Re: [Qemu-trivial] [Qemu-devel] [PATCH v1 4/5] linux-user/uname: Fix GCC 9 build warnings

[Alistair Francis]

On Tue, Apr 30, 2019 at 1:40 PM Eric Blake <address@hidden> wrote:
>
> On 4/30/19 3:09 PM, Alistair Francis wrote:
> > Fix this warning when building with GCC9 on Fedora 30:
> > In function ‘strncpy’,
> >     inlined from ‘sys_uname’ at /home/alistair/qemu/linux-user/uname.c:94:3:
> > /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ 
> > output may be truncated copying 64 bytes from a string of length 64 
> > [-Werror=stringop-truncation]
> >   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos 
> > (__dest));
> >       |          
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Signed-off-by: Alistair Francis <address@hidden>
> > ---
> >  linux-user/uname.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/linux-user/uname.c b/linux-user/uname.c
> > index 313b79dbad..293b2238f2 100644
> > --- a/linux-user/uname.c
> > +++ b/linux-user/uname.c
> > @@ -90,6 +90,11 @@ int sys_uname(struct new_utsname *buf)
> >     * struct linux kernel uses).
> >     */
> >
> > +#if defined(CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE) && QEMU_GNUC_PREREQ(9, 0)
> > +#pragma GCC diagnostic push
> > +#pragma GCC diagnostic ignored "-Wstringop-truncation"
> > +#endif
>
> Why do we need the pragma?
>
> > +
> >    memset(buf, 0, sizeof(*buf));
>
> We are prezeroing the entire field, at which point...
>
> >    COPY_UTSNAME_FIELD(buf->sysname, uts_buf.sysname);
> >    COPY_UTSNAME_FIELD(buf->nodename, uts_buf.nodename);
>
> ...using strncpy() for a shorter string is wasteful (we're writing the
> tail end twice), and for a long string is warning-prone.  Why not
> rewrite COPY_UTSNAME_FIELD() to use memcpy() for the MIN(strlen(src),
> __NEW_UTS_LEN) and drop the write of the trailing NUL, since it will
> already be NUL from your memset()?

I'm happy to do that, I'll change it to memcpy().

Alistair

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