ASSERT: core/unix/signal.c:493 rc == 0: dr_app_start fail on sigaltstack_syscall

[zhaoqin]

An assert failure is seen on DynamoRIO trying to take over a multi-threaded application using dr_app_start.
Internal Error: DynamoRIO debug check failure: core/unix/signal.c:493 rc == 0

It happens if a thread has already setup the signal stack, and DR tries to take it over in the signal handler. In that case, the sigaltstack_syscall fails and returns -1, ([EPERM] An attempt was made to modify an active stack).

(gdb) p/x $rsp
$9 = 0x7ffff6e4a2d0
(gdb) p/x *newstack
$10 = {ss_sp = 0x54c1d000, ss_flags = 0x0, ss_size = 0xe000}
(gdb) p/x *oldstack
$11 = {ss_sp = 0x0, ss_flags = 0x0, ss_size = 0x0}

[derekbruening]

Clearly we need to add a delayed-takeover test that uses signals and sigaltstack.

2 solutions come to mind:

1. Don't set SA_ONSTACK but then reinstate after takeover init: feels slightly risky but if reinstating is before any swap then we're just assuming orig app stack which we're doing anyway.

2. Swap stacks either temporarily in takeover-init, or make the dstack early like some clone handling paths do and swap early -- though this will complicate the app state snapshot.

[derekbruening]

Xref #722
Xref #2018
Xref #1921