linux-kernel - Re: [PATCH] livepatch: add module locking around kallsyms calls

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Date:	Tue, 2 Jun 2015 11:15:47 +0200 (CEST)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Minfei Huang <mnfhuang@...il.com>
cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	"sjenning@...hat.com" <sjenning@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Vojtěch Pavlík <vojtech@...e.cz>,
	"live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	pmladek@...e.cz
Subject: Re: [PATCH] livepatch: add module locking around kallsyms calls

On Tue, 2 Jun 2015, Minfei Huang wrote:

> On Mon, Jun 1, 2015 at 11:48 PM, Miroslav Benes <mbenes@...e.cz> wrote:
> > The list of loaded modules is walked through in
> > module_kallsyms_on_each_symbol (called by kallsyms_on_each_symbol). The
> > module_mutex lock should be acquired to prevent potential corruptions
> > in the list.
> >
> > This was uncovered with new lockdep asserts in module code introduced by
> > the commit 0be964be0d45 ("module: Sanitize RCU usage and locking") in
> > recent next- trees.
> >
> > Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> > ---
> >  kernel/livepatch/core.c | 18 +++++++++++++-----
> >  1 file changed, 13 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> > index e6c8d54..c40ebcc 100644
> > --- a/kernel/livepatch/core.c
> > +++ b/kernel/livepatch/core.c
> > @@ -179,7 +179,9 @@ static int klp_find_object_symbol(const char *objname, const char *name,
> >                 .count = 0
> >         };
> >
> > +       mutex_lock(&module_mutex);
> >         kallsyms_on_each_symbol(klp_find_callback, &args);
> > +       mutex_unlock(&module_mutex);
> >
> >         if (args.count == 0)
> >                 pr_err("symbol '%s' not found in symbol table\n", name);
> > @@ -219,13 +221,19 @@ static int klp_verify_vmlinux_symbol(const char *name, unsigned long addr)
> >                 .name = name,
> >                 .addr = addr,
> >         };
> > +       int ret;
> >
> > -       if (kallsyms_on_each_symbol(klp_verify_callback, &args))
> > -               return 0;
> > +       mutex_lock(&module_mutex);
> > +       ret = kallsyms_on_each_symbol(klp_verify_callback, &args);
> > +       mutex_unlock(&module_mutex);
> >
> 
> Hi.
> In livepatch code path, returning value 0 may represent the right, but
> sometime represent wrong, like the above function.
> 
> Is it possible that we can wrap such function and return the unified
> value? Thus we can not confuse the returning value any more.

Hi,

I must admit I do not understand. Both klp_find_object_symbol and 
klp_verify_vmlinux_symbol return 0 on success or -EINVAL. It is true that 
kallsyms_on_each_symbol and module_kallsyms_on_each symbol are different. 
That is why our kallsyms callbacks are different. See the implementation 
of those. But that is the API. Is this what you are worried about?

> Otherwise annotation is appreciate.

Thanks,
Miroslav
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