perf namespaces: Add helper nsinfo__is_in_root_namespace()

Refactors code for gathering PID infos, it creates the function
nsinfo__get_nspid() to parse process 'status' node in folder '/proc'.

Base on the refactoring, this patch introduces a new helper
nsinfo__is_in_root_namespace(), it returns true when the caller runs in
the root PID namespace.
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: John Garry <john.garry@huawei.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonatan Goldschmidt <yonatan.goldschmidt@granulate.io>
Cc: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org
Cc: netdev@vger.kernel.org
Link: http://lore.kernel.org/lkml/20211212134721.1721245-2-leo.yan@linaro.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/namespaces.c

@@ -60,17 +60,49 @@ void namespaces__free(struct namespaces *namespaces)
 	free(namespaces);
 }
 
+static int nsinfo__get_nspid(struct nsinfo *nsi, const char *path)
+{
+	FILE *f = NULL;
+	char *statln = NULL;
+	size_t linesz = 0;
+	char *nspid;
+
+	f = fopen(path, "r");
+	if (f == NULL)
+		return -1;
+
+	while (getline(&statln, &linesz, f) != -1) {
+		/* Use tgid if CONFIG_PID_NS is not defined. */
+		if (strstr(statln, "Tgid:") != NULL) {
+			nsi->tgid = (pid_t)strtol(strrchr(statln, '\t'),
+						     NULL, 10);
+			nsi->nstgid = nsi->tgid;
+		}
+
+		if (strstr(statln, "NStgid:") != NULL) {
+			nspid = strrchr(statln, '\t');
+			nsi->nstgid = (pid_t)strtol(nspid, NULL, 10);
+			/*
+			 * If innermost tgid is not the first, process is in a different
+			 * PID namespace.
+			 */
+			nsi->in_pidns = (statln + sizeof("NStgid:") - 1) != nspid;
+			break;
+		}
+	}
+
+	fclose(f);
+	free(statln);
+	return 0;
+}
+
 int nsinfo__init(struct nsinfo *nsi)
 {
 	char oldns[PATH_MAX];
 	char spath[PATH_MAX];
 	char *newns = NULL;
-	char *statln = NULL;
-	char *nspid;
 	struct stat old_stat;
 	struct stat new_stat;
-	FILE *f = NULL;
-	size_t linesz = 0;
 	int rv = -1;
 
 	if (snprintf(oldns, PATH_MAX, "/proc/self/ns/mnt") >= PATH_MAX)
@@ -100,34 +132,9 @@ int nsinfo__init(struct nsinfo *nsi)
 	if (snprintf(spath, PATH_MAX, "/proc/%d/status", nsi->pid) >= PATH_MAX)
 		goto out;
 
-	f = fopen(spath, "r");
-	if (f == NULL)
-		goto out;
-
-	while (getline(&statln, &linesz, f) != -1) {
-		/* Use tgid if CONFIG_PID_NS is not defined. */
-		if (strstr(statln, "Tgid:") != NULL) {
-			nsi->tgid = (pid_t)strtol(strrchr(statln, '\t'),
-						     NULL, 10);
-			nsi->nstgid = nsi->tgid;
-		}
-
-		if (strstr(statln, "NStgid:") != NULL) {
-			nspid = strrchr(statln, '\t');
-			nsi->nstgid = (pid_t)strtol(nspid, NULL, 10);
-			/* If innermost tgid is not the first, process is in a different
-			 * PID namespace.
-			 */
-			nsi->in_pidns = (statln + sizeof("NStgid:") - 1) != nspid;
-			break;
-		}
-	}
-	rv = 0;
+	rv = nsinfo__get_nspid(nsi, spath);
 
 out:
-	if (f != NULL)
-		(void) fclose(f);
-	free(statln);
 	free(newns);
 	return rv;
 }
@@ -299,3 +306,12 @@ int nsinfo__stat(const char *filename, struct stat *st, struct nsinfo *nsi)
 
 	return ret;
 }
+
+bool nsinfo__is_in_root_namespace(void)
+{
+	struct nsinfo nsi;
+
+	memset(&nsi, 0x0, sizeof(nsi));
+	nsinfo__get_nspid(&nsi, "/proc/self/status");
+	return !nsi.in_pidns;
+}

tools/perf/util/namespaces.h

@@ -59,6 +59,8 @@ void nsinfo__mountns_exit(struct nscookie *nc);
 char *nsinfo__realpath(const char *path, struct nsinfo *nsi);
 int nsinfo__stat(const char *filename, struct stat *st, struct nsinfo *nsi);
 
+bool nsinfo__is_in_root_namespace(void);
+
 static inline void __nsinfo__zput(struct nsinfo **nsip)
 {
 	if (nsip) {