conf: avoid memleak on NULL path

I noticed that the apparmor code could request metadata even
for a cdrom with no media, which would cause a memory leak of
the hash table used to look for loops in the backing chain.
But even before that, we blindly dereferenced the path for
printing a debug statement, so it is just better to enforce
that this is only used on non-NULL names.

* src/util/virstoragefile.c (virStorageFileGetMetadata): Assume
non-NULL path.
* src/util/virstoragefile.h: Annotate this.
* src/security/virt-aa-helper.c (get_files): Fix caller.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/security/virt-aa-helper.c

@@ -943,13 +943,16 @@ get_files(vahControl * ctl)
 
     for (i = 0; i < ctl->def->ndisks; i++) {
         virDomainDiskDefPtr disk = ctl->def->disks[i];
+        const char *src = virDomainDiskGetSource(disk);
 
+        if (!src)
+            continue;
         /* XXX - 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.
          */
         if (!disk->backingChain) {
             bool probe = ctl->allowDiskFormatProbing;
-            disk->backingChain = virStorageFileGetMetadata(virDomainDiskGetSource(disk),
+            disk->backingChain = virStorageFileGetMetadata(src,
                                                            virDomainDiskGetFormat(disk),
                                                            -1, -1, probe);
         }

src/util/virstoragefile.c

@@ -1144,7 +1144,7 @@ virStorageFileGetMetadata(const char *path, int format,
     virHashTablePtr cycle = virHashCreate(5, NULL);
     virStorageFileMetadataPtr ret;
 
-    if (!cycle || !path)
+    if (!cycle)
         return NULL;
 
     if (format <= VIR_STORAGE_FILE_NONE)

src/util/virstoragefile.h

@@ -257,7 +257,8 @@ int virStorageFileProbeFormatFromBuf(const char *path, char *buf,
 virStorageFileMetadataPtr virStorageFileGetMetadata(const char *path,
                                                     int format,
                                                     uid_t uid, gid_t gid,
-                                                    bool allow_probe);
+                                                    bool allow_probe)
+    ATTRIBUTE_NONNULL(1);
 virStorageFileMetadataPtr virStorageFileGetMetadataFromFD(const char *path,
                                                           int fd,
                                                           int format);