[PATCH iproute2] lib/namespace: don't try to mount rw /sys over a ro one

[Lubomir Rintel]

It will fail with EPERM on Linux 4.15.

Signed-off-by: Lubomir Rintel <lkund...@v3.sk>
---
 lib/namespace.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/lib/namespace.c b/lib/namespace.c
index 30b51388..6f3356d0 100644
--- a/lib/namespace.c
+++ b/lib/namespace.c
@@ -7,6 +7,7 @@
  *             2 of the License, or (at your option) any later version.
  */
 
+#include <sys/statvfs.h>
 #include <fcntl.h>
 #include <dirent.h>
 #include <limits.h>
@@ -46,6 +47,8 @@ int netns_switch(char *name)
 {
        char net_path[PATH_MAX];
        int netns;
+       unsigned long mountflags = 0;
+       struct statvfs fsstat;
 
        snprintf(net_path, sizeof(net_path), "%s/%s", NETNS_RUN_DIR, name);
        netns = open(net_path, O_RDONLY | O_CLOEXEC);
@@ -73,12 +76,25 @@ int netns_switch(char *name)
                        strerror(errno));
                return -1;
        }
+
        /* Mount a version of /sys that describes the network namespace */
-       if (umount2("/sys", MNT_DETACH) < 0) {
-               fprintf(stderr, "umount of /sys failed: %s\n", strerror(errno));
+
+       if (statvfs("/sys", &fsstat) < 0) {
+               fprintf(stderr, "could not stat /sys (not mounted?): 
%s\n",strerror(errno));
                return -1;
        }
-       if (mount(name, "/sys", "sysfs", 0, NULL) < 0) {
+       if (fsstat.f_flag & ST_RDONLY) {
+               /* If /sys is not writable (e.g. in a container), we can't
+                * unmount the old /sys instance, but we can still mount a new
+                * read-only instance over it. */
+               mountflags = MS_RDONLY;
+       } else {
+               if (umount2("/sys", MNT_DETACH) < 0) {
+                       fprintf(stderr, "umount of /sys failed: %s\n", 
strerror(errno));
+                       return -1;
+               }
+       }
+       if (mount(name, "/sys", "sysfs", mountflags, NULL) < 0) {
                fprintf(stderr, "mount of /sys failed: %s\n",strerror(errno));
                return -1;
        }
-- 
2.14.3