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[Qemu-devel] [Bug 1803872] Re: [PATCH v3 4/5] migration: Use QEMU_NONSTR

From: Eric Blake
Subject: [Qemu-devel] [Bug 1803872] Re: [PATCH v3 4/5] migration: Use QEMU_NONSTRING for non NUL-terminated arrays
Date: Tue, 18 Dec 2018 21:36:58 -0000

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 

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

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  gcc 8.2 reports stringop-truncation when building qemu

Status in QEMU:

Bug description:
  QEMU 3.0

  block/sheepdog.c: In function 'find_vdi_name':
  block/sheepdog.c:1239:5: error: 'strncpy' specified bound 256 equals 
destination size [-Werror=stringop-truncation]
       strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);

  If this is the intended behavior, please suppress the warning. For

  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wstringop-truncation"
      strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
  #pragma GCC diagnostic pop

  This also happens on other sources, for example hw/acpi/core.c, so
  another option is to suppress it globally on CFLAGS (-Wno-stringop-

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