linux-kernel - Re: [PATCH 2/2] livepatch: Fix the bug if the function name is larger than KSYM_NAME

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Date:	Tue, 14 Apr 2015 20:41:21 +0200
From:	Petr Mladek <pmladek@...e.cz>
To:	Minfei Huang <mhuang@...hat.com>
Cc:	Minfei Huang <minfei.huang@...mail.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>, sjenning@...hat.com,
	jkosina@...e.cz, vojtech@...e.cz, live-patching@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] livepatch: Fix the bug if the function name is
 larger than KSYM_NAME_LEN-1

On Wed 2015-04-15 01:01:39, Minfei Huang wrote:
> On 04/14/15 at 06:27pm, Petr Mladek wrote:
> > On Tue 2015-04-14 23:55:36, Minfei Huang wrote:
> > > On 04/14/15 at 10:11P, Josh Poimboeuf wrote:
> > > > On Tue, Apr 14, 2015 at 01:45:49PM +0800, Minfei Huang wrote:
> > > > > On 04/14/15 at 12:32P, Josh Poimboeuf wrote:
> > > > > > On Tue, Apr 14, 2015 at 01:29:50PM +0800, Minfei Huang wrote:
> > > > > > > 
> > > > > > > For end user, they may know litter about restriction of kallsyms and
> > > > > > > livepatch. How can they know the restriction that function name is
> > > > > > > limited to 127?
> > > > > > 
> > > > > > As I mentioned above, I think kallsyms.c should fail the build if it
> > > > > > encounters a symbol longer than KSYM_NAME_LEN.
> > > > > > 
> > > > > 
> > > > > I dont think it is a good idea to handle this case like that. The
> > > > > function name is only for human recognization. Why the compiler fails
> > > > > to build it?
> > > > 
> > > > Well, the function name isn't only for human recognition.  kpatch-build
> > > > generates patch modules automatically.  It assumes that the compiled
> > > > function name matches the kallsyms name.  And I'd guess that a lot of
> > > > other code (both in-kernel and user space tools) make the same
> > > > assumption.
> > > > 
> > > > Not to mention that most humans would also make the same assumption...
> > > 
> > > Yes. The assumption is correct for most case.
> > > 
> > > It is significance for livepatch to support extra module, because in my
> > > opinion kernel is more stable than the third module.
> > > 
> > > So it is more important, if the livepatch can patch all sorts of patch.
> > > For dynamic function name, I think it is simple to avoid it.
> > 
> > Do you have some really existing module with such a crazy long
> > function names or is this debate pure theoretical, please?
> > 
> 
> No, I do not have such running module which function name is exceed to
> 127.
> 
> Again, we can not predict what end user do to name the function name. I
> think the overlength function name is valid for linux kernel, if the
> module can be installed.

My position on this is that using >127 length function names is
insane. I would be scared to use such a module on a production system.
If we refuse patching, we actually do a favor for the user.
Instead of fixing live patch for such a scenario, we should suggest
the user to use more trustful modules.

Best Regards,
Petr
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