bpf: Add bpf_get_attach_cookie() BPF helper to access bpf_cookie value

Add new BPF helper, bpf_get_attach_cookie(), which can be used by BPF programs
to get access to a user-provided bpf_cookie value, specified during BPF
program attachment (BPF link creation) time.

Naming is hard, though. With the concept being named "BPF cookie", I've
considered calling the helper:
  - bpf_get_cookie() -- seems too unspecific and easily mistaken with socket
    cookie;
  - bpf_get_bpf_cookie() -- too much tautology;
  - bpf_get_link_cookie() -- would be ok, but while we create a BPF link to
    attach BPF program to BPF hook, it's still an "attachment" and the
    bpf_cookie is associated with BPF program attachment to a hook, not a BPF
    link itself. Technically, we could support bpf_cookie with old-style
    cgroup programs.So I ultimately rejected it in favor of
    bpf_get_attach_cookie().

Currently all perf_event-backed BPF program types support
bpf_get_attach_cookie() helper. Follow-up patches will add support for
fentry/fexit programs as well.

While at it, mark bpf_tracing_func_proto() as static to make it obvious that
it's only used from within the kernel/trace/bpf_trace.c.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210815070609.987780-7-andrii@kernel.org

tools/include/uapi/linux/bpf.h

@@ -4856,6 +4856,21 @@ union bpf_attr {
  * 		Get address of the traced function (for tracing and kprobe programs).
  * 	Return
  * 		Address of the traced function.
+ *
+ * u64 bpf_get_attach_cookie(void *ctx)
+ * 	Description
+ * 		Get bpf_cookie value provided (optionally) during the program
+ * 		attachment. It might be different for each individual
+ * 		attachment, even if BPF program itself is the same.
+ * 		Expects BPF program context *ctx* as a first argument.
+ *
+ * 		Supported for the following program types:
+ *			- kprobe/uprobe;
+ *			- tracepoint;
+ *			- perf_event.
+ * 	Return
+ *		Value specified by user at BPF link creation/attachment time
+ *		or 0, if it was not specified.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -5032,6 +5047,7 @@ union bpf_attr {
 	FN(timer_start),		\
 	FN(timer_cancel),		\
 	FN(get_func_ip),		\
+	FN(get_attach_cookie),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper