Syscall API: mquery()

• To provide W^X on i386, segments of each shared library are mapped with specific offsets

  • e.g., data segment is placed exactly 1GB above the code segment

• ld.so implements that for dlopen() by using mquery() to probe the address space for a base address where everything will fit:

          base = 0
          foreach segment {
             ret = mquery(base + segment->offset, segment->size)
             if (base == 0)
                base = ret;
             else if (ret == MAP_FAILED)
                /* get hint, reset base to match */
                goto begin;
          }