libbpf: auto-resolve programs/libraries when necessary for uprobes

bpf_program__attach_uprobe_opts() requires a binary_path argument
specifying binary to instrument.  Supporting simply specifying
"libc.so.6" or "foo" should be possible too.

Library search checks LD_LIBRARY_PATH, then /usr/lib64, /usr/lib.
This allows users to run BPF programs prefixed with
LD_LIBRARY_PATH=/path2/lib while still searching standard locations.
Similarly for non .so files, we check PATH and /usr/bin, /usr/sbin.

Path determination will be useful for auto-attach of BPF uprobe programs
using SEC() definition.
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1648654000-21758-2-git-send-email-alan.maguire@oracle.com

tools/lib/bpf/libbpf.c

@@ -10517,6 +10517,46 @@ static int perf_event_uprobe_open_legacy(const char *probe_name, bool retprobe,
 	return pfd;
 }
 
+/* Get full path to program/shared library. */
+static int resolve_full_path(const char *file, char *result, size_t result_sz)
+{
+	const char *search_paths[2];
+	int i;
+
+	if (strstr(file, ".so")) {
+		search_paths[0] = getenv("LD_LIBRARY_PATH");
+		search_paths[1] = "/usr/lib64:/usr/lib";
+	} else {
+		search_paths[0] = getenv("PATH");
+		search_paths[1] = "/usr/bin:/usr/sbin";
+	}
+
+	for (i = 0; i < ARRAY_SIZE(search_paths); i++) {
+		const char *s;
+
+		if (!search_paths[i])
+			continue;
+		for (s = search_paths[i]; s != NULL; s = strchr(s, ':')) {
+			char *next_path;
+			int seg_len;
+
+			if (s[0] == ':')
+				s++;
+			next_path = strchr(s, ':');
+			seg_len = next_path ? next_path - s : strlen(s);
+			if (!seg_len)
+				continue;
+			snprintf(result, result_sz, "%.*s/%s", seg_len, s, file);
+			/* ensure it is an executable file/link */
+			if (access(result, R_OK | X_OK) < 0)
+				continue;
+			pr_debug("resolved '%s' to '%s'\n", file, result);
+			return 0;
+		}
+	}
+	return -ENOENT;
+}
+
 LIBBPF_API struct bpf_link *
 bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
 				const char *binary_path, size_t func_offset,
@@ -10524,6 +10564,7 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
 {
 	DECLARE_LIBBPF_OPTS(bpf_perf_event_opts, pe_opts);
 	char errmsg[STRERR_BUFSIZE], *legacy_probe = NULL;
+	char full_binary_path[PATH_MAX];
 	struct bpf_link *link;
 	size_t ref_ctr_off;
 	int pfd, err;
@@ -10536,12 +10577,23 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
 	ref_ctr_off = OPTS_GET(opts, ref_ctr_offset, 0);
 	pe_opts.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
 
+	if (binary_path && !strchr(binary_path, '/')) {
+		err = resolve_full_path(binary_path, full_binary_path,
+					sizeof(full_binary_path));
+		if (err) {
+			pr_warn("prog '%s': failed to resolve full path for '%s'\n",
+				prog->name, binary_path);
+			return libbpf_err_ptr(err);
+		}
+		binary_path = full_binary_path;
+	}
+
 	legacy = determine_uprobe_perf_type() < 0;
 	if (!legacy) {
 		pfd = perf_event_open_probe(true /* uprobe */, retprobe, binary_path,
 					    func_offset, pid, ref_ctr_off);
 	} else {
-		char probe_name[512];
+		char probe_name[PATH_MAX + 64];
 
 		if (ref_ctr_off)
 			return libbpf_err_ptr(-EINVAL);