Re: Fork problem with hexchat if cygserver is running

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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On Aug  8 10:24, Corinna Vinschen wrote:
> On Aug  8 09:27, Michael Haubenwallner wrote:
> > On 8/7/19 7:41 PM, Ken Brown wrote:
> > > Roughly 1 out of 3 times that I try to use hexchat, I get a fork failure:
> > > [...]
> > >    363   26064 [main] hexchat 12399 C:\cygwin64\bin\hexchat.exe: *** fatal error 
> > > in forked process - fixup_shms_after_fork: NtMapViewOfSection (0x7FF4EE130000), 
> > > status 0xC0000018.  Terminating.
> > > 
> > > [status 0xC0000018 is STATUS_CONFLICTING_ADDRESSES.]
> > > 
> > > This was under cygwin-3.0.7-1.  It also happens with cygwin1.dll
> > > built from the current master branch, and it also happens with
> > > cygwin-3.0.6-1.  Not being familiar with this part of the Cygwin
> > > code, my first thought was to do a bisection.  But I haven't yet
> > > found a good revision to start with.  I will still try to do that,
> > > but in the meantime I thought I should report it.
> > 
> > I doubt there is a commit that introduces this problem.  Instead, this feels
> > like an address conflict with some (internal) memory allocated for some Windows
> > (or even Cygwin) object.
> > So I'd wonder if early memory reservation like is done for dynamically loaded
> > DLLs may help for SHMs as well.
> 
> That sounds like a good idea for a start, but I don't think so.  The
> interesting thing here is not that it fails, but that it fails with the
> above address:
> 
>   NtMapViewOfSection (0x7FF4EE130000)
>                       ^^^^^^^^^^^^^^
> 
> Note that this address is a 48 bit address, starting with 0x7f.
> 
> Up to the current Cygwin 3.0.7, Cygwin's mmap only uses 44 bit addresses
> below 0x0700:00000000, top-down.  The reason is that older 64 bit
> Windows versions only support a 44 bit address space.  Starting with
> Windows 8.1, Windows supports a 48 bit address space, and Cygwin 3.1
> will support that as well, by utilizing the address space top-down
> from 0x7000:00000000.
> 
> However, the above address is beyond even that, in the middle of the
> address space utilized by Windows itself.  Combine that with ASLR...
> 
> Given that, my guess is that we're actually running into the
> problem that the shmat() call does not utilize the mmap address
> allocator, so the chosen address has a high probability to collide
> with Windows' own stuff.
> 
> IMHO, the fix would be to make the mmap_alloc object global, so it
> can be utilized by shmat() to create more sane addresses.
> 
> Does that make sense?

I've sent a preliminary (untested) patch to the cygwin-patches ML.
Can you two please review it?

Ken, can you also test it in your scenario, please?


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer

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