allow to associate
arbitrary user-provided u64 value which will be accessible from BPF
program with bpf_get_attach_cookie(). With multi-attach kprobes this
because extremely crucial feature to support, otherwise it's both
expensive, inconvenient and complicated to be able to distinguish
between different instances of the same multi-attach kprobe
invocation. So with that, what would be the interface to specify these
BPF cookies for this multi-attach kprobe, if we are going through
perf_event_attr. Probably picking yet another unused field and
union-izing it with a pointer. It will work, but makes the interface
even more overloaded. While for LINK_CREATE we can just add another
pointer to a u64[] with the same size as number of kfunc names and
offsets.

I'm not sure we could bypass perf event easily.. perhaps introduce
BPF_PROG_TYPE_RAW_KPROBE as we did for tracepoints or just new
type for multi kprobe attachment like BPF_PROG_TYPE_MULTI_KPROBE
that might be that way we'd have full control over the API

Indeed. The existing kprobe prog type has this api:
 * Return: BPF programs always return an integer which is interpreted by
 * kprobe handler as:
 * 0 - return from kprobe (event is filtered out)
 * 1 - store kprobe event into ring buffer

that part we cannot change.
No one was using that filtering feature. It often was in a way.
New MULTI_KPROBE prog type should not have it.