Add ability for users to specify custom u64 value when creating BPF link for
perf_event-backed BPF programs (kprobe/uprobe, perf_event, tracepoints).

This is useful for cases when the same BPF program is used for attaching and
processing invocation of different tracepoints/kprobes/uprobes in a generic
fashion, but such that each invocation is distinguished from each other (e.g.,
BPF program can look up additional information associated with a specific
kernel function without having to rely on function IP lookups). This enables
new use cases to be implemented simply and efficiently that previously were
possible only through code generation (and thus multiple instances of almost
identical BPF program) or compilation at runtime (BCC-style) on target hosts
(even more expensive resource-wise). For uprobes it is not even possible in
some cases to know function IP before hand (e.g., when attaching to shared
library without PID filtering, in which case base load address is not known
for a library).

This is done by storing u64 user_ctx in struct bpf_prog_array_item,
corresponding to each attached and run BPF program. Given cgroup BPF programs
already use 2 8-byte pointers for their needs and cgroup BPF programs don't
have (yet?) support for user_ctx, reuse that space through union of
cgroup_storage and new user_ctx field.

Make it available to kprobe/tracepoint BPF programs through bpf_trace_run_ctx.
This is set by BPF_PROG_RUN_ARRAY, used by kprobe/uprobe/tracepoint BPF
program execution code, which luckily is now also split from
BPF_PROG_RUN_ARRAY_CG. This run context will be utilized by a new BPF helper
giving access to this user context value from inside a BPF program. Generic
perf_event BPF programs will access this value from perf_event itself through
passed in BPF program context.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
  drivers/media/rc/bpf-lirc.c    |  4 ++--
  include/linux/bpf.h            | 16 +++++++++++++++-
  include/linux/perf_event.h     |  1 +
  include/linux/trace_events.h   |  6 +++---
  include/uapi/linux/bpf.h       |  7 +++++++
  kernel/bpf/core.c              | 29 ++++++++++++++++++-----------
  kernel/bpf/syscall.c           |  2 +-
  kernel/events/core.c           | 21 ++++++++++++++-------
  kernel/trace/bpf_trace.c       |  8 +++++---
  tools/include/uapi/linux/bpf.h |  7 +++++++
  10 files changed, 73 insertions(+), 28 deletions(-)

diff --git a/drivers/media/rc/bpf-lirc.c b/drivers/media/rc/bpf-lirc.c
index afae0afe3f81..7490494273e4 100644
--- a/drivers/media/rc/bpf-lirc.c
+++ b/drivers/media/rc/bpf-lirc.c

@@ -160,7 +160,7 @@ static int lirc_bpf_attach(struct rc_dev *rcdev, struct bpf_prog *prog)
  		goto unlock;
  	}
  
-	ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
+	ret = bpf_prog_array_copy(old_array, NULL, prog, 0, &new_array);
  	if (ret < 0)
  		goto unlock;
  

  void bpf_trace_run1(struct bpf_prog *prog, u64 arg1);

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 00b1267ab4f0..bc1fd54a8f58 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -1448,6 +1448,13 @@ union bpf_attr {
  				__aligned_u64	iter_info;	/* extra bpf_iter_link_info */
  				__u32		iter_info_len;	/* iter_info length */
  			};
+			struct {
+				/* black box user-provided value passed through
+				 * to BPF program at the execution time and
+				 * accessible through bpf_get_user_ctx() BPF helper
+				 */
+				__u64		user_ctx;
+			} perf_event;