When entering the scheduler from the signal handler, tell

the kernel to (re)use the alternate signal stack.  In this
case, we don't return normally from the signal handler,
so the kernel still thinks we are using the signal stack.
The fixes a nasty bug where the signal handler can start
fiddling with the stack of a thread while the handler is
actually running on the same stack.

MFC candidate

4 files changed, 38 insertions, 19 deletions

diff --git a/lib/libkse/thread/thr_init.c b/lib/libkse/thread/thr_init.c
index 35731c4..be671a1 100644
--- a/lib/libkse/thread/thr_init.c
+++ b/lib/libkse/thread/thr_init.c
@@ -92,7 +92,6 @@ _thread_init(void)
 	int		mib[2];
 	struct clockinfo clockinfo;
 	struct sigaction act;
-	struct sigaltstack alt;
 
 	/* Check if this function has already been called: */
 	if (_thread_initial)
@@ -281,11 +280,15 @@ _thread_init(void)
 		/* Clear the signal queue: */
 		memset(_thread_sigq, 0, sizeof(_thread_sigq));
 
-		/* Create and install an alternate signal stack: */
-		alt.ss_sp = malloc(SIGSTKSZ);	/* recommended stack size */
-		alt.ss_size = SIGSTKSZ;
-		alt.ss_flags = 0;
-		if (_thread_sys_sigaltstack(&alt, NULL) != 0)
+		/*
+		 * Create and install an alternate signal stack of
+		 * the recommended size:
+		 */
+		_thread_sigstack.ss_sp = malloc(SIGSTKSZ);
+		_thread_sigstack.ss_size = SIGSTKSZ;
+		_thread_sigstack.ss_flags = 0;
+		if ((_thread_sigstack.ss_sp == NULL) ||
+		    (_thread_sys_sigaltstack(&_thread_sigstack, NULL) != 0))
 			PANIC("Unable to install alternate signal stack");
 
 		/* Enter a loop to get the existing signal status: */
diff --git a/lib/libkse/thread/thr_kern.c b/lib/libkse/thread/thr_kern.c
index 4d92385..2b7b270 100644
--- a/lib/libkse/thread/thr_kern.c
+++ b/lib/libkse/thread/thr_kern.c
@@ -71,6 +71,7 @@ thread_run_switch_hook(pthread_t thread_out, pthread_t thread_in);
 
 /* Static variables: */
 static int	last_tick = 0;
+static int	called_from_handler = 0;
 
 /*
  * This is called when a signal handler finishes and wants to
@@ -106,20 +107,12 @@ _thread_kern_sched(ucontext_t *scp)
 
 	/* Check if this function was called from the signal handler: */
 	if (scp != NULL) {
-		/*
-		 * The signal handler should have saved the state of
-		 * the current thread.  Restore the process signal
-		 * mask.
-		 */
-		if (_thread_sys_sigprocmask(SIG_SETMASK,
-		    &_process_sigmask, NULL) != 0)
-			PANIC("Unable to restore process mask after signal");
+		called_from_handler = 1;
 		/*
 		 * We're running on the signal stack; just call the
 		 * kernel scheduler directly.
 		 */
 		DBG_MSG("Entering scheduler due to signal\n");
-		_thread_kern_scheduler();
 	} else {
 		/* Save the state of the current thread: */
 		if (_setjmp(_thread_run->ctx.jb) == 0) {
@@ -162,9 +155,9 @@ _thread_kern_sched(ucontext_t *scp)
 			}
 			return;
 		}
-		/* Switch to the thread scheduler: */
-		___longjmp(_thread_kern_sched_jb, 1);
 	}
+	/* Switch to the thread scheduler: */
+	___longjmp(_thread_kern_sched_jb, 1);
 }
 
 void
@@ -188,6 +181,26 @@ _thread_kern_scheduler(void)
 	if ((_thread_run->flags & PTHREAD_FLAGS_PRIVATE) == 0)
 		_last_user_thread = _thread_run;
 
+	if (called_from_handler != 0) {
+		called_from_handler = 0;
+
+		/*
+		 * The signal handler should have saved the state of
+		 * the current thread.  Restore the process signal
+		 * mask.
+		 */
+		if (_thread_sys_sigprocmask(SIG_SETMASK,
+		    &_process_sigmask, NULL) != 0)
+			PANIC("Unable to restore process mask after signal");
+
+		/*
+		 * Since the signal handler didn't return normally, we
+		 * have to tell the kernel to reuse the signal stack.
+		 */
+		if (_thread_sys_sigaltstack(&_thread_sigstack, NULL) != 0)
+			PANIC("Unable to restore alternate signal stack");
+	}
+
 	/* Are there pending signals for this thread? */
 	if (_thread_run->check_pending != 0) {
 		_thread_run->check_pending = 0;
diff --git a/lib/libkse/thread/thr_private.h b/lib/libkse/thread/thr_private.h
index e8fff12..9d6bc31 100644
--- a/lib/libkse/thread/thr_private.h
+++ b/lib/libkse/thread/thr_private.h
@@ -1127,6 +1127,9 @@ SCLASS	volatile int	_sigq_check_reqd
 #endif
 ;
 
+/* The signal stack. */
+SCLASS struct sigaltstack _thread_sigstack;
+
 /* Thread switch hook. */
 SCLASS pthread_switch_routine_t _sched_switch_hook
 #ifdef GLOBAL_PTHREAD_PRIVATE
@@ -1268,7 +1271,7 @@ int     _thread_sys_sigsuspend(const sigset_t *);
 int     _thread_sys_siginterrupt(int, int);
 int     _thread_sys_sigpause(int);
 int     _thread_sys_sigreturn(ucontext_t *);
-int     _thread_sys_sigaltstack(const struct sigaltstack *, struct sigstack *);
+int     _thread_sys_sigaltstack(const struct sigaltstack *, struct sigaltstack *);
 int     _thread_sys_sigstack(const struct sigstack *, struct sigstack *);
 int     _thread_sys_sigvec(int, struct sigvec *, struct sigvec *);
 void    _thread_sys_psignal(unsigned int, const char *);
diff --git a/lib/libkse/thread/thr_sig.c b/lib/libkse/thread/thr_sig.c
index 3bcd9c1..3ac9740 100644
--- a/lib/libkse/thread/thr_sig.c
+++ b/lib/libkse/thread/thr_sig.c
@@ -1066,7 +1066,7 @@ thread_sigframe_add(pthread_t thread, int sig, int has_args)
 	/*
 	 * Set up the context:
 	 */
-	stackp += sizeof(double);
+	stackp -= sizeof(double);
 	_setjmp(thread->ctx.jb);
 	SET_STACK_JB(thread->ctx.jb, stackp);
 	SET_RETURN_ADDR_JB(thread->ctx.jb, _thread_sig_wrapper);