RE: [PATCH v2 1/2] module: update dependencies at try_module_get()

2022-05-01 Thread David Laight 
From: Mauro Carvalho Chehab
> Sent: 30 April 2022 14:38
> 
> Em Sat, 30 Apr 2022 14:04:59 +0200
> Greg KH  escreveu:
> 
> > On Sat, Apr 30, 2022 at 11:30:58AM +0100, Mauro Carvalho Chehab wrote:
> 
> > Did you run checkpatch on this?  Please do :)
> >
> > > +
> > > + if (mod == this)
> > > + return 0;
> >
> > How can this happen?
> > When people mistakenly call try_module_get(THIS_MODULE)?
> 
> Yes. There are lots of place where this is happening:
> 
>   $ git grep try_module_get\(THIS_MODULE|wc -l
>   82
> 
> > We should
> > throw up a big warning when that happens anyway as that's always wrong.
> >
> > But that's a different issue from this change, sorry for the noise.
> 
> It sounds very weird to use try_module_get(THIS_MODULE).
> 
> We could add a WARN_ON() there - or something similar - but I would do it
> on a separate patch.

You could add a compile-time check.
But a run-time one seems unnecessary.
Clearly try_module_get(THIS_MODULE) usually succeeds.

I think I can invent a case where it can fail:
The module count must be zero, and a module unload in progress.
The thread doing the unload is blocked somewhere.
Another thread makes a callback into the module for some request
that (for instance) would need to create a kernel thread.
It tries to get a reference for the thread.
So try_module_get(THIS_MODULE) is the right call - and will fail here.

David

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Re: [PATCH v2 1/2] module: update dependencies at try_module_get()

2022-04-30 Thread Mauro Carvalho Chehab 
Em Sat, 30 Apr 2022 14:04:59 +0200
Greg KH  escreveu:

> On Sat, Apr 30, 2022 at 11:30:58AM +0100, Mauro Carvalho Chehab wrote:

> Did you run checkpatch on this?  Please do :)
> 
> > +
> > +   if (mod == this)
> > +   return 0;  
> 
> How can this happen?
> When people mistakenly call try_module_get(THIS_MODULE)?  

Yes. There are lots of place where this is happening:

$ git grep try_module_get\(THIS_MODULE|wc -l
82

> We should
> throw up a big warning when that happens anyway as that's always wrong.
> 
> But that's a different issue from this change, sorry for the noise.

It sounds very weird to use try_module_get(THIS_MODULE).

We could add a WARN_ON() there - or something similar - but I would do it 
on a separate patch.

> 
> > +
> > +   mutex_lock(&module_mutex);
> > +
> > +   ret = ref_module(this, mod);
> > +
> > +#ifdef CONFIG_MODULE_UNLOAD
> > +   if (ret)
> > +   goto ret;
> > +
> > +   ret = sysfs_create_link(mod->holders_dir,
> > +   &this->mkobj.kobj, this->name);  
> 
> Meta comment, why do we only create links if we can unload things?

Good question. I don't know for certain. This is the already existing 
pattern at add_usage_links() - see kernel/module/sysfs.c.

Also, lsmod uses sysfs links when showing dependencies.

Regards,
Mauro

Re: [PATCH v2 1/2] module: update dependencies at try_module_get()

2022-04-30 Thread Greg KH 
On Sat, Apr 30, 2022 at 11:30:58AM +0100, Mauro Carvalho Chehab wrote:
> Sometimes, device drivers are bound into each other via try_module_get(),
> making such references invisible when looking at /proc/modules or lsmod.
> 
> Add a function to allow setting up module references for such
> cases, and call it when try_module_get() is used.
> 
> Reviewed-by: Dan Williams 
> Signed-off-by: Mauro Carvalho Chehab 
> ---
> 
> See [PATCH v2 0/2] at: 
> https://lore.kernel.org/all/cover.1651314499.git.mche...@kernel.org/
> 
>  include/linux/module.h |  4 +++-
>  kernel/module/main.c   | 35 +--
>  2 files changed, 36 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/module.h b/include/linux/module.h
> index 46d4d5f2516e..836851baaad4 100644
> --- a/include/linux/module.h
> +++ b/include/linux/module.h
> @@ -620,7 +620,9 @@ extern void __module_get(struct module *module);
>  
>  /* This is the Right Way to get a module: if it fails, it's being removed,
>   * so pretend it's not there. */
> -extern bool try_module_get(struct module *module);
> +extern bool __try_module_get(struct module *module, struct module *this);
> +
> +#define try_module_get(mod) __try_module_get(mod, THIS_MODULE)
>  
>  extern void module_put(struct module *module);
>  
> diff --git a/kernel/module/main.c b/kernel/module/main.c
> index 05a42d8fcd7a..9f4416381e65 100644
> --- a/kernel/module/main.c
> +++ b/kernel/module/main.c
> @@ -631,6 +631,35 @@ static int ref_module(struct module *a, struct module *b)
>   return 0;
>  }
>  
> +static int ref_module_dependency(struct module *mod,
> +struct module *this)

This can be on one line, right?

> +{
> + int ret;
> +
> + if (!this || !this->name) {
> + return -EINVAL;
> + }

Did you run checkpatch on this?  Please do :)

> +
> + if (mod == this)
> + return 0;

How can this happen?

When people mistakenly call try_module_get(THIS_MODULE)?  We should
throw up a big warning when that happens anyway as that's always wrong.

But that's a different issue from this change, sorry for the noise.

> +
> + mutex_lock(&module_mutex);
> +
> + ret = ref_module(this, mod);
> +
> +#ifdef CONFIG_MODULE_UNLOAD
> + if (ret)
> + goto ret;
> +
> + ret = sysfs_create_link(mod->holders_dir,
> + &this->mkobj.kobj, this->name);

Meta comment, why do we only create links if we can unload things?

thanks,

greg k-h