> -----Original Message----- > From: Junio C Hamano [mailto:[email protected]] > Sent: Tuesday, April 18, 2017 10:51 PM > To: Jonathan Nieder <[email protected]> > Cc: David Turner <[email protected]>; [email protected]; > [email protected] > Subject: Re: [PATCH v3 2/2] xgethostname: handle long hostnames > > Jonathan Nieder <[email protected]> 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.
