Re: [PATCH 2/2] livepatch: Fix the bug if the function name is larger than KSYM_NAME_LEN-1

From: Miroslav Benes
Date: Mon Apr 27 2015 - 04:41:54 EST


On Sun, 26 Apr 2015, Minfei Huang wrote:

> 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))

This means that in your scenario described above count would be >0 here
and kallsyms symbol would not be resolved... which is the same situation
as of now without your patch. And you can find this objection above as
well.

I still think this needs to be fixed somewhere else and you can find hints
and points in the thread.

Maybe someone else feels differently and will say so...

Cheers,
Miroslav
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/