Re: [Qemu-devel] [PATCH v3 4/5] migration: Use QEMU_NONSTRING for non NUL-terminated arrays

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Philippe Mathieu-Daudé (philmd@redhat.com) wrote:
> GCC 8 added a -Wstringop-truncation warning:
> 
>   The -Wstringop-truncation warning added in GCC 8.0 via r254630 for
>   bug 81117 is specifically intended to highlight likely unintended
>   uses of the strncpy function that truncate the terminating NUL
>   character from the source string.
> 
> This new warning leads to compilation failures:
> 
>     CC      migration/global_state.o
>   qemu/migration/global_state.c: In function 'global_state_store_running':
>   qemu/migration/global_state.c:45:5: error: 'strncpy' specified bound 100 equals destination size [-Werror=stringop-truncation]
>        strncpy((char *)global_state.runstate, state, sizeof(global_state.runstate));
>        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   make: *** [qemu/rules.mak:69: migration/global_state.o] Error 1
> 
> Use the QEMU_NONSTRING attribute, since this array is intended to store
> character arrays that do not necessarily contain a terminating NUL.
> 
> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> 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 8e8ab5c51e..6e19333422 100644
> --- a/migration/global_state.c
> +++ b/migration/global_state.c
> @@ -21,7 +21,7 @@
>  
>  typedef struct {
>      uint32_t size;
> -    uint8_t runstate[100];
> +    uint8_t runstate[100] QEMU_NONSTRING;

Hmm; global_state_post_load needs to be fixed for this;  it
uses s->runsate and ends up passing it to both a trace
and a qapi_enum_parse - so it's really treating it as a string.
That code is unsafe anyway since it's assuming the received
runstate would be terminated.

Dave

>      RunState state;
>      bool received;
>  } GlobalState;
> -- 
> 2.17.2
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK