fix race condition in raise - just mask signals

a signal handler could fork after the pid/tid were read, causing the
wrong process to be signalled. i'm not sure if this is supposed to
have UB or not, but raise is async-signal-safe, so it probably is
allowed. the current solution is slightly expensive so this
implementation is likely to be changed in the future.

src/signal/raise.c

@@ -2,17 +2,17 @@
 #include <errno.h>
 #include "syscall.h"
 
+int __sigprocmask(int, const sigset_t *, sigset_t *);
+
 int raise(int sig)
 {
 	int pid, tid, ret;
-	/* Getting the pid/tid pair is not atomic, and could give wrong
-	 * result if a fork occurs in a signal handler between the two
-	 * syscalls. Use the tgkill syscall's ESRCH semantics to detect
-	 * this condition and retry. */
-	do {
-		tid = syscall0(__NR_gettid);
-		pid = syscall0(__NR_getpid);
-		ret = syscall3(__NR_tgkill, pid, tid, sig);
-	} while (ret<0 && errno == ESRCH);
+	sigset_t set;
+	sigfillset(&set);
+	__sigprocmask(SIG_BLOCK, &set, &set);
+	tid = syscall0(__NR_gettid);
+	pid = syscall0(__NR_getpid);
+	ret = syscall3(__NR_tgkill, pid, tid, sig);
+	__sigprocmask(SIG_SETMASK, &set, 0);
 	return ret;
 }