RFR(XS): 8202181: Correctly specify size of hostname buffer in Unix Inet*AddressImpl_getLocalHostName implementations

Fri Apr 27 07:35:04 UTC 2018

Hi all,

thanks for looking into this. Here are a few comments

First of all, there are no real life issues I have seen with this. It is just something that occurred to me when working with the code. But, why not fix it, even it is a corner case that might never happen.

@Thomas: As for the zero termination of the hostname result after the call to gethostname: Yes, we should unconditionally terminate the result, which we do. Unfortunately this part of code cannot be moved outside the solaris #ifdef because the part in the #ifdef contains variable declarations. And you know - the C compatibility issue...

I looked again into the macro definitions for for HOST_NAME_MAX and NI_MAXHOST. HOST_NAME_MAX is mentioned in the gethostname docs ([1] and [2]). Glibc docs indicate it is 64 Byte or 255 Byte. So it looks like it is a quite small buffer, compared to NI_MAXHOST from netdb.h, which is the value that getnameinfo likes to work with, as per [3]. Posix genameinfo doc ([4]) does not mention NI_MAXHOST but Linux doc says it is 1025 which is what we'd define if it is not set.

Taking this input I have updated my webrev to round things up a little bit: http://cr.openjdk.java.net/~clanger/webrevs/8202181.1/ 

I moved the definition of NI_MAXHOST into net_util_md.h and updated the comment a little bit to make clearer why it is there. In Inet4AddressImpl.c and Inet6AddressImpl.c I also fixed the other places where getnameinfo is called to use sizeof(buffer) instead of NI_MAXHOST.

Best regards
Christoph

[1] http://man7.org/linux/man-pages/man2/gethostname.2.html
[2] http://pubs.opengroup.org/onlinepubs/009695399/functions/gethostname.html 
[3] http://man7.org/linux/man-pages/man3/getnameinfo.3.html 
[4] http://pubs.opengroup.org/onlinepubs/9699919799/functions/getnameinfo.html 

> -----Original Message-----
> From: vyom tewari [mailto:vyom.tewari at oracle.com]
> Sent: Freitag, 27. April 2018 08:38
> To: Thomas Stüfe <thomas.stuefe at gmail.com>
> Cc: Langer, Christoph <christoph.langer at sap.com>; net-
> dev at openjdk.java.net
> Subject: Re: RFR(XS): 8202181: Correctly specify size of hostname buffer in
> Unix Inet*AddressImpl_getLocalHostName implementations
> 
> 
> 
> On Friday 27 April 2018 10:58 AM, Thomas Stüfe wrote:
> > On Fri, Apr 27, 2018 at 5:57 AM, vyom tewari <vyom.tewari at oracle.com>
> wrote:
> >> Hi Christoph,
> >>
> >>
> >> On Tuesday 24 April 2018 04:45 PM, Langer, Christoph wrote:
> >>
> >> Hi Vyom,
> >>
> >> I think, it is intentional to handle case where return "hostname" is to
> >> large to
> >> fit in  array.  if you see the man page(http://man7.org/linux/man-
> >> pages/man2/gethostname.2.html) it says that it is unspecified whether
> >> returned buffer includes a terminating null byte.
> >>
> >> current code will put null in case of large "hostname", What do you think ?
> >>
> >> yes, I had read the man page and saw this point of the spec. But exactly
> for
> >> this purpose there's this code:
> >>
> >> // make sure string is null-terminated
> >> hostname[NI_MAXHOST] = '\0';
> >>
> >> If we only hand 'NI_MAXHOST' as size value into gethostname, then the
> >> function might only write NI_MAXHOST - 1 characters of the hostname
> into the
> >> buffer.
> >>
> >> doc says it will copy len bytes into buffer and will not copy null character
> >> into the buffer.
> >>
> >> ################################
> >>
> >> C library/kernel differences
> >>         The GNU C library does not employ the gethostname() system call;
> >>         instead, it implements gethostname() as a library function that calls
> >>         uname(2) and copies up to len bytes from the returned nodename
> field
> >>         into name.  Having performed the copy, the function then checks if
> >>         the length of the nodename was greater than or equal to len, and if
> >>         it is, then the function returns -1 with errno set to ENAMETOOLONG;
> >>         in this case, a terminating null byte is not included in the returned
> >>         name.
> >>
> ##########################################################
> ##
> >>
> > This is shared code, so we should refer to Posix, not linux specific man
> pages.
> >
> >
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/gethostname
> .html
> >
> > <quote>
> >
> > DESCRIPTION
> >
> > The gethostname() function shall return the standard host name for the
> > current machine. The namelen argument shall specify the size of the
> > array pointed to by the name argument. The returned name shall be
> > null-terminated, except that if namelen is an insufficient length to
> > hold the host name, then the returned name shall be truncated and it
> > is unspecified whether the returned name is null-terminated.
> >
> > Host names are limited to {HOST_NAME_MAX} bytes.
> >
> > RETURN VALUE
> >
> > Upon successful completion, 0 shall be returned; otherwise, -1 shall
> > be returned.
> >
> > </quote>
> >
> > Note that there is no indication what happens if the buffer is too
> > small. It may zero-terminate, it may not. It may return an error, it
> > may not. Decision is left to the platform implementors.
> >
> > So from that, I would pass in a large-enough buffer and always
> > zero-terminate on success. According to Posix, a large-enough buffer
> > means HOST_NAME_MAX bytes.
> >
> > I do not understand why we use NI_MAXHOST instead (and we we define
> it
> > to an arbitrary 1025 byte if undefined). Were there platforms where
> > HOST_NAME_MAX was too short? If yes, one should at least check that
> > NI_MAXHOST >= HOST_NAME_MAX.
> Even i noticed this, why we use our own NI_MAXHOST instead
> HOST_NAME_MAX ?
> >> Just for curiosity, are we facing any issues with the current code ?. Your
> >> code change looks logical but if nothing is broken then why to change
> code.
> >>
> > If it can be proven by looking at the API description that it is
> > broken for some corner case, why keep it broken?
>   :) Agreed, as i said Christoph change is logically correct  but i
> don't know the history behind current code, so just wanted to be sure
> that  we are not missing any corner case.
> 
> Thanks,
> Vyom
> 
> >
> > Thanks, Thomas
> >
> >> Thanks,
> >> Vyom
> >>
> >> Best regards
> >> Christoph
> >>
> >>