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References: <CALLhcm4QGY+eP0_CRiSbJwQ12kOetvTK=6-AtC17x7d+QhGKTw AT mail DOT gmail DOT com> <20190603115456 DOT GG3437 AT calimero DOT vinschen DOT de> <CALLhcm4qaha=XuBWEqag7QH1veA82UrB94emFtFMd26dZ_ZGZQ AT mail DOT gmail DOT com> <20190604131836 DOT GS3437 AT calimero DOT vinschen DOT de> <CALLhcm4sHGmFwki+RuUOUA-GfnXzmOUX4Tr8Es-=291Bo+A4dg AT mail DOT gmail DOT com> <20190604144948 DOT GT3437 AT calimero DOT vinschen DOT de>
In-Reply-To: <20190604144948.GT3437@calimero.vinschen.de>
From: Stanislav Kascak <stanislav DOT kascak AT gmail DOT com>
Date: Tue, 4 Jun 2019 18:01:53 +0200
Message-ID: <CALLhcm5rKTVCD3Hb69CzC-94-86-PRpUofA9OEAy_mci1GnHQA@mail.gmail.com>
Subject: Re: possible problem with memory allocation using calloc/mmap/munmap
To: cygwin AT cygwin DOT com, Stanislav Kascak <stanislav DOT kascak AT gmail DOT com>
Content-Type: text/plain; charset="UTF-8"

> > > > > > It seems that when mmap() is called with length argument exceeding
> > > > > > size of file, only memory to fit that file is allocated. munmap()
> > > > > > however frees the full specified length. [...]
> > > > > [...]
> > > > > I know this situation is unsatisfying, but I have no easy workaround
> > > > > to allow this.  Cygwin could add the anonymous mapping on the next
> > > > > 64K boundary on 64 bit, but that would result in a hole in the mapping
> > > > > which seemed like a rather bad idea when porting mmap to 64 bit.
> > > > >
> > > > > Ken's also right that munmap is doing the right thing here.  If
> > > > > anything's wrong, it's mmap's workaround for mappings beyond the file
> > > > > length.  If only 64 bit would allow 4K-aligned mappings :(
> > > >
> > > > Thanks for the answer. It is appreciated.
> > > > I understand the problem and difficulty to resolve it. Maybe returning
> > > > an error from mmap (and putting a comment to code for its reason)
> > > > would be sufficient. mmap caller could just adjust requested
> > > > allocation size to file size. Without error, caller has no way of
> > > > knowing memory was not allocated and segfault is then thrown in an
> > > > unrelated memory segment which makes the root cause hard to track
> > > > down. But, I do not know all the implication that could result from
> > > > that, so evaluation of this approach is up to you.
> > > [...]
> > > Eventually Cygwin adds another mapping to fullfill the entire mapping
> > > request:
> > >
> > >  |-- file 4K --|-- filler 60K --|-- filler 192K --|
> > >
> > > The problem on WOW64 and real 64 bit is that it's impossible to map
> > > the first filler.  However, this area in the VM will *never* be
> > > allocated by other application functions due to the allocation
> > > granularity of 64K!
> > >
> > > So my workaround for 64 bit and WOW64 is to just skip allocating the
> > > first filler:
> > >
> > >  |-- file 4K --|-- THE VOID 60K --|-- filler 192K --|
> > >
> > > The advantage is now that the following munmap of 256K will only
> > > unmap the map for the file and the filler, but not the region you
> > > calloced before, which formerly was accidentally mapped to the
> > > filler region.  This just can't happen anymore now.
> > >
> > > Would that be feasible?  If so I can push my patch and create a
> > > developer snapshot for testing.
> >
> > Two questions arise when I'm thinking about workaround solution:
> > - what happens if caller tries to write to |-- THE VOID 60K --|. Since
> > this is unallocated, would there be a segfault?
>
> Accessing the VOID would raise SIGSEGV, while accessing the filler
> raises SIGBUS.  The latter is also used to implement MAP_NORESERVE,
> which the VOID can't support.

I played around a bit and I can confirm it would be consistent with
current behavior:
memwrite <0 - filesize) - no error, written to file
memwrite <filesize - 4k) - no error, no file change
memwrite <4k, 64k) - SIGSEGV
memwrite <64k, mmap alloc size) - SIGSEGV or another mem alloc
overwrite (depending on whether there is another allocation)
With workaround last line would be fixed to SIGBUS (along with proper
allocation length). I believe this is completely OK.

>
> > - is it possible that some subsequent mem alloc request would return
> > region from |-- THE VOID 60K --| which could again cause segfault
> > after munmap?
>
> No, as stated above.  Allocations are restricted to Windows' 64K
> allocation granularity.

I apologize. I missed that sentence. So, your workaround seems fine.

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