Bug 1791029 - Deal with lstat potentially lying in nsLocalFileUnix. r=xpcom-reviewers,nika

Differential Revision: https://phabricator.services.mozilla.com/D158796

xpcom/io/nsLocalFileUnix.cpp

@@ -1911,9 +1911,13 @@ nsLocalFile::GetNativeTarget(nsACString& aResult) {
     return NS_ERROR_OUT_OF_MEMORY;
   }
 
-  if (readlink(mPath.get(), target.BeginWriting(), (size_t)size) < 0) {
+  ssize_t written = readlink(mPath.get(), target.BeginWriting(), size_t(size));
+  if (written < 0) {
     return NSRESULT_FOR_ERRNO();
   }
+  // Target might have changed since the lstat call, or lstat might lie, see bug
+  // 1791029.
+  target.Truncate(written);
 
   nsresult rv = NS_OK;
   nsCOMPtr<nsIFile> self(this);
@@ -1961,12 +1965,13 @@ nsLocalFile::GetNativeTarget(nsACString& aResult) {
       break;
     }
 
-    int32_t linkLen =
+    ssize_t linkLen =
         readlink(flatRetval.get(), newTarget.BeginWriting(), size);
     if (linkLen == -1) {
       rv = NSRESULT_FOR_ERRNO();
       break;
     }
+    newTarget.Truncate(linkLen);
     target = newTarget;
   }