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

[Eric Blake]

On 12/18/18 11:51 AM, Philippe Mathieu-Daudé 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.

>   typedef struct {
>       uint32_t size;
> -    uint8_t runstate[100];
> +    uint8_t runstate[100] QEMU_NONSTRING;

Since 100 bytes for runstate[] is larger than any string possible in our 
current enum string values, could we instead add an assert that 
strlen(state) < sizeof(global_state.runstate), and then use strpadcpy() 
to make our intent obvious while still shutting up the compiler warning, 
but without having to deal with the fallout of marking runstate as a 
non-string?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org