virtiofsd: passthrough_ll: add lo_map for ino/fh indirection

A layer of indirection is needed because passthrough_ll cannot expose
pointers or file descriptor numbers to untrusted clients.  Malicious
clients could send invalid pointers or file descriptors in order to
crash or exploit the file system daemon.

lo_map provides an integer key->value mapping.  This will be used for
ino and fh fields in the patches that follow.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/passthrough_ll.c

@@ -74,6 +74,21 @@ struct _uintptr_to_must_hold_fuse_ino_t_dummy_struct {
 };
 #endif
 
+struct lo_map_elem {
+    union {
+        /* Element values will go here... */
+        ssize_t freelist;
+    };
+    bool in_use;
+};
+
+/* Maps FUSE fh or ino values to internal objects */
+struct lo_map {
+    struct lo_map_elem *elems;
+    size_t nelems;
+    ssize_t freelist;
+};
+
 struct lo_inode {
     struct lo_inode *next; /* protected by lo->mutex */
     struct lo_inode *prev; /* protected by lo->mutex */
@@ -130,6 +145,115 @@ static struct lo_data *lo_data(fuse_req_t req)
     return (struct lo_data *)fuse_req_userdata(req);
 }
 
+__attribute__((unused)) static void lo_map_init(struct lo_map *map)
+{
+    map->elems = NULL;
+    map->nelems = 0;
+    map->freelist = -1;
+}
+
+__attribute__((unused)) static void lo_map_destroy(struct lo_map *map)
+{
+    free(map->elems);
+}
+
+static int lo_map_grow(struct lo_map *map, size_t new_nelems)
+{
+    struct lo_map_elem *new_elems;
+    size_t i;
+
+    if (new_nelems <= map->nelems) {
+        return 1;
+    }
+
+    new_elems = realloc(map->elems, sizeof(map->elems[0]) * new_nelems);
+    if (!new_elems) {
+        return 0;
+    }
+
+    for (i = map->nelems; i < new_nelems; i++) {
+        new_elems[i].freelist = i + 1;
+        new_elems[i].in_use = false;
+    }
+    new_elems[new_nelems - 1].freelist = -1;
+
+    map->elems = new_elems;
+    map->freelist = map->nelems;
+    map->nelems = new_nelems;
+    return 1;
+}
+
+__attribute__((unused)) static struct lo_map_elem *
+lo_map_alloc_elem(struct lo_map *map)
+{
+    struct lo_map_elem *elem;
+
+    if (map->freelist == -1 && !lo_map_grow(map, map->nelems + 256)) {
+        return NULL;
+    }
+
+    elem = &map->elems[map->freelist];
+    map->freelist = elem->freelist;
+
+    elem->in_use = true;
+
+    return elem;
+}
+
+__attribute__((unused)) static struct lo_map_elem *
+lo_map_reserve(struct lo_map *map, size_t key)
+{
+    ssize_t *prev;
+
+    if (!lo_map_grow(map, key + 1)) {
+        return NULL;
+    }
+
+    for (prev = &map->freelist; *prev != -1;
+         prev = &map->elems[*prev].freelist) {
+        if (*prev == key) {
+            struct lo_map_elem *elem = &map->elems[key];
+
+            *prev = elem->freelist;
+            elem->in_use = true;
+            return elem;
+        }
+    }
+    return NULL;
+}
+
+__attribute__((unused)) static struct lo_map_elem *
+lo_map_get(struct lo_map *map, size_t key)
+{
+    if (key >= map->nelems) {
+        return NULL;
+    }
+    if (!map->elems[key].in_use) {
+        return NULL;
+    }
+    return &map->elems[key];
+}
+
+__attribute__((unused)) static void lo_map_remove(struct lo_map *map,
+                                                  size_t key)
+{
+    struct lo_map_elem *elem;
+
+    if (key >= map->nelems) {
+        return;
+    }
+
+    elem = &map->elems[key];
+    if (!elem->in_use) {
+        return;
+    }
+
+    elem->in_use = false;
+
+    elem->freelist = map->freelist;
+    map->freelist = key;
+}
+
 static struct lo_inode *lo_inode(fuse_req_t req, fuse_ino_t ino)
 {
     if (ino == FUSE_ROOT_ID) {