threads/79887: [patch] freopen() isn't thread-safe

[David Xu]

The following reply was made to PR threads/79887; it has been noted by GNATS.

From: David Xu <davidxu at freebsd.org>
To: John Baldwin <jhb at freebsd.org>
Cc: bug-followup at freebsd.org, tejblum at yandex-team.ru
Subject: Re: threads/79887: [patch] freopen() isn't thread-safe
Date: Wed, 08 Dec 2010 10:43:35 +0800

 John Baldwin wrote:
 > David,
 > 
 > I think the submitter's analysis is correct that the only place that can set 
 > the close function pointer is funopen() and that for that case (and any other 
 > "fake" files), the file descriptor will be -1.  If the fd is >= 0, then it 
 > must be a file-descriptor-backed FILE, and relying on dup2() to close the fd 
 > is ok.
 > 
 > As the manpage notes, the most common usage is to redirect stderr or stdout by 
 > doing 'freopen("/dev/null", "w", stderr)'.  The bug allows some other random
 > code that is calling open() in another thread to have that open() return 2 
 > during the window where fd '2' is closed during freopen().  That other file 
 > descriptor then gets trounced by the dup2() call in freopen() to point to 
 > something else.
 > 
 > The code likely uses _close() rather than close() directly to be cleaner.  
 > Given that this is stdio, I don't think we are really worried about the 
 > performance impact of one extra wrapper function.
 > 
 > I think the original patch is most likely correct.
 > 
 
 The patch works, I just don't like the design of the 
 (*fp->_close)(fp->_cookie)
 it seems the patch make freopen bypass it.
 I think the patch can be committed, but I am busy and have
 no time to do it by myself.