Re: [Qemu-devel] Hacks for building on gcc 7 / Fedora 26

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

On Thu, Jul 20, 2017 at 05:15:49PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (berrange@redhat.com) wrote:
> > On Fri, Apr 07, 2017 at 03:38:47PM +0100, Dr. David Alan Gilbert wrote:
> > > Hi,
> > >    Fedora 26 has gcc 7.0.1 which has the normal compliment
> > > of new fussy warnings; so far I've posted :
> > > 
> > > tests/check-qdict: Fix missing brackets
> > > slirp/smb: Replace constant strings by glib string
> > > 
> > > that fix one actual mistake and work around something it's being
> > > fussy over.
> > > 
> > > But I've also got a pile of hacks, attached below that I'm
> > > not too sure what I'll do with them yet, but they're attached
> > > for anyone else trying to build.  Note they're smoke-only-tested.
> > > 
> > > I also have gcc bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80346
> > > filed for what I reckon is a couple of overly pessimistic warnings.
> > 
> > 
> > > diff --git a/include/qemu/iov.h b/include/qemu/iov.h
> > > index bd9fd55b0a..ebb0221140 100644
> > > --- a/include/qemu/iov.h
> > > +++ b/include/qemu/iov.h
> > > @@ -46,7 +46,7 @@ static inline size_t
> > >  iov_from_buf(const struct iovec *iov, unsigned int iov_cnt,
> > >               size_t offset, const void *buf, size_t bytes)
> > >  {
> > > -    if (__builtin_constant_p(bytes) && iov_cnt &&
> > > +    if (__builtin_constant_p(bytes) && iov_cnt && bytes <= INT_MAX &&
> > >          offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
> > >          memcpy(iov[0].iov_base + offset, buf, bytes);
> > >          return bytes;
> > > @@ -59,7 +59,7 @@ static inline size_t
> > >  iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
> > >             size_t offset, void *buf, size_t bytes)
> > >  {
> > > -    if (__builtin_constant_p(bytes) && iov_cnt &&
> > > +    if (__builtin_constant_p(bytes) && iov_cnt && bytes <= INT_MAX &&
> > >          offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
> > >          memcpy(buf, iov[0].iov_base + offset, bytes);
> > >          return bytes;
> 
> tbh I don't know what the right fix for this is;  the gcc discussion
> confused me as to why it thinks it can be a valid case.

Even if gcc is broken in issuing a warning here, we still need to
make it quiet so people on F26 and similarly new distros can build
without warnings.

IMHO your patch is ok, or we could be alittle more explicit about
catching just the case where you pass -1 for bytes, and have

  && bytes != -1


Regards,
Daniel
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