Re: [PATCH] kprobes: bugfix: try_module_get even if calling_mod is NULL

[Masami Hiramatsu <mhiramat@redhat.com>]

Andrew Morton wrote:
> I do not understand this description "Get probed module even if the
> caller is in the kernel core code".
> 
> What bug is being fixed here?  What is the kernel behaviour before and
> after the patch?

When someone called register_*probe() from kernel-core code(not from
module) and that probes a kernel module, users can remove the probed
module because kprobe doesn't increment reference counter of the module.
(on the other hand, if the kernel-module calls register_*probe,
kprobe increments refcount of the probed module.)

Currently, we have no register_*probe() calling from kernel-core(except
smoke-test, but the smoke-test doesn't probe module), so there is no
real bugs. But the logic is wrong(or not fair) and it can causes a
problem when someone might want to probe module from kernel.

After this patch is applied, even if someone put register_*probe() call
in the kernel-core code, it increments the reference counter of the
probed module, and it prevents user to remove the module until stopping
probing it.

> Was the bug present in 2.6.27, 2.6.26 etc?  Or was it a post-2.6.28
> regression?

Hmm, it might be an enhancement, because currently the kernel doesn't
have real bugs.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com