Re: [Qemu-devel] sheepdog build warning

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Dec 17, 2018 at 11:16:05PM -0500, Michael S. Tsirkin wrote:
> On Mon, Dec 17, 2018 at 11:03:11PM -0500, Michael S. Tsirkin wrote:
> > mingw32 build on fedora fails with this warning:
> > 
> > /scm/qemu/block/sheepdog.c: In function 'find_vdi_name':
> > /scm/qemu/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);
> >      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > cc1: all warnings being treated as errors
> > make: *** [/scm/qemu/rules.mak:69: block/sheepdog.o] Error 1
> > make: *** Waiting for unfinished jobs....
> > 
> > 
> > Reading the code one sees it's working as intended:
> > 
> > 
> > static int find_vdi_name(BDRVSheepdogState *s, const char *filename,
> >                          uint32_t snapid, const char *tag, uint32_t *vid,
> >                          bool lock, Error **errp)
> > {
> >     int ret, fd;
> >     SheepdogVdiReq hdr;
> >     SheepdogVdiRsp *rsp = (SheepdogVdiRsp *)&hdr;
> >     unsigned int wlen, rlen = 0;
> >     char buf[SD_MAX_VDI_LEN + SD_MAX_VDI_TAG_LEN];
> > 
> >     fd = connect_to_sdog(s, errp);
> >     if (fd < 0) {
> >         return fd;
> >     }
> > 
> >     /* This pair of strncpy calls ensures that the buffer is zero-filled,
> >      * which is desirable since we'll soon be sending those bytes, and
> >      * don't want the send_req to read uninitialized data.
> >      */
> >     strncpy(buf, filename, SD_MAX_VDI_LEN);
> >     strncpy(buf + SD_MAX_VDI_LEN, tag, SD_MAX_VDI_TAG_LEN);
> > 
> > 
> > .....
> > 
> > }
> > 
> > 
> > 
> > so this seems to be the case of GCC developers deciding that
> > strncpy is simply a bad API and a correct use of it should be
> > warned against.
> > 
> > I propose either
> > 
> > 1. simply adding
> > 
> > #pragma GCC diagnostic ignored "-Wstringop-truncation"
> > 
> > in osdep.
> > 
> > 2. adding an inline wrapper with said pragma in there.
> > 
> > 3. -Wno-stringop-truncation is the makefile
> > 
> > Thoughts?
> > 
> > -- 
> 
> 
> So here's approach 2. However I note that a newer gcc 8.2.1
> does not give this warning. Maybe detect at configure time
> and suppress (option 3)?

I'm not 

> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 3bf48bcdec..64d8258529 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -134,6 +134,22 @@ extern int daemon(int, int);
>  #define assert(x)  g_assert(x)
>  #endif
>  
> +/*
> + * GCC 8.0 declared war on strncpy. Admittedly it's a tricky interface
> + * with unintuitive semantics, but we use it widely.
> + */
> +#if defined(__GNUC__) && __GNUC__ >= 8
> +static inline void qemu_strncpy(char *to, const char *from, int n)
> +{
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wstringop-truncation"
> +    strncpy(to, from, n);
> +#pragma GCC diagnostic pop
> +}
> +
> +#define strncpy qemu_strncpy
> +#endif

I don't think we should disable the warning for all uses of strncpy
as that defeats the protection this warning offers for bad usage.

IMHO, just push/pop the warning pragma inline in the sheepdog code
which triggers it.

Regards,
Daniel
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