Provide CBS_get_last_u8().

This will be used in the TLSv1.3 record layer.

From BoringSSL.

ok tb@

2 files changed, 19 insertions, 2 deletions

diff --git a/src/lib/libssl/bs_cbs.c b/src/lib/libssl/bs_cbs.c
index ab76b78927..627c609bb9 100644
--- a/src/lib/libssl/bs_cbs.c
+++ b/src/lib/libssl/bs_cbs.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bs_cbs.c,v 1.21 2021/10/31 06:48:54 jsing Exp $       */
+/*      $OpenBSD: bs_cbs.c,v 1.22 2021/12/15 17:23:34 jsing Exp $       */
 /*
  * Copyright (c) 2014, Google Inc.
  *
@@ -191,6 +191,17 @@ CBS_get_u32(CBS *cbs, uint32_t *out)
 }
 
 int
+CBS_get_last_u8(CBS *cbs, uint8_t *out)
+{
+        if (cbs->len == 0)
+                return 0;
+
+        *out = cbs->data[cbs->len - 1];
+        cbs->len--;
+        return 1;
+}
+
+int
 CBS_get_bytes(CBS *cbs, CBS *out, size_t len)
 {
         const uint8_t *v;
diff --git a/src/lib/libssl/bytestring.h b/src/lib/libssl/bytestring.h
index 9e55dd44d6..4ab2828d09 100644
--- a/src/lib/libssl/bytestring.h
+++ b/src/lib/libssl/bytestring.h
@@ -1,4 +1,4 @@
-/*      $OpenBSD: bytestring.h,v 1.19 2021/05/16 10:58:27 jsing Exp $   */
+/*      $OpenBSD: bytestring.h,v 1.20 2021/12/15 17:23:34 jsing Exp $   */
 /*
  * Copyright (c) 2014, Google Inc.
  *
@@ -134,6 +134,12 @@ int CBS_get_u24(CBS *cbs, uint32_t *out);
 int CBS_get_u32(CBS *cbs, uint32_t *out);
 
 /*
+ * CBS_get_last_u8 sets |*out| to the last uint8_t from |cbs| and shortens
+ * |cbs|. It returns one on success and zero on error.
+ */
+int CBS_get_last_u8(CBS *cbs, uint8_t *out);
+
+/*
  * CBS_get_bytes sets |*out| to the next |len| bytes from |cbs| and advances
  * |cbs|. It returns one on success and zero on error.
  */