misc: Refactor ssh_strerror to check return values

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/misc.c

@@ -1953,17 +1953,25 @@ char *ssh_strreplace(const char *src, const char *pattern, const char *replace)
  */
 char *ssh_strerror(int err_num, char *buf, size_t buflen)
 {
-#if defined(_WIN32)
+#if defined(__linux__) && defined(__GLIBC__) && defined(_GNU_SOURCE)
-    strerror_s(buf, buflen, err_num);
-    return buf;
-#elif defined(__linux__) && defined(__GLIBC__) && defined(_GNU_SOURCE)
     /* GNU extension on Linux */
     return strerror_r(err_num, buf, buflen);
 #else
-    /* POSIX version available for example on FreeBSD */
+    int rv;
-    strerror_r(err_num, buf, buflen);
+
-    return buf;
+#if defined(_WIN32)
+    rv = strerror_s(buf, buflen, err_num);
+#else
+    /* POSIX version available for example on FreeBSD or in musl libc */
+    rv = strerror_r(err_num, buf, buflen);
 #endif /* _WIN32 */
+
+    /* make sure the buffer is initialized and terminated with NULL */
+    if (-rv == ERANGE) {
+        buf[0] = '\0';
+    }
+    return buf;
+#endif /* defined(__linux__) && defined(__GLIBC__) && defined(_GNU_SOURCE) */
 }
 
 /** @} */

tests/unittests/torture_misc.c

@@ -732,6 +732,34 @@ static void torture_ssh_strreplace(void **state)
     assert_null(replaced_string);
 }
 
+static void torture_ssh_strerror(void **state)
+{
+    char buf[1024];
+    size_t bufflen = sizeof(buf);
+    char *out = NULL;
+
+    (void) state;
+
+    out = ssh_strerror(ENOENT, buf, 1); /* too short */
+    assert_string_equal(out, "\0");
+
+    out = ssh_strerror(256, buf, bufflen); /* unknown error code */
+    /* This error is always different:
+     * Freebd: "Unknown error: 256"
+     * MinGW/Win: "Unknown error"
+     * Linux/glibc: "Unknown error 256"
+     * Alpine/musl: "No error information"
+     */
+    assert_non_null(out);
+
+    out = ssh_strerror(ENOMEM, buf, bufflen);
+    /* This actually differs too for glibc/musl:
+     * musl: "Out of memory"
+     * everything else: "Cannot allocate memory"
+     */
+    assert_non_null(out);
+}
+
 int torture_run_tests(void) {
     int rc;
     struct CMUnitTest tests[] = {
@@ -755,6 +783,7 @@ int torture_run_tests(void) {
         cmocka_unit_test(torture_ssh_mkdirs),
         cmocka_unit_test(torture_ssh_quote_file_name),
         cmocka_unit_test(torture_ssh_strreplace),
+        cmocka_unit_test(torture_ssh_strerror),
     };
 
     ssh_init();