lkml.org 
[lkml]   [2010]   [Sep]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 03/11] jump label: Base patch for jump label
On Thu, Sep 23, 2010 at 10:37:58AM -0400, Mathieu Desnoyers wrote:
> * Steven Rostedt (rostedt@goodmis.org) wrote:
> > From: Jason Baron <jbaron@redhat.com>
> >
> > base patch to implement 'jump labeling'. Based on a new 'asm goto' inline
> > assembly gcc mechanism, we can now branch to labels from an 'asm goto'
> > statment. This allows us to create a 'no-op' fastpath, which can subsequently
> > be patched with a jump to the slowpath code. This is useful for code which
> > might be rarely used, but which we'd like to be able to call, if needed.
> > Tracepoints are the current usecase that these are being implemented for.
> >
> [...]
> > +/***
> > + * jump_label_update - update jump label text
> > + * @key - key value associated with a a jump label
> > + * @type - enum set to JUMP_LABEL_ENABLE or JUMP_LABEL_DISABLE
> > + *
> > + * Will enable/disable the jump for jump label @key, depending on the
> > + * value of @type.
> > + *
> > + */
> > +
> > +void jump_label_update(unsigned long key, enum jump_label_type type)
> > +{
> > + struct jump_entry *iter;
> > + struct jump_label_entry *entry;
> > + struct hlist_node *module_node;
> > + struct jump_label_module_entry *e_module;
> > + int count;
> > +
> > + mutex_lock(&jump_label_mutex);
> > + entry = get_jump_label_entry((jump_label_t)key);
> > + if (entry) {
> > + count = entry->nr_entries;
> > + iter = entry->table;
> > + while (count--) {
> > + if (kernel_text_address(iter->code))
>
> As I pointed out in another thread, I'm concerned about the use of
> kernel_text_address without module mutex here. kernel_text_address calls
> is_module_text_address(), which calls __module_text_address() with
> preemption off.
>
> __module_text_address() looks like:
>
> struct module *__module_address(unsigned long addr)
> {
> struct module *mod;
>
> if (addr < module_addr_min || addr > module_addr_max)
> return NULL;
>
> list_for_each_entry_rcu(mod, &modules, list)
> if (within_module_core(addr, mod)
> || within_module_init(addr, mod))
> return mod;
> return NULL;
> }
>
> struct module *__module_text_address(unsigned long addr)
> {
> struct module *mod = __module_address(addr);
> if (mod) {
> /* Make sure it's within the text section. */
> if (!within(addr, mod->module_init, mod->init_text_size)
> && !within(addr, mod->module_core, mod->core_text_size))
> mod = NULL;
> }
> return mod;
> }
>
> So the test for the address being in the module core is already
> problematic, since we hold preempt off only within
> is_module_text_address(). The is_module_text_address() caller is then
> free to write to this address even after the module has been unloaded
> and the module unload grace period ended.
>
> Even worse, such grace period is not waited for at module load time
> within:
>
> init_module()
> module_free(mod, mod->module_init);
> mod->module_init = NULL;
> mod->init_size = 0;
> mod->init_text_size = 0;
> (done with module_mutex held, while the module is already in the
> module list)
>
> We'd probably have to hold the module mutex around the
> is_module_text_address() call and address use (which can be a pain), or
> to correctly address this part of init_module() with RCU and require
> that preempt off is held across both __module_text_address() call site
> and the actual use of that pointer (which does not fit with jump label,
> which need to sleep, so we'd have to move module.c to a preemptable
> rcu_read_lock/synchronize_rcu() C.S.).
>
> Thoughts ?
>

I was thinking about the rcu_read_lock/synchronize_rcu() for this race.
We can hold the rcu_read_lock() across the is_module_text_address()
check in the jump label code, and then we can do in module.c:

mod->module_init = NULL;
synchronize_rcu();
module_free(mod, mod->module_init);
.
.
.

or we could push the rcu_read_lock() further down into
is_module_address()?

thanks,

-Jason




\
 
 \ /
  Last update: 2010-09-27 20:09    [from the cache]
©2003-2011 Jasper Spaans