libbpf: Use weak hidden modifier for USDT BPF-side API functions

Use __weak __hidden for bpf_usdt_xxx() APIs instead of much more
confusing `static inline __noinline`. This was previously impossible due
to libbpf erroring out on CO-RE relocations pointing to eliminated weak
subprogs. Now that previous patch fixed this issue, switch back to
__weak __hidden as it's a more direct way of specifying the desired
behavior.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220408181425.2287230-3-andrii@kernel.org

tools/lib/bpf/usdt.bpf.h

@@ -103,7 +103,7 @@ int __bpf_usdt_spec_id(struct pt_regs *ctx)
 }
 
 /* Return number of USDT arguments defined for currently traced USDT. */
-static inline __noinline
+__weak __hidden
 int bpf_usdt_arg_cnt(struct pt_regs *ctx)
 {
 	struct __bpf_usdt_spec *spec;
@@ -124,7 +124,7 @@ int bpf_usdt_arg_cnt(struct pt_regs *ctx)
  * Returns 0 on success; negative error, otherwise.
  * On error *res is guaranteed to be set to zero.
  */
-static inline __noinline
+__weak __hidden
 int bpf_usdt_arg(struct pt_regs *ctx, __u64 arg_num, long *res)
 {
 	struct __bpf_usdt_spec *spec;
@@ -204,7 +204,7 @@ int bpf_usdt_arg(struct pt_regs *ctx, __u64 arg_num, long *res)
  * utilizing BPF cookies internally, so user can't use BPF cookie directly
  * for USDT programs and has to use bpf_usdt_cookie() API instead.
  */
-static inline __noinline
+__weak __hidden
 long bpf_usdt_cookie(struct pt_regs *ctx)
 {
 	struct __bpf_usdt_spec *spec;