JIT compilers allocate executable memory by using mmap() (not malloc(), since malloc()-allocated memory is not guaranteed (or even likely) to be executable). When you map memory with mmap(), you can decide the protection bits (PROT_READ, PROT_WRITE, PROT_EXEC, and they can be OR'd together in any combination).
malloc()ed memory is executable if you mprotect() it to be so. malloc()ing memory and blindly executing it is definitely a bug OTOH. malloc() is mmap() based a lot of systems nowadays anyway, so malloc() vs mmap() is just a choice of convenience of API.
> malloc()ed memory is executable if you mprotect() it to be so.
True, but since mprotect() can only operate on full pages your mprotect() calls will most likely affect memory beyond what malloc gave you. For example if remove write or execute permissions your program will probably crash, and it's best to avoid having pages that are both writable and executable.
For example, from the V8 sources: http://www.google.com/codesearch#W9JxUuHYyMg/trunk/src/platf...