Comment 6 for bug 1279620

Created attachment 7435
adapted from example in http://pubs.opengroup.org/onlinepubs/009695399/functions/makecontext.html

I'm attaching a simple demo program for makecontext/swapcontext that I found somewhere with the addition of code to print the alternate signal stack before and after calling swapcontext. On aarch64 it prints this:

start f2
start f1
finish f2
finish f1
{ss_sp: (nil), ss_flags: 2, ss_size: 0}
{ss_sp: 0x7ffbe931f8, ss_flags: 0, ss_size: 8192}

It turns out that because setcontext is implemented in terms of the rt_sigreturn syscall it ends up copying the uc_stack data from the passed context into the (kernel) task's sigaltstack parameters. Hilarity ensues. Specifically it means that programs linked against the gccgo runtime sometimes handle signals with SP pointing at memory that another thread is using for its stack, with predictably bad results.