From 59488f25ad9d2e804f85a48d38e38df9e94ae61d Mon Sep 17 00:00:00 2001
From: Xin LI <delphij@FreeBSD.org>
Date: Mon, 20 Aug 2018 02:17:55 +0000
Subject: [PATCH] In r331279 the code used ENOSYS to check the existence of
 getrandom(2). This will only work if the caller already handles SIGSYS, which
 is not always the case.

Address this by checking osreldate instead. Note that because there
was not __FreeBSD_version bump when the system call was added, use
1200061 (r332100) which is the first bump after the introduction of
the system call.

PR:		230762
Reported by:	Jenkins via Mark Millard
Reviewed by:	cem
Differential Revision:	https://reviews.freebsd.org/D16807
---
 lib/libc/gen/getentropy.c | 37 +++++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 8 deletions(-)

diff --git a/lib/libc/gen/getentropy.c b/lib/libc/gen/getentropy.c
index 2e0f649a15be..4e91afc6a6a1 100644
--- a/lib/libc/gen/getentropy.c
+++ b/lib/libc/gen/getentropy.c
@@ -34,10 +34,14 @@ __FBSDID("$FreeBSD$");
 #include <sys/sysctl.h>
 
 #include <errno.h>
+#include <stdbool.h>
 #include <stdlib.h>
 
 #include "libc_private.h"
 
+/* First __FreeBSD_version bump after introduction of getrandom(2) (r331279) */
+#define GETRANDOM_FIRST 1200061
+
 extern int __sysctl(int *, u_int, void *, size_t *, void *, size_t);
 
 static size_t
@@ -99,21 +103,38 @@ int
 getentropy(void *buf, size_t buflen)
 {
 	ssize_t rd;
+	bool have_getrandom;
 
 	if (buflen > 256) {
 		errno = EIO;
 		return (-1);
 	}
 
+	have_getrandom = (__getosreldate() >= GETRANDOM_FIRST);
+
 	while (buflen > 0) {
-		rd = getrandom(buf, buflen, 0);
-		if (rd == -1) {
-			if (errno == EINTR)
-				continue;
-			else if (errno == ENOSYS || errno == ECAPMODE)
-				return (getentropy_fallback(buf, buflen));
-			else
-				return (-1);
+		if (have_getrandom) {
+			rd = getrandom(buf, buflen, 0);
+			if (rd == -1) {
+				switch (errno) {
+				case ECAPMODE:
+					/*
+					 * Kernel >= r331280 and < r337999
+					 * will return ECAPMODE when the
+					 * caller is already in capability
+					 * mode, fallback to traditional
+					 * method in this case.
+					 */
+					have_getrandom = false;
+					continue;
+				case EINTR:
+					continue;
+				default:
+					return (-1);
+				}
+			}
+		} else {
+			return (getentropy_fallback(buf, buflen));
 		}
 
 		/* This cannot happen. */