qemu: eliminate "Ignoring open failure" when using root-squash NFS

This eliminates the warning message reported in:

 https://bugzilla.redhat.com/show_bug.cgi?id=624447

It was caused by a failure to open an image file that is not
accessible by root (the uid libvirtd is running as) because it's on a
root-squash NFS share, owned by a different user, with permissions of
660 (or maybe 600).

The solution is to use virFileOpenAs() rather than open(). The
codepath that generates the error is during qemuSetupDiskCGroup(), but
the actual open() is in a lower-level generic function called from
many places (virDomainDiskDefForeachPath), so some other pieces of the
code were touched just to add dummy (or possibly useful) uid and gid
arguments.

Eliminating this warning message has the nice side effect that the
requested operation may even succeed (which in this case isn't
necessary, but shouldn't hurt anything either).

src/conf/domain_conf.c

@@ -13554,6 +13554,7 @@ done:
 int virDomainDiskDefForeachPath(virDomainDiskDefPtr disk,
                                 bool allowProbing,
                                 bool ignoreOpenFailure,
+                                uid_t uid, gid_t gid,
                                 virDomainDiskDefPathIterator iter,
                                 void *opaque)
 {
@@ -13610,15 +13611,14 @@ int virDomainDiskDefForeachPath(virDomainDiskDefPtr disk,
             goto cleanup;
         }
 
-        if ((fd = open(path, O_RDONLY)) < 0) {
+        if ((fd = virFileOpenAs(path, O_RDONLY, 0, uid, gid, 0)) < 0) {
             if (ignoreOpenFailure) {
                 char ebuf[1024];
                 VIR_WARN("Ignoring open failure on %s: %s", path,
-                         virStrerror(errno, ebuf, sizeof(ebuf)));
+                         virStrerror(-fd, ebuf, sizeof(ebuf)));
                 break;
             } else {
-                virReportSystemError(errno,
-                                     _("unable to open disk path %s"),
+                virReportSystemError(-fd, _("unable to open disk path %s"),
                                      path);
                 goto cleanup;
             }

src/conf/domain_conf.h

@@ -1974,6 +1974,7 @@ typedef int (*virDomainDiskDefPathIterator)(virDomainDiskDefPtr disk,
 int virDomainDiskDefForeachPath(virDomainDiskDefPtr disk,
                                 bool allowProbing,
                                 bool ignoreOpenFailure,
+                                uid_t uid, gid_t gid,
                                 virDomainDiskDefPathIterator iter,
                                 void *opaque);
 

src/qemu/qemu_cgroup.c

@@ -96,6 +96,7 @@ int qemuSetupDiskCgroup(struct qemud_driver *driver,
     return virDomainDiskDefForeachPath(disk,
                                        driver->allowDiskFormatProbing,
                                        true,
+                                       driver->user, driver->group,
                                        qemuSetupDiskPathAllow,
                                        &data);
 }
@@ -137,6 +138,7 @@ int qemuTeardownDiskCgroup(struct qemud_driver *driver,
     return virDomainDiskDefForeachPath(disk,
                                        driver->allowDiskFormatProbing,
                                        true,
+                                       driver->user, driver->group,
                                        qemuTeardownDiskPathDeny,
                                        &data);
 }

src/security/security_dac.c

@@ -186,6 +186,7 @@ virSecurityDACSetSecurityImageLabel(virSecurityManagerPtr mgr,
     return virDomainDiskDefForeachPath(disk,
                                        virSecurityManagerGetAllowDiskFormatProbing(mgr),
                                        false,
+                                       priv->user, priv->group,
                                        virSecurityDACSetSecurityFileLabel,
                                        mgr);
 }

src/security/security_selinux.c

@@ -689,9 +689,16 @@ SELinuxSetSecurityImageLabel(virSecurityManagerPtr mgr,
     if (disk->type == VIR_DOMAIN_DISK_TYPE_NETWORK)
         return 0;
 
+    /* XXX On one hand, it would be nice to have the driver's uid:gid
+     * here so we could retry opens with it. On the other hand, it
+     * probably doesn't matter because in practice that's only useful
+     * for files on root-squashed NFS shares, and NFS doesn't properly
+     * support selinux anyway.
+     */
     return virDomainDiskDefForeachPath(disk,
                                        allowDiskFormatProbing,
                                        true,
+                                       -1, -1, /* current process uid:gid */
                                        SELinuxSetSecurityFileLabel,
                                        secdef);
 }

src/security/virt-aa-helper.c

@@ -910,10 +910,14 @@ get_files(vahControl * ctl)
         /* XXX passing ignoreOpenFailure = true to get back to the behavior
          * from before using virDomainDiskDefForeachPath. actually we should
          * be passing ignoreOpenFailure = false and handle open errors more
-         * careful than just ignoring them */
+         * careful than just ignoring them.
+         * XXX2 - if we knew the qemu user:group here we could send it in
+         *        so that the open could be re-tried as that user:group.
+         */
         int ret = virDomainDiskDefForeachPath(ctl->def->disks[i],
                                               ctl->allowDiskFormatProbing,
                                               true,
+                                              -1, -1 /* current uid:gid */
                                               add_file_path,
                                               &buf);
         if (ret != 0)