fix #648 by ignoring EINVAL on-remount of proc

When using --mount-proc=/some/path then unshare fails if the path provided is not already mounted due to the mount(2) call to change the propagation of the mount.
In such a case mount(2) returns EINVAL, which however is used for a variety of other errors.
If this error is ignored mistakenly the effects however should be negligible since:

1. the mount of proc afterwards happens regardless, errors of which are not ignored
2. the propagation change of root uses MS_REC, which should already change the propagation of all mounts recursively

Furthermore /proc is not touched if --mount-proc specifies a different mount point.
This should not cause too much unexpected behaviour due to point 2 from above in any case.
Specifying --mount-proc with a different path also means that unshare(3) is not instructed to touch /proc, thus /proc not being touched should not be unexpected.
As a side note, if unshare is called with /proc as an (implicit) parameter to --mount-proc then /proc is a stacked mount, meaning if /proc is unmounted within the namespace the host /proc will be visible again, thus not touching /proc with a different parameter does not constitute more information leakage than the alternative, quite contrary it may even be the desired behaviour.

Signed-off-by: benaryorg <binary@benary.org>

diff --git a/sys-utils/unshare.c b/sys-utils/unshare.c
index 39e2895afd665d44077d832c3c0f06a455e97b46..8e61221da82417a13a099826a0a669723afa89fb 100644 (file)
--- a/sys-utils/unshare.c
+++ b/sys-utils/unshare.c
@@ -650,8 +650,18 @@ int main(int argc, char *argv[])
                err(EXIT_FAILURE, _("cannot chdir to '%s'"), newdir);
 
        if (procmnt) {
-               if (!newroot && mount("none", procmnt, NULL, MS_PRIVATE|MS_REC, NULL) != 0)
-                       err(EXIT_FAILURE, _("cannot change %s filesystem propagation"), procmnt);
+               /* When not changing root and using the default propagation flags
+                  then the recursive propagation change of root will
+                  automatically change that of an existing proc mount. */
+               if (!newroot && propagation != (MS_PRIVATE|MS_REC)) {
+                       int rc = mount("none", procmnt, NULL, MS_PRIVATE|MS_REC, NULL);
+
+                       /* Custom procmnt means that proc is very likely not mounted, causing EINVAL.
+                          Ignoring the error in this specific instance is considered safe. */
+                       if(rc != 0 && errno != EINVAL)
+                               err(EXIT_FAILURE, _("cannot change %s filesystem propagation"), procmnt);
+               }
+
                if (mount("proc", procmnt, "proc", MS_NOSUID|MS_NOEXEC|MS_NODEV, NULL) != 0)
                        err(EXIT_FAILURE, _("mount %s failed"), procmnt);
        }