Re: [BUG] mprotect() on Windows NT 5+

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Jul 15 18:19, Xavier Joubert wrote:
> Hello dear Cygwin developpers !
> 
> 
> This is my first post here, so I would like to begin by sending you a big thank 
> for Cygwin ! This is a great tool to port programs to Windows.

Thanks :-)

> I think I found a bug in mprotect() implementation. This call is unable to set 
> some protections (PROT_READ|PROT_WRITE for example) on a given memory area, 
> while it can set some others (PROT_NONE or PROT_READ for example) on the same 
> memory area. From my tests, it seems to happen only on WNT5+ (ie. Windows 2000 
> and Windows XP - detailed results below).

Thanks for the helpful testcase.

The cause is a limitation in newer Windows NT versions, which make sense. 
Well, sort of.  The protection modes PAGE_READWRITE and PAGE_WRITECOPY are
mutually exlusive, which is enforced in calls to VirtualProtect since W2K.

Since your example uses MAP_PRIVATE, which Cygwin maps to PAGE_WRITECOPY,
trying to protect with MAP_WRITE, which internally maps to PAGE_READWRTE,
fails on W2K and later.  I've checked in a fix, so that mprotect tests for
the original protection mode of the first page in the area, and uses
READWRITE or WRITECOPY, whichever matches the original protection.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Co-Project Leader          mailto:cygwin@cygwin.com
Red Hat, Inc.

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