lib/namespace: avoid double-mounting a /sys

This partly reverts 8f0807023d067e2bb585a2ae8da93e59689d10f1, bringing
back the umount(/sys) attempt.

In a LXC container we're unable to umount the sysfs instance, nor mount
a read-write one. We still are able to create a new read-only instance.

Nevertheless, it still makes sense to attempt the umount() even though
the sysfs is mounted read-only. Otherwise we may end up attempting to
mount a sysfs with the same flags as is already mounted, resulting in
an EBUSY error (meaning "Already mounted").

Perhaps this is not a very likely scenario in real world, but we hit
it in NetworkManager test suite and makes netns_switch() somewhat more
robust. It also fixes the case, when /sys wasn't mounted at all.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

diff --git a/lib/namespace.c b/lib/namespace.c
index 43e0fe349ec360ed1719959dcd8ce42aa840b977..06ae0a48c2243afa4847013a022504dc420a1b45 100644 (file)
--- a/lib/namespace.c
+++ b/lib/namespace.c
@@ -82,19 +82,13 @@ int netns_switch(char *name)
 
        /* Mount a version of /sys that describes the network namespace */
 
-       if (statvfs("/sys", &fsstat) < 0) {
-               fprintf(stderr, "could not stat /sys (not mounted?): %s\n",strerror(errno));
-               return -1;
-       }
-       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 (umount2("/sys", MNT_DETACH) < 0) {
+               /* If this fails, perhaps there wasn't a sysfs instance mounted. Good. */
+               if (statvfs("/sys", &fsstat) == 0) {
+                       /* We couldn't umount the sysfs, we'll attempt to overlay it.
+                        * A read-only instance can't be shadowed with a read-write one. */
+                       if (fsstat.f_flag & ST_RDONLY)
+                               mountflags = MS_RDONLY;
                }
        }
        if (mount(name, "/sys", "sysfs", mountflags, NULL) < 0) {