Hi everyone
I am trying to understand if its possible to add functions dynamically
to the kernel source based on the presence of certain modules in the
running kernel image.
I did try what brian suggested with the function pointer and yeah it
does work. But I could not understand what peter was trying to say about
modular users since I suppose he mentioned one module (B in this case)
using a function pointer defined in (or by) module A. In my case, since
it is the kernel that is gonna use the function, I need to make sure
that the module doesn't get unloaded while we are using the function.
By the way Jan I couldnt find bridge.c although there is an object file
called bridge.o within /net/bridge. Nonetheless, I did check
nf_nat_ftp.c as an example to see if something similar is being done
there too. What I did find out is that nf_nat_ftp_hook is rcu assigned
in nf_nat_ftp.c however this function pointer is being used with
nf_contrack_ftp.c (im talkin 2.6.27.45 here). In this case, it still is
module1 <- module2 and not kernel <- module. Could you please elaborate
a little how it affects the latter case. Or perhaps u were commenting on
the former case since peter brought that to our notice.
thanks again for your replies and keen to hear your input
Regards,
Aijaz Baig
>
>
> ---------- Forwarded message ----------
> From: Brian Gerst <[email protected]>
> Date: Tue, Aug 24, 2010 at 4:46 PM
> Subject: Re: help needed with EXPORT_SYMBOL
> To: [email protected]
>
>
>
> On Tue, Aug 24, 2010 at 12:44 AM, Aijaz Baig <[email protected]>
> wrote:
> > Hello everyone,
> >
> > thanks for sending in the replies with so many options. I am still a
> > relative newbie so please do bear with me guys if I sound a bit
> naive at
> > times.
> >
> > I'll first start with brian's suggestion. He said:
> > "Use an exported function pointer in the main kernel as a hook that
> the
> > module sets when it is loaded. Note, you must use module_get and
> > module_put around the call to the module to prevent it from
> unloading
> > while in use."
> >
> > I have never used such a technique and do not understand what you
> mean.
> > Could you please elaborate a little. Do you mean I should NF_HOOK
> within
> > the kernel code and then make the function that I would like to
> execute
> > as the hookfn? Well..if this is what u meant then wouldn't it also
> cause
> > all hooks at that particular position to be executed as well?
> >
> > And then peter and finally jan came up with, perhaps better
> suggestions
> > (i really cannot judge the quality of any of these suggestions..m
> too
> > much of a noob to do that). I got confused in the XY explanation by
> jan
> > since there both X and Y are modules it seems and I was talking
> about a
> > module exporting a certain symbol to be used by the kernel. Peter
> did
> > suggest notification chains. Is there a simple example that I can
> look
> > at? Will it allow the kernel to see symbols defined within the
> module?
> >
> > Lastly, will I be able to use kallsyms_lookup() to do what I can? I
> > notice that it replaced EXPORT_SYMTAB sometime back. So if yes I can
> use
> > it, does anyone know how to fill in the very first parameter of this
> > function which says address (of what exactly and how am I supposed
> to
> > know this address?)
> >
> > Keen to hear from you and once again, thank you for your inputs,
>
>
> You still haven't explained why you want to lookup a symbol in a
> module from the main kernel. There is probably a better way of doing
> it.
>
> --
> Brian Gerst
>
>
On Wednesday 2010-08-25 11:27, Aijaz Baig wrote:
>By the way Jan I couldnt find bridge.c although there is an object file
>called bridge.o within /net/bridge.
$ git grep -e br_handle_frame_hook v2.6.35 net/bridge/
See that, which is used kernel<-module. This frame hook has been changed
in 2.6.36-rc1 which is why looking at 2.6.35 would be the short route.
On Wed, 2010-08-25 at 14:57 +0530, Aijaz Baig wrote:
> I am trying to understand if its possible to add functions dynamically
> to the kernel source based on the presence of certain modules in the
> running kernel image.
>
> I did try what brian suggested with the function pointer and yeah it
> does work. But I could not understand what peter was trying to say about
> modular users since I suppose he mentioned one module (B in this case)
> using a function pointer defined in (or by) module A. In my case, since
> it is the kernel that is gonna use the function, I need to make sure
> that the module doesn't get unloaded while we are using the function.
Right, so there's two problems there:
- the exposed function pointer
- unload serialization
The problem with the exposed function pointer is that two competing
modules can set the function pointer:
extern void (*ptr)(void);
static void (*old_ptr)(void);
void func(void)
{
/* do something */
}
module_init()
{
old_ptr = ptr;
ptr = func;
}
module_exit()
{
ptr = old_ptr;
synchronize_stuff();
}
Now suppose two modules A and B both have that template, then load
module A, at that time ptr == A::func, A::old_ptr = NULL, right?
Then load B, then ptr == B:func, B::old_ptr = A::func.
then unload A, then ptr == NULL; /* hey where's B gone? */
Suppose module_exit() had read:
module_exit()
{
if (ptr == func)
ptr = old_ptr;
}
Then after A got unloaded, you'd have: ptr = B::func, B::old_ptr ==
A:func, and the world goes *bang* when you unload B.
If you'd have exposed the thing like:
static void (*callback)(void);
int set_foo_callback(void (*func)(void))
{
if (callback && func)
return -EBUSY;
callback = func;
synchronize_world();
return 0;
}
EXPORT_SYMBOL_GPL(set_foo_callback);
Then there'd be no confusion, as loading B while A was already loaded
would fail since set_foo_callback() would fail with -EBUSY.
Now the synchronization issue, above represented by synchronize_world();
that will have to do something that ensures all current users of the ptr
have gone, taking a reference on the module will likely result in an
pinned module, since what will drop the ref again?
Module unload does a fairly heavy stop-machine thing, which might be
sufficient for some, but a sensible solution really depends on the
problem domain, RCU could be used in various ways.
On Wed, 25 Aug 2010 14:57:58 +0530 Aijaz Baig wrote:
> By the way Jan I couldnt find bridge.c although there is an object file
> called bridge.o within /net/bridge. Nonetheless, I did check
See net/bridge/Makefile:
obj-$(CONFIG_BRIDGE) += bridge.o
bridge-y := br.o br_device.o br_fdb.o br_forward.o br_if.o br_input.o \
br_ioctl.o br_notify.o br_stp.o br_stp_bpdu.o \
br_stp_if.o br_stp_timer.o br_netlink.o
and see Documentation/kbuild/makefiles.txt.
---
~Randy
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