On Mon, Feb 29, 2016 at 1:29 PM, Stas Sergeev [off-list ref] wrote:

This patch implements the SS_AUTODISARM flag that can be ORed with
SS_ONSTACK when forming ss_flags.
When this flag is set, sigaltstack will be disabled when entering
the signal handler; more precisely, after saving sas to uc_stack.
When leaving the signal handler, the sigaltstack is restored by
uc_stack.
When this flag is used, it is safe to switch from sighandler with
swapcontext(). Without this flag, the subsequent signal will corrupt
the state of the switched-away sighandler.

This looks reasonable to me with one exception: how does a user
program detect the presence of this feature?  Anyone else have any
thoughts?