diff -r 084b21cd0228 -r 899ecf76b570 src/os_cpu/linux_x86/vm/os_linux_x86.cpp --- a/src/os_cpu/linux_x86/vm/os_linux_x86.cpp Mon Sep 23 08:56:19 2013 -0700 +++ b/src/os_cpu/linux_x86/vm/os_linux_x86.cpp Wed Sep 25 13:58:13 2013 +0200 @@ -876,3 +876,46 @@ #endif } #endif + + +/* + * IA32 only: execute code at a high address in case buggy NX emulation is present. I.e. avoid CS limit + * updates (JDK-8023956). + */ +void os::workaround_expand_exec_shield_cs_limit() { +#if defined(IA32) + size_t page_size = os::vm_page_size(); + /* + * Take the highest VA the OS will give us and exec + * + * Although using -(pagesz) as mmap hint works on newer kernel as you would + * think, older variants affected by this work-around don't (search forward only). + * + * On the affected distributions, we understand the memory layout to be: + * + * TASK_LIMIT= 3G, main stack base close to TASK_LIMT. + * + * A few pages south main stack will do it. + * + * If we are embedded in an app other than launcher (initial != main stack), + * we don't have much control or understanding of the address space, just let it slide. + */ + char* hint = (char*) (Linux::initial_thread_stack_bottom() - + ((StackYellowPages + StackRedPages + 1) * page_size)); + char* codebuf = os::reserve_memory(page_size, hint); + if ( (codebuf == NULL) || (!os::commit_memory(codebuf, page_size, true)) ) { + return; // No matter, we tried, best effort. + } + if (PrintMiscellaneous && (Verbose || WizardMode)) { + tty->print_cr("[CS limit NX emulation work-around, exec code at: %p]", codebuf); + } + + // Some code to exec: the 'ret' instruction + codebuf[0] = 0xC3; + + // Call the code in the codebuf + __asm__ volatile("call *%0" : : "r"(codebuf)); + + // keep the page mapped so CS limit isn't reduced. +#endif +}