openssl_strerror_r: Fix handling of GNU strerror_r

GNU strerror_r may return either a pointer to a string that the function
stores in buf, or a pointer to some (immutable) static string in which case
buf is unused.

In such a case we need to set buf manually.
Reviewed-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8371)

(cherry picked from commit e3b35d2b29e9446af83fcaa534e67e7b04a60d7a)

crypto/o_str.c

@@ -223,7 +223,26 @@ int openssl_strerror_r(int errnum, char *buf, size_t buflen)
 #if defined(_MSC_VER) && _MSC_VER>=1400
     return !strerror_s(buf, buflen, errnum);
 #elif defined(_GNU_SOURCE)
-    return strerror_r(errnum, buf, buflen) != NULL;
+    char *err;
+
+    /*
+     * GNU strerror_r may not actually set buf.
+     * It can return a pointer to some (immutable) static string in which case
+     * buf is left unused.
+     */
+    err = strerror_r(errnum, buf, buflen);
+    if (err == NULL)
+        return 0;
+    /*
+     * If err is statically allocated, err != buf and we need to copy the data.
+     * If err points somewhere inside buf, OPENSSL_strlcpy can handle this,
+     * since src and dest are not annotated with __restrict and the function
+     * reads src byte for byte and writes to dest.
+     * If err == buf we do not have to copy anything.
+     */
+    if (err != buf)
+        OPENSSL_strlcpy(buf, err, buflen);
+    return 1;
 #elif (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L) || \
       (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE >= 600)
     /*
@@ -234,6 +253,7 @@ int openssl_strerror_r(int errnum, char *buf, size_t buflen)
     return !strerror_r(errnum, buf, buflen);
 #else
     char *err;
+
     /* Fall back to non-thread safe strerror()...its all we can do */
     if (buflen < 2)
         return 0;
@@ -241,8 +261,7 @@ int openssl_strerror_r(int errnum, char *buf, size_t buflen)
     /* Can this ever happen? */
     if (err == NULL)
         return 0;
-    strncpy(buf, err, buflen - 1);
-    buf[buflen - 1] = '\0';
+    OPENSSL_strlcpy(buf, err, buflen);
     return 1;
 #endif
 }