linux-kernel - Re: [PATCH v5 2/3] KASLR: Handle memory limit specified by memmap and mem option

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Message-ID: <20170516023719.GB29117@x1>
Date:   Tue, 16 May 2017 10:37:19 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Dou Liyang <douly.fnst@...fujitsu.com>
Cc:     tglx@...utronix.de, keescook@...omium.org, mingo@...nel.org,
        m.mizuma@...fujitsu.com, linux-kernel@...r.kernel.org,
        dyoung@...hat.com, dan.j.williams@...el.com, hpa@...or.com,
        x86@...nel.org
Subject: Re: [PATCH v5 2/3] KASLR: Handle memory limit specified by memmap
 and mem option

On 05/16/17 at 09:43am, Dou Liyang wrote:
> 
> 
> At 05/16/2017 09:12 AM, Baoquan He wrote:
> > On 05/16/17 at 08:56am, Dou Liyang wrote:
> > > Hi Baoquan,
> > > 
> > > At 05/13/2017 01:46 PM, Baoquan He wrote:
> > > > Option mem= will limit the max address a system can use and any memory
> > > > region above the limit will be removed.
> > > > 
> > > > Furthermore, memmap=nn[KMG] which has no offset specified has the same
> > > > behaviour as mem=.
> > > > 
> > > > KASLR needs to consider this when choosing the random position for
> > > > decompressing the kernel. Do it now.
> > > > 
> > > > Signed-off-by: Baoquan He <bhe@...hat.com>
> > > > Tested-by: Masayoshi Mizuma <m.mizuma@...fujitsu.com>
> > > > ---
> > > >  arch/x86/boot/compressed/kaslr.c | 68 +++++++++++++++++++++++++++++-----------
> > > >  1 file changed, 50 insertions(+), 18 deletions(-)
> > > > 
> > > >  	return -EINVAL;
> > > > @@ -173,9 +184,14 @@ static void mem_avoid_memmap(char *str)
> > > >  		if (rc < 0)
> > > >  			break;
> > > >  		str = k;
> > > > -		/* A usable region that should not be skipped */
> > > > -		if (size == 0)
> > > > +
> > > > +		if (start == 0) {
> > > > +			/* Store the specified memory limit if size > 0 */
> > > > +			if (size > 0)
> > > > +				mem_limit = size;
> > > 
> > > Baoquan,
> > > 
> > > I am not sure about setting the value of mem_limit to mem_size directly.
> > > 
> > > If the command line has both the "memmap" and "mem", such as
> > >  ... mem=2G memmap=4G ...
> > > 
> > > ...in that code, the mem_limit may be 4G not 2G.
> > 
> > No, could you tell why you want to add both "memmap=nnKMG" and "mem=" at
> > the same time? As you sid, what if I add "mem=4G mem=2G mem=1G"?
> 
> Just for testing :)
> 
> Ok, thanks, I see. We should be responsible for our command line. don't
> need to consider with these situations in kernel.

Thanks for testing and looking into this patchset, Liyang. I understand
your worry, this patch is trying to have the same behaviour as in
arch/x86/kernel/e820.c. Seems the liability of specifying them right lays
on users' shoulder.

Thanks
Baoquan