How is it possible for JVM to run generated machine code from a Java application via JIT?

Question

How is it possible that JVM compiles bytecode into native code dynamically and then executes it? I can imagine that it is possible to write data values into memory but if I remember correctly, a program can't write to the memory that contains instructions (otherwise viruses could use this feature and proliferate quickly).

Answer 1

Very few architectures implement the level of memory protection (only the OS has write access to memory areas containing code) you're talking about, the ones where Java uses a JIT definitely don't.

And viruses do use this feature and even more to proliferate quickly. But when you think about it, there's nothing inherently dangerous in a process modifying its own code. It isn't any more dangerous than being able to write to a file and then load a library.

Answer 2

Normally, you can't write to the section of memory that contains code directly, but there are ways to override that. For JITs, often what is done is to have some read-write data space on the heap and then use an operation such as mprotect to make it executable.

Answer 3

OSes do provide facilities to allocate "executable" memory. The JVM needs to allocate the target memory in a different way than standard malloc() when generating JITed code.

For example, on Windows, use VirtualAlloc with PAGE_EXECUTE . Similar functions exist in Linux, AIX, etc...