RE: [PATCH v3 2/2] xgethostname: handle long hostnames

[David Turner <David.Turner@twosigma.com>]

> -----Original Message-----
> From: Junio C Hamano [mailto:gitster@pobox.com]
> Sent: Tuesday, April 18, 2017 10:51 PM
> To: Jonathan Nieder <jrnieder@gmail.com>
> Cc: David Turner <David.Turner@twosigma.com>; git@vger.kernel.org;
> l.s.r@web.de
> Subject: Re: [PATCH v3 2/2] xgethostname: handle long hostnames
> 
> Jonathan Nieder <jrnieder@gmail.com> writes:
> 
> > Hi,
> >
> > David Turner wrote:
> >
> >> If the full hostname doesn't fit in the buffer supplied to
> >> gethostname, POSIX does not specify whether the buffer will be
> >> null-terminated, so to be safe, we should do it ourselves.  Introduce
> >> new function, xgethostname, which ensures that there is always a \0
> >> at the end of the buffer.
> >
> > I think we should detect the error instead of truncating the hostname.
> > That (on top of your patch) would look like the following.
> >
> > Thoughts?
> > Jonathan
> >
> > diff --git i/wrapper.c w/wrapper.c
> > index d837417709..e218bd3bef 100644
> > --- i/wrapper.c
> > +++ w/wrapper.c
> > @@ -660,11 +660,13 @@ int xgethostname(char *buf, size_t len)  {
> >  	/*
> >  	 * If the full hostname doesn't fit in buf, POSIX does not
> > -	 * specify whether the buffer will be null-terminated, so to
> > -	 * be safe, do it ourselves.
> > +	 * guarantee that an error will be returned. Check for ourselves
> > +	 * to be safe.
> >  	 */
> >  	int ret = gethostname(buf, len);
> > -	if (!ret)
> > -		buf[len - 1] = 0;
> > +	if (!ret && !memchr(buf, 0, len)) {
> > +		errno = ENAMETOOLONG;
> > +		return -1;
> > +	}
> 
> Hmmmm.  "Does not specify if the buffer will be NUL-terminated"
> would mean that it is OK for the platform gethostname() to stuff
> sizeof(buf)-1 first bytes of the hostname in the buffer and then truncate by
> placing '\0' at the end of the buf, and we would not notice truncation with the
> above change on such a platform, no?

My read of the docs is that not only is that OK, but it is also permitted
for the platform to put sizeof(buf) bytes into the buffer and *not* 
put \0 at the end.

So in order to do a dynamic approach, we would have to allocate some
buffer, then run gethostname, then check if the penultimate element 
of the buffer was written to, and if so, allocate a larger buffer.  Yucky,
but possible.