Re: [Qemu-trivial] [Qemu-devel] [PATCH v1 1/5] util/qemu-sockets: Fix GCC 9 build warnings

[Alistair Francis]

On Tue, Apr 30, 2019 at 2:16 PM Eric Blake <address@hidden> wrote:
>
> On 4/30/19 3:25 PM, Eric Blake wrote:
> > On 4/30/19 3:08 PM, Alistair Francis wrote:
> >> Fix this warning when building with GCC9 on Fedora 30:
> >> In function ‘strncpy’,
> >>     inlined from ‘unix_connect_saddr.isra.0’ at util/qemu-sockets.c:925:5:
> >> /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ 
> >> specified bound 108 equals destination size [-Werror=stringop-truncation]
> >>   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos 
> >> (__dest));
> >>       |          
> >> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >> In function ‘strncpy’,
> >>     inlined from ‘unix_listen_saddr.isra.0’ at util/qemu-sockets.c:880:5:
> >>
>
> >> -    strncpy(un.sun_path, path, sizeof(un.sun_path));
> >> +    strncpy(un.sun_path, path, sizeof(un.sun_path) - 1);
> >
> > NACK. Linux allows you to use the full width of un.sun_path (a NUL
> > terminator is required if you copy less than that, but not if you use
> > the full width). Rather, we may need to mark path as a potential
> > nonstring to silence the warning, or use memcpy instead of strncpy, or
> > some other workaround.  (Sadly, this is one of those odd places where
> > strncpy is actually the right function to use, but there are so many
> > other places where strncpy is used incorrectly that it has turned into a
> > battle to use it here)
>
> We don't have control over un (that's from the libc system headers), but
> does adding the QEMU_NONSTRING attribute to our declaration of path
> serve to silence the warning?

I don't think that would fix it, I'll double check though.

>
> In short, I think most of this series should look at the use of the
> QEMU_NONSTRING macro, as that macro goes hand-in-hand with strncpy() for
> informing the compiler exactly when we know that we are copying
> something that has fixed length and may or may not be NUL-terminated.

I have changed the series to use memcpy() and QEMU_NONSTRING. I'll
send a v2 out soon with all your comments addressed.

Alistair

>
> --
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
>