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

[Jiri Olsa]

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 <k...@templeofstupid.com>
Fixes: 843ff37bb59e ("perf symbols: Find symbols in different mount namespace")
Link: http://lkml.kernel.org/n/tip-zg3vz7kjr86cco7lo91v8...@git.kernel.org
Signed-off-by: Jiri Olsa <jo...@kernel.org>
---
 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