winpr: fix CreateProcess signal handler issues

The calling thread of CreateProcess can be in any library and
can have arbitrary signal masks and handlers.

We now save the caller's mask and  block all signals before forking.
After fork:
- child resets the handlers and unblocks all signals.
- parent restores the caller's original signal mask.

winpr/libwinpr/thread/process.c

@@ -80,6 +80,14 @@
 #include "../handle/handle.h"
 #include "../security/security.h"
 
+#ifndef NSIG
+#ifdef SIGMAX
+#define NSIG SIGMAX
+#else
+#define NSIG 64
+#endif
+#endif
+
 char** EnvironmentBlockToEnvpA(LPCH lpszEnvironmentBlock)
 {
 	char* p;
@@ -210,6 +218,9 @@ BOOL _CreateProcessExA(HANDLE hToken, DWORD dwLogonFlags,
 	WINPR_ACCESS_TOKEN* token;
 	LPTCH lpszEnvironmentBlock;
 	BOOL ret = FALSE;
+	sigset_t oldSigMask;
+	sigset_t newSigMask;
+	BOOL restoreSigMask = FALSE;
 
 	pid = 0;
 	numArgs = 0;
@@ -241,6 +252,10 @@ BOOL _CreateProcessExA(HANDLE hToken, DWORD dwLogonFlags,
 	if (NULL == filename)
 		goto finish;
 
+	/* block all signals so that the child can safely reset the caller's handlers */
+	sigfillset(&newSigMask);
+	restoreSigMask = !pthread_sigmask(SIG_SETMASK, &newSigMask, &oldSigMask);
+
 	/* fork and exec */
 
 	pid = fork();
@@ -254,16 +269,33 @@ BOOL _CreateProcessExA(HANDLE hToken, DWORD dwLogonFlags,
 	if (pid == 0)
 	{
 		/* child process */
+#ifndef __sun
+		int maxfd;
+#endif
+		int fd;
+		int sig;
+		sigset_t set;
+		struct sigaction act;
+
+		/* set default signal handlers */
+		memset(&act, 0, sizeof(act));
+		act.sa_sigaction = SIG_DFL;
+		act.sa_flags = 0;
+		sigemptyset(&act.sa_mask);
+		for (sig = 1; sig < NSIG; sig++)
+			sigaction(sig, &act, NULL);
+		/* unblock all signals */
+		sigfillset(&set);
+		pthread_sigmask(SIG_UNBLOCK, &set, NULL);
+
 #ifdef __sun
 	closefrom(3);
 #else
-	int maxfd;
 #ifdef F_MAXFD // on some BSD derivates
 	maxfd = fcntl(0, F_MAXFD);
 #else
 	maxfd = sysconf(_SC_OPEN_MAX);
 #endif
-	int fd;
 	for(fd=3; fd<maxfd; fd++)
 		close(fd);
 #endif // __sun
@@ -324,6 +356,12 @@ BOOL _CreateProcessExA(HANDLE hToken, DWORD dwLogonFlags,
 	ret = TRUE;
 
 finish:
+
+	/* restore caller's original signal mask */
+	if (restoreSigMask)
+		pthread_sigmask(SIG_SETMASK, &oldSigMask, NULL);
+
+
 	free(filename);
 
 	if (pArgs)