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

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Message-ID: <BLU436-SMTP114B39F2B3546650AB5BCCB94EA0@phx.gbl>
Date:	Sun, 26 Apr 2015 21:05:39 +0800
From:	Minfei Huang <minfei.huang@...mail.com>
To:	Miroslav Benes <mbenes@...e.cz>
CC:	Minfei Huang <mhuang@...hat.com>, Petr Mladek <pmladek@...e.cz>,
	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 04/15/15 at 01:58P, Miroslav Benes wrote:
> On Wed, 15 Apr 2015, Minfei Huang wrote:
> 
> > On 04/15/15 at 10:30P, Miroslav Benes wrote:
> > > On Wed, 15 Apr 2015, Minfei Huang wrote:
> > > 
> > > > 
> > > > Yes, the function name can be changed, before the extra module is
> > > > installed to the production system.
> > > > 
> > > > We discuss around and around, there are still some confusion with it.
> > > > 
> > > > 1) How does end user know that livepatch can _not_ support the function
> > > >     which length is larger than 127. We can not enforce the end user
> > > >     to know the livepatch and kallsyms code in detail.
> > > > 2) How does end user use livepatch to patch running extra module, once
> > > >     the module is running in the production system, if the function name
> > > >     is insane.
> > > > 3) The error message is ambiguity, if we try to patch the overlength
> > > >     function. We can give the error message clearly, once the function
> > > >     name is overlength.
> > > > 
> > > > I think it is better that we can take more time on the people who will
> > > > use livepatch frequently.
> > > 
> > > Just my two cents, even if we admit that such change is worth it (and I 
> > > am still not convinced that it is the case), I think it would make sense 
> > > to fix it somewhere in kallsyms as Josh proposed. I suspect that when 
> > 
> > Ohhh...
> > 
> > Fixing kallsyms to restrict the function name length maybe is not a good
> > idea. I have no idea how we should do this, except for the coding
> > problems.
> 
> Well we do it now via scripts/kallsyms.c when vmlinux is built. Try it. We 
> apparently do not do it when kernel modules are built out of the tree (as 
> you demonstrated before). So the question is whether we should do it also 
> there. That is one thing we try to tell you.
> 
> The other one is that 128 characters long function names are insane. 
> Probably that is what KSYM_NAME_LEN is for in the first place. Maybe you 
> could even try to add the check to checkpatch.pl.
> 
> > > function names longer than KSYM_NAME_LEN were common there would be many 
> > > similar problems elsewhere in the kernel.
> > > 
> > > That is you can prepare a patch to kallsyms and submit it there. Not sure 
> > > who is the maintainer but he might have an opinion about this...
> > > 
> > > Thanks,
> > > Miroslav
> > 
> > Hold on, I get a scenario that livepatch may do fatal error. I am fine
> > that livepatch do not support overlength function name, because it can
> > not corrupt the kernel.
> > 
> > Once there is a function name A is larger than 127, and another function
> > name B is as longer as 127, it is disaster that we want to patch
> > function B, if function name of first 127 is same between A and B.
> 
> True, but see above.
> 
> > Livepatch may find the function of A to patch it. So this patch(2/2) may
> > be needed to fix the issue.
> 
> Hm, but this patch is not the solution for the issue, or is it? You would 
> check only those first KSYM_NAME_LEN characters, but that would not 
> differentiate between A and B. Or maybe I do not follow.
> 

Hello, guys.

Do I need to post a patch to fix the above issue? Applied following
patch, livepatch will fail to patch the patch, since there are more than
two symbols to be matched.
If so, I would like to post an official patch to the maillist.

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 284e269..67b237f 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -152,7 +152,7 @@ static int klp_find_callback(void *data, const char *name,
 	if ((mod && !args->objname) || (!mod && args->objname))
 		return 0;
 
-	if (strcmp(args->name, name))
+	if (strncmp(args->name, name, KSYM_NAME_LEN-1))
 		return 0;
 
 	if (args->objname && strcmp(args->objname, mod->name))


Thanks
Minfei


> Thanks
> Miroslav
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