[PATCH] perf tools: Restore proper cwd on return from mnt ns

From: Jiri Olsa
Date: Thu Nov 01 2018 - 13:00:08 EST


When reporting on 'record' server we try to retrieve/use
the mnt namespace of the profiled tasks. We use following
API with cookie to hold the return namespace, roughly:

nsinfo__mountns_enter(struct nsinfo *nsi, struct nscookie *nc)
setns(newns, 0);
...
new ns related open..
...
nsinfo__mountns_exit(struct nscookie *nc)
setns(nc->oldns)

Once finished we setns to old namespace, which also sets the
current working directory (cwd) to "/", trashing the cwd we had.

This is mostly fine, because we use absolute paths almost everywhere,
but it screws up perf diff:

# perf diff
failed to open perf.data: No such file or directory (try 'perf record' first)
...

Adding the current working directory to be part of the cookie
and restoring it in the nsinfo__mountns_exit call.

Cc: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>
Fixes: 843ff37bb59e ("perf symbols: Find symbols in different mount namespace")
Link: http://lkml.kernel.org/n/tip-zg3vz7kjr86cco7lo91v8yhj@xxxxxxxxxxxxxx
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
tools/perf/util/namespaces.c | 18 ++++++++++++++++--
tools/perf/util/namespaces.h | 1 +
2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/namespaces.c b/tools/perf/util/namespaces.c
index cf8bd123cf73..fb0458b7e6aa 100644
--- a/tools/perf/util/namespaces.c
+++ b/tools/perf/util/namespaces.c
@@ -18,6 +18,7 @@
#include <stdio.h>
#include <string.h>
#include <unistd.h>
+#include <asm/bug.h>

struct namespaces *namespaces__new(struct namespaces_event *event)
{
@@ -186,6 +187,7 @@ void nsinfo__mountns_enter(struct nsinfo *nsi,
char curpath[PATH_MAX];
int oldns = -1;
int newns = -1;
+ char *oldcwd = NULL;

if (nc == NULL)
return;
@@ -199,9 +201,13 @@ void nsinfo__mountns_enter(struct nsinfo *nsi,
if (snprintf(curpath, PATH_MAX, "/proc/self/ns/mnt") >= PATH_MAX)
return;

+ oldcwd = get_current_dir_name();
+ if (!oldcwd)
+ return;
+
oldns = open(curpath, O_RDONLY);
if (oldns < 0)
- return;
+ goto errout;

newns = open(nsi->mntns_path, O_RDONLY);
if (newns < 0)
@@ -210,11 +216,14 @@ void nsinfo__mountns_enter(struct nsinfo *nsi,
if (setns(newns, CLONE_NEWNS) < 0)
goto errout;

+ nc->oldcwd = oldcwd;
nc->oldns = oldns;
nc->newns = newns;
return;

errout:
+ if (oldcwd)
+ free(oldcwd);
if (oldns > -1)
close(oldns);
if (newns > -1)
@@ -223,11 +232,16 @@ void nsinfo__mountns_enter(struct nsinfo *nsi,

void nsinfo__mountns_exit(struct nscookie *nc)
{
- if (nc == NULL || nc->oldns == -1 || nc->newns == -1)
+ if (nc == NULL || nc->oldns == -1 || nc->newns == -1 || !nc->oldcwd)
return;

setns(nc->oldns, CLONE_NEWNS);

+ if (nc->oldcwd) {
+ WARN_ON_ONCE(chdir(nc->oldcwd));
+ free(nc->oldcwd);
+ }
+
if (nc->oldns > -1) {
close(nc->oldns);
nc->oldns = -1;
diff --git a/tools/perf/util/namespaces.h b/tools/perf/util/namespaces.h
index cae1a9a39722..d5f46c09ea31 100644
--- a/tools/perf/util/namespaces.h
+++ b/tools/perf/util/namespaces.h
@@ -38,6 +38,7 @@ struct nsinfo {
struct nscookie {
int oldns;
int newns;
+ char *oldcwd;
};

int nsinfo__init(struct nsinfo *nsi);
--
2.17.2