err/err.c: add err_clear_last_constant_time.

Expected usage pattern is to unconditionally set error and then
wipe it if there was no actual error.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit f658a3b64d8750642f4975090740865f770c2a1b)

Resolved conflicts:
	crypto/err/err.c

(Merged from https://github.com/openssl/openssl/pull/7735)

1 files changed, 21 insertions, 0 deletions

diff --git a/crypto/err/err.c b/crypto/err/err.c
index 08c27a3e83..638cbf235b 100644
--- a/crypto/err/err.c
+++ b/crypto/err/err.c
@@ -19,6 +19,7 @@
 #include <openssl/bio.h>
 #include <openssl/opensslconf.h>
 #include <internal/thread_once.h>
+#include "internal/constant_time_locl.h"
 
 static void err_load_strings(int lib, ERR_STRING_DATA *str);
 
@@ -822,3 +823,23 @@ int ERR_pop_to_mark(void)
     es->err_flags[es->top] &= ~ERR_FLAG_MARK;
     return 1;
 }
+
+void err_clear_last_constant_time(int clear)
+{
+    ERR_STATE *es;
+    int top;
+
+    es = ERR_get_state();
+    if (es == NULL)
+        return;
+
+    top = es->top;
+
+    es->err_flags[top] &= ~(0 - clear);
+    es->err_buffer[top] &= ~(0UL - clear);
+    es->err_file[top] = (const char *)((uintptr_t)es->err_file[top] &
+                                       ~((uintptr_t)0 - clear));
+    es->err_line[top] |= 0 - clear;
+
+    es->top = (top + ERR_NUM_ERRORS - clear) % ERR_NUM_ERRORS;
+}