On 6/7/19 12:49 PM, Yu-cheng Yu wrote:

quoted

This also gives us an excellent opportunity to make it read-only as seen from
userspace to prevent exploits from just poking it full of ones before
redirecting execution.

GLIBC sets bits only for legacy code, and then makes the bitmap read-only.  That
avoids most issues:

  To populate bitmap pages, mprotect() is required.
  Reading zero bitmap pages would not waste more physical memory, right?

Huh, how does glibc know about all possible past and future legacy code
in the application?