perf symbols: Find symbols in different mount namespace

Teach perf how to resolve symbols from binaries that are in a different
mount namespace from the tool.  This allows perf to generate meaningful
stack traces even if the binary resides in a different mount namespace
from the tool.

Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
Tested-by: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1499305693-1599-2-git-send-email-kjlx@templeofstupid.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/namespaces.c

@@ -9,9 +9,13 @@
 #include "namespaces.h"
 #include "util.h"
 #include "event.h"
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sched.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 
 struct namespaces *namespaces__new(struct namespaces_event *event)
 {
@@ -35,3 +39,126 @@ void namespaces__free(struct namespaces *namespaces)
 {
 	free(namespaces);
 }
+
+void nsinfo__init(struct nsinfo *nsi)
+{
+	char oldns[PATH_MAX];
+	char *newns = NULL;
+	struct stat old_stat;
+	struct stat new_stat;
+
+	if (snprintf(oldns, PATH_MAX, "/proc/self/ns/mnt") >= PATH_MAX)
+		return;
+
+	if (asprintf(&newns, "/proc/%d/ns/mnt", nsi->pid) == -1)
+		return;
+
+	if (stat(oldns, &old_stat) < 0)
+		goto out;
+
+	if (stat(newns, &new_stat) < 0)
+		goto out;
+
+	/* Check if the mount namespaces differ, if so then indicate that we
+	 * want to switch as part of looking up dso/map data.
+	 */
+	if (old_stat.st_ino != new_stat.st_ino) {
+		nsi->need_setns = true;
+		nsi->mntns_path = newns;
+		newns = NULL;
+	}
+
+out:
+	free(newns);
+}
+
+struct nsinfo *nsinfo__new(pid_t pid)
+{
+	struct nsinfo *nsi = calloc(1, sizeof(*nsi));
+
+	if (nsi != NULL) {
+		nsi->pid = pid;
+		nsi->need_setns = false;
+		nsinfo__init(nsi);
+		refcount_set(&nsi->refcnt, 1);
+	}
+
+	return nsi;
+}
+
+void nsinfo__delete(struct nsinfo *nsi)
+{
+	zfree(&nsi->mntns_path);
+	free(nsi);
+}
+
+struct nsinfo *nsinfo__get(struct nsinfo *nsi)
+{
+	if (nsi)
+		refcount_inc(&nsi->refcnt);
+	return nsi;
+}
+
+void nsinfo__put(struct nsinfo *nsi)
+{
+	if (nsi && refcount_dec_and_test(&nsi->refcnt))
+		nsinfo__delete(nsi);
+}
+
+void nsinfo__mountns_enter(struct nsinfo *nsi, struct nscookie *nc)
+{
+	char curpath[PATH_MAX];
+	int oldns = -1;
+	int newns = -1;
+
+	if (nc == NULL)
+		return;
+
+	nc->oldns = -1;
+	nc->newns = -1;
+
+	if (!nsi || !nsi->need_setns)
+		return;
+
+	if (snprintf(curpath, PATH_MAX, "/proc/self/ns/mnt") >= PATH_MAX)
+		return;
+
+	oldns = open(curpath, O_RDONLY);
+	if (oldns < 0)
+		return;
+
+	newns = open(nsi->mntns_path, O_RDONLY);
+	if (newns < 0)
+		goto errout;
+
+	if (setns(newns, CLONE_NEWNS) < 0)
+		goto errout;
+
+	nc->oldns = oldns;
+	nc->newns = newns;
+	return;
+
+errout:
+	if (oldns > -1)
+		close(oldns);
+	if (newns > -1)
+		close(newns);
+}
+
+void nsinfo__mountns_exit(struct nscookie *nc)
+{
+	if (nc == NULL || nc->oldns == -1 || nc->newns == -1)
+		return;
+
+	setns(nc->oldns, CLONE_NEWNS);
+
+	if (nc->oldns > -1) {
+		close(nc->oldns);
+		nc->oldns = -1;
+	}
+
+	if (nc->newns > -1) {
+		close(nc->newns);
+		nc->newns = -1;
+	}
+}