Can you enter x64 32-bit “long compatibility sub-mode” outside of kernel mode?

Question

This might be an exact duplicate of Is it possible to execute 32-bit code in 64-bit process by doing mode-switching?, but that question is from a year ago and only has one a

Answer 1

Yes, you can. It's even doable using fully supported interfaces. Use modify_ldt to install a 32-bit code segment into the LDT, then set up a far pointer to your 32-bit code, then do an indirect jump to it using ljumpl *(%eax) in AT&T notation.

You'll face all kinds of snafus, though. The high bits of your stack pointer are likely to get destroyed. You probably need a data segment if you actually want to run real code. And you'll need to do another far jump to get back to 64-bit mode.

A fully worked-out example is in my linux-clock-tests in test_vsyscall.cc. (It's a little broken on any released kernel: int cc will crash. You should change that to something else more clever, like "nop". Look in intcc32.