Date: Wed, 27 Apr 2016 14:17:45 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Removing stupid, spurious UB in stdio (bikeshed time) 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<NULL, and while this should > > obviously be false (since < implies !=), NULL<NULL is actually UB. > > [snip] > > So what to do? > > Well, since NULL-NULL and NULL<NULL are well-defined in C++, ... ;) Ha ha. > 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
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