Fully allocate file backing writable maps (#389)

When ftruncate() is used on a filesystem supporting sparse files,
space in the file is not actually allocated.  Then, when the file
is mmap'd and libffi writes to the mapping, SIGBUS is thrown to
the calling application.  Instead, always fully allocate the file
that will back writable maps.

src/closures.c

@@ -723,6 +723,36 @@ open_temp_exec_file (void)
   return fd;
 }
 
+/* We need to allocate space in a file that will be backing a writable
+   mapping.  Several problems exist with the usual approaches:
+   - fallocate() is Linux-only
+   - posix_fallocate() is not available on all platforms
+   - ftruncate() does not allocate space on filesystems with sparse files
+   Failure to allocate the space will cause SIGBUS to be thrown when
+   the mapping is subsequently written to.  */
+static int
+allocate_space (int fd, off_t offset, off_t len)
+{
+  static size_t page_size;
+
+  /* Obtain system page size. */
+  if (!page_size)
+    page_size = sysconf(_SC_PAGESIZE);
+
+  unsigned char buf[page_size];
+  memset (buf, 0, page_size);
+
+  while (len > 0)
+    {
+      off_t to_write = (len < page_size) ? len : page_size;
+      if (write (fd, buf, to_write) < to_write)
+        return -1;
+      len -= to_write;
+    }
+
+  return 0;
+}
+
 /* Map in a chunk of memory from the temporary exec file into separate
    locations in the virtual memory address space, one writable and one
    executable.  Returns the address of the writable portion, after
@@ -744,7 +774,7 @@ dlmmap_locked (void *start, size_t length, int prot, int flags, off_t offset)
 
   offset = execsize;
 
-  if (ftruncate (execfd, offset + length))
+  if (allocate_space (execfd, offset, length))
     return MFAIL;
 
   flags &= ~(MAP_PRIVATE | MAP_ANONYMOUS);