linux-kernel - Re: [PATCH v7 0/5] Add Intel IOMMU debugfs support

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Date:   Mon, 19 Mar 2018 09:37:14 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     "Raj, Ashok" <ashok.raj@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Gayatri Kammela <gayatri.kammela@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v7 0/5] Add Intel IOMMU debugfs support

On Thu, 15 Mar 2018 14:18:54 +0100
Joerg Roedel <joro@...tes.org> wrote:

> On Thu, Feb 15, 2018 at 08:38:11AM -0800, Jacob Pan wrote:
> > Just wondering if your concern is on the implementation or the
> > debugfs idea in general. Perhaps have some common IOMMU debugfs?  
> 
> My concern mainly is that we add interfaces which reveal
> potentially security relevant information
I don;t think security is any worse than existing kernel page table in
debugfs. i.e. /sys/kernel/debug/page_tables
This is a debug feature.
> to user-space and that tools
> come up using it so that this also becomes kABI and we can't easily
> change it anymore and this whole stuff turns into a maintence
> nightmare.
> 
Agreed, perhaps we can address that by only dumping user readable data
which avoid having a parser tool that relies on stable kABI?

> So that is definitly not something I'd like to see enabled in the
> distros, and its better to avoid it at all and search for better ways
> to debug upcoming issues.
> 
We can make it "def_bool n" so only used by advanced customers who can
recompile kernel.
> BPF tracers and tracing in general comes to mind here...
> 
my concern is that tracing is suitable for dynamic debugging, but these
context info are mostly static. Perhaps I am missing some tracing
features.

Thanks,

Jacob
> 
> 	Joerg
> 

[Jacob Pan]