On Thu, Mar 15, 2007 at 05:53:04PM -0800, Andrew Morton wrote:
> My, what a lot of code you have here. I note that nobody can be assed even
> reviewing it. Now why is that?
I hope, Al could find some time again.
> On Sun, 11 Mar 2007 20:04:56 +0300 Alexey Dobriyan <[EMAIL PROTECTED]> wrote:
> > Fix following races:
> > ===========================================
> > 1. Write via ->write_proc sleeps in copy_from_user(). Module disappears
> > meanwhile. Or, more generically, system call done on /proc file, method
> > supplied by module is called, module dissapeares meanwhile.
> >
> > pde = create_proc_entry()
> > if (!pde)
> > return -ENOMEM;
> > pde->write_proc = ...
> > open
> > write
> > copy_from_user
> > pde = create_proc_entry();
> > if (!pde) {
> > remove_proc_entry();
> > return -ENOMEM;
> > /* module unloaded */
> > }
>
> We usually fix that race by pinning the module: make whoever registered the
> proc entries also register their THIS_MODULE, do a try_module_get() on it
> before we start to play with data structures which the module owns.
>
> Can we do that here?
We can, but it will be unreliable:
Typical proc entry creation sequence is
pde = create_proc_entry(...);
if (pde)
pde->owner = THIS_MODULE;
Right after create_proc_entry() ->owner is NULL, so try_module_get()
won't do anything, but proc_delete_inode() could put module which was
never getted.
This should fixable by always setting ->owner before proc entry is
glued to proc entries tree. Something like this:
#define create_proc_entry(...) __create_proc_entry(..., THIS_MODULE)
However, I think it's not enough: delete_module(2) first waits for
refcount becoming zero, only then calls modules's exit function which
starts removing proc entries. In between, proc entries are accessible
and fully-functional, so try_module_get() can again get module and
module_put(pde->owner) can happen AFTER module dissapears.
What will it put?
And how can you fix that? The only way I know is to REMOVE ->owner
completely, once we agree on this pde_users/pde_unload_lock stuff.
> And is the above race fix related to the below one in any fashion?
> > ==========================================
> > 2. bogo-revoke aka proc_kill_inodes()
> >
> > remove_proc_entry vfs_read
> > proc_kill_inodes [check ->f_op validness]
> > [check ->f_op->read validness]
> > [verify_area, security permissions checks]
> > ->f_op = NULL;
> > if (file->f_op->read)
> > /* ->f_op dereference, boom */
>
> So you fixed this via sort-of-refcounting on pde->pde_users.
>
> hmm.
Probably, you're right and they are independently fixable. It's all
about following 3 lines after all. My turn to hmm...
> > - proc_kill_inodes(de);
> > + if (!S_ISREG(de->mode))
> > + proc_kill_inodes(de);
> > + spin_lock(&pde->pde_unload_lock);
> > + pde->pde_users--;
> > + if (pde->pde_unload_completion && pde->pde_users == 0)
> > + complete(pde->pde_unload_completion);
> > +out_unlock:
> > + spin_unlock(&pde->pde_unload_lock);
>
> The above six lines happen rather a lot - perhaps it could be placed in a
> helper funtion?
OK. Should I send incremental updates or full patch again?
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