linux-kernel - Re: mmap'ed memory in core files ?

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Message-ID: <20080702105027.GA1111@frolo.macqel>
Date:	Wed, 2 Jul 2008 12:50:27 +0200
From:	Philippe De Muyter <phdm@...qel.be>
To:	Michael Kerrisk <mtk.manpages@...il.com>
Cc:	linux-kernel@...r.kernel.org, libdc1394-devel@...ts.sourceforge.net
Subject: Re: mmap'ed memory in core files ?

Hi Michael,

On Tue, Jul 01, 2008 at 08:16:11PM +0200, Michael Kerrisk wrote:
> On 7/1/08, Philippe De Muyter <phdm@...qel.be> wrote:
> > Hello everybody,
> >
> >  I develop video acquisition software using the video1394 interface.
> >  The images grabbed by the camera and iee1394 bus are kept in kernel
> >  memory and made available to the user program through a mmap call done
> >  in the libdc1394 library :
> >
> >  dma_ring_buffer= mmap(0, vmmap.nb_buffers * vmmap.buf_size,
> >                 PROT_READ|PROT_WRITE,MAP_SHARED, craw->capture.dma_fd, 0);
> >
> >  Sometimes, my program crashes and produces a core file :)  It seems to
> >  me that the core file does not contain the mmap'ed memory and hence
> >  I cannot replay my program with the same image for debugging purpose.
> >
> >  Is it possible to configure the kernel through /proc, or through the mmap
> >  system call to have that mmapped segment in the core file, or do I need
> >  to modify the kernel itself to obtain the behaviour I want ?  If I
> >  need to modify the kernel, can some kind soul provide me some pointers ?
> 
> 
> Have a look at the section "Controlling which mappings are written to
> the core dump" in a recent core.5 man page:
> http://www.kernel.org/doc/man-pages/online/pages/man5/core.5.html

thanks for the info.  I didn't know about /proc/PID/coredump_filter.

that part was promising :

       bit 2  Dump file-backed private mappings.
       bit 3  Dump file-backed shared mappings.

    The default value of coredump_filter is 0x3; this reflects traditional
    Linux behavior and means that only anonymous memory segments are dumped.

Unfortunately, the part that applies to me (I have tested it) is the next one :

    Memory-mapped I/O pages such as frame buffer are never dumped, [...],
    regardless of the coredump_filter value.

Is that a design decision, or a mere finding of the way it is implemented
now ?

So, back to my original question :

Can some kind soul provide me some pointers to the way I should modify
the kernel to make the inclusion of the video1394 mmapped segment in
core files possible ?

best regards

Philippe
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