libbpf: Add auto-attach for uprobes based on section name

Now that u[ret]probes can use name-based specification, it makes
sense to add support for auto-attach based on SEC() definition.
The format proposed is

        SEC("u[ret]probe/binary:[raw_offset|[function_name[+offset]]")

For example, to trace malloc() in libc:

        SEC("uprobe/libc.so.6:malloc")

...or to trace function foo2 in /usr/bin/foo:

        SEC("uprobe//usr/bin/foo:foo2")

Auto-attach is done for all tasks (pid -1).  prog can be an absolute
path or simply a program/library name; in the latter case, we use
PATH/LD_LIBRARY_PATH to resolve the full path, falling back to
standard locations (/usr/bin:/usr/sbin or /usr/lib64:/usr/lib) if
the file is not found via environment-variable specified locations.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1648654000-21758-4-git-send-email-alan.maguire@oracle.com

tools/testing/selftests/bpf/progs/test_bpf_cookie.c

@@ -37,14 +37,14 @@ int handle_kretprobe(struct pt_regs *ctx)
 	return 0;
 }
 
-SEC("uprobe/trigger_func")
+SEC("uprobe")
 int handle_uprobe(struct pt_regs *ctx)
 {
 	update(ctx, &uprobe_res);
 	return 0;
 }
 
-SEC("uretprobe/trigger_func")
+SEC("uretprobe")
 int handle_uretprobe(struct pt_regs *ctx)
 {
 	update(ctx, &uretprobe_res);

tools/testing/selftests/bpf/progs/test_task_pt_regs.c

@@ -14,7 +14,7 @@ char current_regs[PT_REGS_SIZE] = {};
 char ctx_regs[PT_REGS_SIZE] = {};
 int uprobe_res = 0;
 
-SEC("uprobe/trigger_func")
+SEC("uprobe")
 int handle_uprobe(struct pt_regs *ctx)
 {
 	struct task_struct *current;

tools/testing/selftests/bpf/progs/trigger_bench.c

@@ -54,7 +54,7 @@ int bench_trigger_fmodret(void *ctx)
 	return -22;
 }
 
-SEC("uprobe/self/uprobe_target")
+SEC("uprobe")
 int bench_trigger_uprobe(void *ctx)
 {
 	__sync_add_and_fetch(&hits, 1);