From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/9959 Path: news.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: Removing stupid, spurious UB in stdio (bikeshed time) Date: Wed, 27 Apr 2016 14:17:45 -0400 Message-ID: <20160427181745.GT21636@brightrain.aerifal.cx> References: <20160426221815.GA24105@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1461781084 6817 80.91.229.3 (27 Apr 2016 18:18:04 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 27 Apr 2016 18:18:04 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-9972-gllmg-musl=m.gmane.org@lists.openwall.com Wed Apr 27 20:18:04 2016 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1avU2F-00050h-Pm for gllmg-musl@m.gmane.org; Wed, 27 Apr 2016 20:18:04 +0200 Original-Received: (qmail 3782 invoked by uid 550); 27 Apr 2016 18:18:00 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 3761 invoked from network); 27 Apr 2016 18:17:59 -0000 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:9959 Archived-At: On Wed, Apr 27, 2016 at 10:56:25AM +0300, Alexander Monakov wrote: > On Tue, 26 Apr 2016, Rich Felker wrote: > > There's a lot of nonsense-UB in stdio due to buffer comparisons along > > the lines of "f->rpos < f->rend". The intent of these comparisons is > > to simultaneously check that the buffer is initialized for the proper > > mode (read or write) and that there's data left in it (for reading) or > > space left (to write) or buffered data to be written out (for write), > > etc. > > > > Unfortunately, when the buffer is uninitialized for the mode being > > checked, the comparison becomes NULL > obviously be false (since < implies !=), NULL > [snip] > > So what to do? > > Well, since NULL-NULL and NULL Sorry that I don't offer a more substantial comment; let me just chime in > on the point that a writeup documenting stdio design, like you say, OK. > > I think a good place to start might be coming up with and documenting a > > clear model for how stdio's buffer internals are supposed to work, what > > operations are allowed, what invariants hold, etc. based on the above > > analysis of current UB issues and what the code is doing. > > would be nice to have; you recently noted that setvbuf has restrictions, > and if there are other non-obvious stuff (especially if musl-specific), > having it written down should be useful. Are you talking about the C-standard-imposed restriction that you can only use setvbuf as the first operation on a new FILE? Or something else I said that I'm not remembering? I was thinking more about musl's internally-imposed contracts on internal code (users of the buffer pointers). Of course external contracts for the stdio API have a role in determining what the internal interfaces need to be capable of. Rich