libthr: Fix LIBPTHREAD_QUEUE_FIFO range

LIBPTHREAD_QUEUE_FIFO is used in bitwise offset, we set the limit to
0~31 so that it won't cause undefined behavior. Also, create a new
helper function to do getenv++strtol directly and check the boundary of
the valid value.

Reviewed by:    kib
MFC after:      2 weeks
Differential Revision: https://reviews.freebsd.org/D60487

lib/libthr/libthr.3

@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 1, 2021
+.Dd October 8, 2026
 .Dt LIBTHR 3
 .Os
 .Sh NAME
@@ -199,7 +199,7 @@ The value is the count of loop operations.
 The integer value of the variable specifies how often blocked
 threads are inserted at the head of the sleep queue, instead of its tail.
 Bigger values reduce the frequency of the FIFO discipline.
-The value must be between 0 and 255.
+The value must be between 0 and 31.
 .It Dv LIBPTHREAD_UMTX_MIN_TIMEOUT
 The minimal amount of time, in nanoseconds, the thread is required to sleep
 for pthread operations specifying a timeout.

lib/libthr/thread/thr_init.c

@@ -475,6 +475,24 @@ __thr_get_main_stack_lim(size_t *lim)
 	return (false);
 }
 
+static bool
+getenv_int(const char *name, int mn, int mx, int *result)
+{
+	const char *env;
+	char *endptr;
+	long val;
+
+	env = getenv(name);
+	if (env == NULL || *env == '\0')
+		return (false);
+	val = strtol(env, &endptr, 10);
+	if (*endptr != '\0' || errno != 0 || val < mn || val > mx) {
+		return (false);
+	}
+	*result = val;
+	return (true);
+}
+
 static void
 init_private(void)
 {
@@ -519,15 +537,12 @@ init_private(void)
 		_thr_guard_default = _thr_page_size;
 		_pthread_attr_default.guardsize_attr = _thr_guard_default;
 		_pthread_attr_default.stacksize_attr = _thr_stack_default;
-		env = getenv("LIBPTHREAD_SPINLOOPS");
-		if (env)
-			_thr_spinloops = atoi(env);
-		env = getenv("LIBPTHREAD_YIELDLOOPS");
-		if (env)
-			_thr_yieldloops = atoi(env);
-		env = getenv("LIBPTHREAD_QUEUE_FIFO");
-		if (env)
-			_thr_queuefifo = atoi(env);
+		(void)getenv_int("LIBPTHREAD_SPINLOOPS", 0, INT_MAX,
+		    &_thr_spinloops);
+		(void)getenv_int("LIBPTHREAD_YIELDLOOPS", 0, INT_MAX,
+		    &_thr_yieldloops);
+		(void)getenv_int("LIBPTHREAD_QUEUE_FIFO", 0, 31,
+		    &_thr_queuefifo);
 		env = getenv("LIBPTHREAD_UMTX_MIN_TIMEOUT");
 		if (env) {
 			char *endptr;