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

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH v3 4/5] migration: Use QEMU_NONSTRING for non NUL-terminated arrays
Date: Tue, 18 Dec 2018 15:36:58 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

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, 
   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

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