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

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Date:	Wed, 15 Apr 2015 12:24:29 -0400
From:	Justin Keller <justincompsci@...il.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Minfei Huang <minfei.huang@...mail.com>,
	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

I agree that 128+ function names are bad and there is no need for such
long names, epically for compatibility with 80 char/line displays. I
cannot think of an example even near as long as 127 characters. Are
there even such long function names anywhere in the kernel?

Justin

On Wed, Apr 15, 2015 at 7:58 AM, Miroslav Benes <mbenes@...e.cz> 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.
>
> Thanks
> Miroslav
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