SYS_rt_sigreturn calls sigaltstack on ucontext_t.uc_stack

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on July 05, 2011 17:02:38_

it seems that SYS_rt_sigreturn calls sigaltstack on ucontext_t.uc_stack.

xref http://lkml.org/lkml/2004/5/22/146 TODO: investigate:

is this true on all flavors of linux? which versions?

TODO: consequences:

in handle_sigreturn(): need to call handle_sigaltstack() on uc_stack

when initializing: to support taking over while on alt stack, should detect that and modify uc_stack instead of calling SYS_sigaltstack

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=506_