[Qemu-devel] [PATCH v3 5/5] migration: Use strnlen() for fixed-size string

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

GCC 8 introduced the -Wstringop-overflow, which detect buffer overflow
by string-modifying functions declared in <string.h>, such strncpy(),
used in global_state_store_running().

Since the global_state.runstate does not necessarily contains a
terminating NUL character, We had to use the QEMU_NONSTRING attribute.

The GCC manual says about the nonstring attribute:

  However, when the array is declared with the attribute the call to
  strlen is diagnosed because when the array doesn’t contain a
  NUL-terminated string the call is undefined. [...]
  In addition, calling strnlen and strndup with such arrays is safe
  provided a suitable bound is specified, and not diagnosed.

GCC indeed found an incorrect use of strlen(), because this array
is loaded by VMSTATE_BUFFER(runstate, GlobalState) then parsed
using qapi_enum_parse which does not get the buffer length.

Use strnlen() which returns sizeof(s->runstate) if the array is not
NUL-terminated.

This fixes:

    CC      migration/global_state.o
  qemu/migration/global_state.c: In function 'global_state_pre_save':
  qemu/migration/global_state.c:109:15: error: 'strlen' argument 1 declared attribute 'nonstring' [-Werror=stringop-overflow=]
       s->size = strlen((char *)s->runstate) + 1;
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~
  qemu/migration/global_state.c:24:13: note: argument 'runstate' declared here
       uint8_t runstate[100] QEMU_NONSTRING;
               ^~~~~~~~
  cc1: all warnings being treated as errors
  make: *** [qemu/rules.mak:69: migration/global_state.o] Error 1

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 migration/global_state.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/migration/global_state.c b/migration/global_state.c
index 6e19333422..c19030ef62 100644
--- a/migration/global_state.c
+++ b/migration/global_state.c
@@ -106,7 +106,7 @@ static int global_state_pre_save(void *opaque)
     GlobalState *s = opaque;
 
     trace_migrate_global_state_pre_save((char *)s->runstate);
-    s->size = strlen((char *)s->runstate) + 1;
+    s->size = strnlen((char *)s->runstate, sizeof(s->runstate)) + 1;
 
     return 0;
 }
-- 
2.17.2