diff --git a/changes/bug21134 b/changes/bug21134
new file mode 100644
index 0000000000..b851718372
--- /dev/null
+++ b/changes/bug21134
@@ -0,0 +1,5 @@
+  o Minor bugfixes (portability):
+    - Do not silently truncate content of files if they are larger
+      than SIZE_MAX bytes. This issue could occur on 32 bit systems
+      with large file support and files which are larger than 4 GB.
+      Fixes bug 21134; bugfix on 0.3.0.1-alpha.
diff --git a/src/common/compat.c b/src/common/compat.c
index ebf05f59e1..16b222904a 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -258,6 +258,12 @@ tor_mmap_file(const char *filename)
   page_size = getpagesize();
   size += (size%page_size) ? page_size-(size%page_size) : 0;
 
+  if (st.st_size > SSIZE_T_CEILING || size < st.st_size) {
+    log_warn(LD_FS, "File \"%s\" is too large. Ignoring.",filename);
+    errno = EFBIG;
+    close(fd);
+    return NULL;
+  }
   if (!size) {
     /* Zero-length file. If we call mmap on it, it will succeed but
      * return NULL, and bad things will happen. So just fail. */