keep referential integrity for file table entries

Before this, some functions such as `uvwasi_path_open()` could
crash, because they acquired a reference to a fd table entry
using `uvwasi_fd_table_get()` and continued to use the result
after calling `uvwasi_fd_table_insert_fd()`; however, the latter
could `realloc()` the fd table, potentially invalidating the
earlier return value of `uvwasi_fd_table_get()`.

Since the previous commit introduced a per-fd allocation anyway,
this modifies the code to allocate the table entry itself as part
of that allocation. This ensures referential integrity for
the table entry.

(Aside -- I’m not sure that it was valid to call `uv_mutex_init()`
and then move that mutex around in memory, either.)

Author: addaleax

include/fd_table.h

@@ -17,12 +17,11 @@ struct uvwasi_fd_wrap_t {
   uvwasi_rights_t rights_base;
   uvwasi_rights_t rights_inheriting;
   int preopen;
-  int valid;
   uv_mutex_t mutex;
 };
 
 struct uvwasi_fd_table_t {
-  struct uvwasi_fd_wrap_t* fds;
+  struct uvwasi_fd_wrap_t** fds;
   uint32_t size;
   uint32_t used;
   uv_rwlock_t rwlock;

src/fd_table.c

@@ -187,7 +187,7 @@ static uvwasi_errno_t uvwasi__fd_table_insert(uvwasi_t* uvwasi,
                                               int preopen,
                                               struct uvwasi_fd_wrap_t** wrap) {
   struct uvwasi_fd_wrap_t* entry;
-  struct uvwasi_fd_wrap_t* new_fds;
+  struct uvwasi_fd_wrap_t** new_fds;
   uvwasi_errno_t err;
   uint32_t new_size;
   int index;
@@ -200,8 +200,11 @@ static uvwasi_errno_t uvwasi__fd_table_insert(uvwasi_t* uvwasi,
 
   mp_len = strlen(mapped_path);
   rp_len = strlen(real_path);
-  mp_copy = uvwasi__malloc(uvwasi, mp_len + rp_len + 2);
-  if (mp_copy == NULL) return UVWASI_ENOMEM;
+  entry = (struct uvwasi_fd_wrap_t*)
+    uvwasi__malloc(uvwasi, sizeof(*entry) + mp_len + rp_len + 2);
+  if (entry == NULL) return UVWASI_ENOMEM;
+
+  mp_copy = (char*)(entry + 1);
   rp_copy = mp_copy + mp_len + 1;
   memcpy(mp_copy, mapped_path, mp_len);
   mp_copy[mp_len] = '\0';
@@ -215,12 +218,13 @@ static uvwasi_errno_t uvwasi__fd_table_insert(uvwasi_t* uvwasi,
     new_size = table->size * 2;
     new_fds = uvwasi__realloc(uvwasi, table->fds, new_size * sizeof(*new_fds));
     if (new_fds == NULL) {
+      uvwasi__free(uvwasi, entry);
       err = UVWASI_ENOMEM;
       goto exit;
     }
 
     for (i = table->size; i < new_size; ++i)
-      new_fds[i].valid = 0;
+      new_fds[i] = NULL;
 
     index = table->size;
     table->fds = new_fds;
@@ -229,20 +233,21 @@ static uvwasi_errno_t uvwasi__fd_table_insert(uvwasi_t* uvwasi,
     /* The table is big enough, so find an empty slot for the new data. */
     index = -1;
     for (i = 0; i < table->size; ++i) {
-      if (table->fds[i].valid != 1) {
+      if (table->fds[i] == NULL) {
         index = i;
         break;
       }
     }
 
     /* index should never be -1. */
     if (index == -1) {
+      uvwasi__free(uvwasi, entry);
       err = UVWASI_ENOSPC;
       goto exit;
     }
   }
 
-  entry = &table->fds[index];
+  table->fds[index] = entry;
 
   r = uv_mutex_init(&entry->mutex);
   if (r != 0) {
@@ -258,7 +263,6 @@ static uvwasi_errno_t uvwasi__fd_table_insert(uvwasi_t* uvwasi,
   entry->rights_base = rights_base;
   entry->rights_inheriting = rights_inheriting;
   entry->preopen = preopen;
-  entry->valid = 1;
   table->used++;
 
   if (wrap != NULL)
@@ -295,7 +299,7 @@ uvwasi_errno_t uvwasi_fd_table_init(uvwasi_t* uvwasi,
   table->size = init_size;
   table->fds = uvwasi__calloc(uvwasi,
                               init_size,
-                              sizeof(struct uvwasi_fd_wrap_t));
+                              sizeof(struct uvwasi_fd_wrap_t*));
 
   if (table->fds == NULL) {
     err = UVWASI_ENOMEM;
@@ -346,11 +350,11 @@ void uvwasi_fd_table_free(uvwasi_t* uvwasi, struct uvwasi_fd_table_t* table) {
     return;
 
   for (i = 0; i < table->size; i++) {
-    entry = &table->fds[i];
-    if (!entry->valid) continue;
+    entry = table->fds[i];
+    if (entry == NULL) continue;
 
-    uvwasi__free(uvwasi, entry->path);
     uv_mutex_destroy(&entry->mutex);
+    uvwasi__free(uvwasi, entry);
   }
 
   uvwasi__free(uvwasi, table->fds);
@@ -455,9 +459,9 @@ uvwasi_errno_t uvwasi_fd_table_get(const struct uvwasi_fd_table_t* table,
     goto exit;
   }
 
-  entry = &table->fds[id];
+  entry = table->fds[id];
 
-  if (entry->valid != 1 || entry->id != id) {
+  if (entry == NULL || entry->id != id) {
     err = UVWASI_EBADF;
     goto exit;
   }
@@ -494,16 +498,16 @@ uvwasi_errno_t uvwasi_fd_table_remove(uvwasi_t* uvwasi,
     goto exit;
   }
 
-  entry = &table->fds[id];
+  entry = table->fds[id];
 
-  if (entry->valid != 1 || entry->id != id) {
+  if (entry == NULL || entry->id != id) {
     err = UVWASI_EBADF;
     goto exit;
   }
 
-  uvwasi__free(uvwasi, entry->path);
   uv_mutex_destroy(&entry->mutex);
-  entry->valid = 0;
+  uvwasi__free(uvwasi, entry);
+  table->fds[id] = NULL;
   table->used--;
   err = UVWASI_ESUCCESS;
 exit: