CS208, Monday 28 Feb 2022

Buffer overflow assignment

• What happens to rsp and the contents of the stack when "call" gets executed?
• Same question, but for ret
• Key idea for this assignment: use ret's behavior to mess with what gets executed and when
• Von Neumann architecture: instructions and data in the same place
• What is this hex2raw thing about?
• Will you need to understand machine language?
• Phases 2 and 3 depend on some common defenses (enabled by default) being disabled when compiling ctarget.
• Love this and want even more? Check out the "rtarget" portion of this assignment as assigned by Aaron Bauer last term

Paged virtual memory basics

• virtual pages
• page number, page starting address, and byte offset
• physical pages
• address translation
  • loading instructions into the CPU
  • accessing memory like "movq 0x08(%rsp), %rax"
• simple page table
• (notational digression: mega vs. mebi, bytes vs. Hertz, common sloppy usage, and people's desire not to sound goofy by saying "kibi" and "gibi" out loud)

Practice

• Pretend
  • pages have size 4KiB
  • we have 64-bit addresses
  • we have 16GiB of physical memory
• What is the starting virtual address of page 6 (i.e. the 7th page)?
• Suppose virtual page 6 is stored at physical page 0xabcd3. What is the virtual address of the 9th byte on the page? What is the physical address of that same byte?
• In general, how do you translate between a virtual address and a physical address?
• How big would a page table have to be?

Page tables

• What does the Linux page table look like?
• Mu

Why bother?

• efficient use of resources
  • load a page only when you need it
  • fast context switching (i.e. scheduling a new process)
  • ...
• security
  • different processes access different pages
  • mark pages read-only
  • ...
• simplified compiling/linking/loading