signals: disable stack overflow in handler.

* signal.c (sig_handler): For a is_cpu_exception signal, we
temporarily disable the stack limit. It might be executing on
the sigaltstack buffer, which is almost certainly below the
stack limit.

* tests/012/stack.tl: New test case. We raise a SIGSEGV
and check that in the handler, the stack limit is disabled,
and that we can executed code.

2 files changed, 15 insertions, 0 deletions

diff --git a/signal.c b/signal.c
index 53e4e499..ad5c9849 100644
--- a/signal.c
+++ b/signal.c
@@ -81,11 +81,14 @@ static void sig_handler(int sig)
   int exc = is_cpu_exception(sig);
   int ic = interrupt_count++;
   int in_interrupt = ic > 0;
+  val *stack_lim;
 
   if (exc) {
     gc = gc_state(0);
+    stack_lim = gc_stack_limit;
     as = async_sig_enabled;
     async_sig_enabled = 1;
+    gc_stack_limit = 0;
   }
 
   sig_reload_cache();
@@ -109,6 +112,7 @@ static void sig_handler(int sig)
   }
 
   if (exc) {
+    gc_stack_limit = stack_lim;
     async_sig_enabled = as;
     gc_state(gc);
   }
diff --git a/tests/012/stack.tl b/tests/012/stack.tl
index a8d06487..879d3d27 100644
--- a/tests/012/stack.tl
+++ b/tests/012/stack.tl
@@ -29,3 +29,14 @@
   (set-stack-limit orig) nil
   (set-stack-limit 65536) orig
   (set-stack-limit orig) 65536)
+
+(set-sig-handler sig-segv
+  (lambda (signal async-p)
+    (assert (null (get-stack-limit)))
+    (throw 'out)))
+
+(test
+  (catch
+    (raise sig-segv)
+    (out () :sig))
+  :sig)