Re: [PATCH v2 1/2] util: Introduce qemu_get_host_name()

[Daniel P . Berrangé]

On Mon, Jun 22, 2020 at 07:53:40PM +0200, Michal Privoznik wrote:
> On 6/22/20 7:38 PM, Daniel P. Berrangé wrote:
> > On Mon, Jun 22, 2020 at 07:26:44PM +0200, Michal Privoznik wrote:
> > > This function offers operating system agnostic way to fetch host
> > > name. It is implemented for both POSIX-like and Windows systems.
> > > 
> > > Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> > > ---
> > >   include/qemu/osdep.h | 10 ++++++++++
> > >   util/oslib-posix.c   | 32 ++++++++++++++++++++++++++++++++
> > >   util/oslib-win32.c   | 13 +++++++++++++
> > >   3 files changed, 55 insertions(+)
> > > 
> > > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > > index ff7c17b857..a795d46b28 100644
> > > --- a/include/qemu/osdep.h
> > > +++ b/include/qemu/osdep.h
> > > @@ -607,4 +607,14 @@ static inline void qemu_reset_optind(void)
> > >   #endif
> > >   }
> > > +/**
> > > + * qemu_get_host_name:
> > > + * @errp: Error object
> > > + *
> > > + * Operating system agnostic way of querying host name.
> > > + *
> > > + * Returns allocated hostname (caller should free), NULL on failure.
> > > + */
> > > +char *qemu_get_host_name(Error **errp);
> > > +
> > >   #endif
> > > diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> > > index 916f1be224..865a3d71a7 100644
> > > --- a/util/oslib-posix.c
> > > +++ b/util/oslib-posix.c
> > > @@ -761,3 +761,35 @@ void sigaction_invoke(struct sigaction *action,
> > >       }
> > >       action->sa_sigaction(info->ssi_signo, &si, NULL);
> > >   }
> > > +
> > > +#ifndef HOST_NAME_MAX
> > > +# ifdef _POSIX_HOST_NAME_MAX
> > > +#  define HOST_NAME_MAX _POSIX_HOST_NAME_MAX
> > > +# else
> > > +#  define HOST_NAME_MAX 255
> > > +# endif
> > > +#endif
> > > +
> > > +char *qemu_get_host_name(Error **errp)
> > > +{
> > > +    long len = -1;
> > > +    char *hostname;
> > > +
> > > +#ifdef _SC_HOST_NAME_MAX
> > > +    len = sysconf(_SC_HOST_NAME_MAX);
> > > +#endif /* _SC_HOST_NAME_MAX */
> > > +
> > > +    if (len < 0) {
> > > +        len = HOST_NAME_MAX;
> > > +    }
> > > +
> > > +    hostname = g_malloc0(len + 1);
> > 
> > Nitpick, generally qemu prefers g_new0
> > 
> > > +
> > > +    if (gethostname(hostname, len) < 0) {
> > > +        error_setg_errno(errp, errno,
> > > +                         "cannot get hostname");
> > > +        return NULL;
> > > +    }
> > 
> > According to my man page, it is undefined by POSIX whether there's a
> > trailing NUL when  hostname exceeds the buffer, so the paranoid thing
> > todo is to add
> > 
> >    hostname[len] = '\0';
> 
> Isn't this guaranteed by allocating len + 1 bytes? I mean, g_malloc0() and
> g_new0() will memset() the memory to zero. And since I tell gethostname()
> the buf is only len bytes long I am guaranteed to have 0 at the end of it,
> aren't I? Maybe I should put a comment just before g_malloc0() or g_new0()
> that documents this thought.

Oh right, yes, I'm mis-reading the code.

Regards,
Daniel
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