stackup

xnu MAXSSIZ got you down? stackup <path_to_binary> 0xdeadbeef makes everything all better.

The Mac OS X kernel 'hardcodes' the stack size (MAXSSIZ) to 64MB.

However, check out this kernel code in the load_main routine in xnu/bsd/kern/mach_loader.c:

if (epc->stacksize) {
   result->prog_stack_size = 1;
   result->user_stack_size = epc->stacksize;
} else {
  result->prog_stack_size = 0;
  result->user_stack_size = MAXSSIZ;
}

where epc is an entry_point_command struct.

In other words, if our binary image contains a non-zero stack size in its entry_point_command struct (LC_MAIN), the kernel will use that instead of the default MAXSSIZ.

So, if for some reason you can't recompile, don't want to, or just simply want to be cool, you can run this command line tool to statically modify your program's runtime max stack size.

Enjoy.

Compile

xcrun --sdk macosx swiftc main.swift -o stackup

Examples

Set stack to 1GB: stackup <path_to_binary> 0x0x40000000

Set stack to 0xdeadbeef: stackup <path_to_binary> 0xdeadbeef

Features

• Static binary translation for program max stack size.
• Variable stack size
• No recompilation or source code necessary
• Hackish

TODO:

1. Enable 32-bit binary translation
2. Enable fat-binary (universal) translation