linux-kernel - Re: [PATCH v3 0/3] pstore: add pstore unregister

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Date:	Tue, 20 Oct 2015 21:11:48 -0700
From:	Geliang Tang <geliangtang@....com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Anton Vorontsov <anton@...msg.org>,
	Colin Cross <ccross@...roid.com>,
	Tony Luck <tony.luck@...el.com>,
	Geliang Tang <geliangtang@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/3] pstore: add pstore unregister

On Tue, Oct 20, 2015 at 08:01:20PM -0700, Kees Cook wrote:
> On Tue, Oct 20, 2015 at 7:52 PM, Geliang Tang <geliangtang@....com> wrote:
> > On Tue, Oct 20, 2015 at 10:19:09AM -0700, Kees Cook wrote:
> >> On Tue, Oct 20, 2015 at 12:39 AM, Geliang Tang <geliangtang@....com> wrote:
> >> > On Mon, Oct 19, 2015 at 10:56:54PM +0000, Luck, Tony wrote:
> >> >> Thanks for looking to close out this TODO item.
> >> >>
> >> >> The thing that scared me about unloading pstore was what happens to
> >> >> a process that is in the middle of reading some /sys/fs/pstore/file-name-here
> >>
> >> Were you able to verify that this reading-while-rmmod case works correctly?
> >>
> >> -Kees
> >
> > $ sudo insmod zlib_deflate.ko
> > $ sudo insmod pstore.ko
> > $ lsmod
> > Module                  Size  Used by
> > pstore                 11225  0
> > zlib_deflate           18292  1 pstore
> >
> > $ sudo mount -t pstore pstore /sys/fs/pstore
> > $ lsmod
> > Module                  Size  Used by
> > pstore                 11225  1
> > zlib_deflate           18292  1 pstore
> >
> > $ sudo insmod reed_solomon.ko
> > $ sudo insmod ramoops.ko mem_address=0x40000000 mem_size=0x400000
> > $ lsmod
> > Module                  Size  Used by
> > ramoops                 9638  0
> > reed_solomon            5150  1 ramoops
> > pstore                 11225  2 ramoops
> > zlib_deflate           18292  1 pstore
> >
> > $ tail -f /sys/fs/pstore/console-ramoops-0 &
> > [1] 3483
> > $ lsmod
> > Module                  Size  Used by
> > ramoops                 9638  0
> > reed_solomon            5150  1 ramoops
> > pstore                 11225  3 ramoops
> > zlib_deflate           18292  1 pstore
> >
> > $ kill -9 3483
> > $ lsmod
> > Module                  Size  Used by
> > ramoops                 9638  0
> > reed_solomon            5150  1 ramoops
> > pstore                 11225  2 ramoops
> > zlib_deflate           18292  1 pstore
> >
> > $ sudo rmmod ramoops
> > $ lsmod
> > Module                  Size  Used by
> > reed_solomon            5150  0
> > pstore                 11225  1
> > zlib_deflate           18292  1 pstore
> 
> What happens if you leave the tail running and try to rmmod ramoops?
> (I assume it'll just refuse.)
> 

It'll refuse if we try to unload pstore module. But we can unload
ramoops module. Because we only increase a reference count of pstore
module when the file is opened. We didn't increase ramoops module's
reference count.

Thanks.
Geliang Tang

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