Re: [RFC PATCH] kernel/module: add a safer implementation of try_module_get()

[Marco Pagani <marpagan@redhat.com>]

On 2024-01-30 21:47, Luis Chamberlain wrote:
> On Tue, Jan 30, 2024 at 08:36:14PM +0100, Marco Pagani wrote:
>> The current implementation of try_module_get() requires the module to
>> exist and be live as a precondition. While this may seem intuitive at
>> first glance, enforcing the precondition can be tricky, considering that
>> modules can be unloaded at any time if not previously taken. For
>> instance, the caller could be preempted just before calling
>> try_module_get(), and while preempted, the module could be unloaded and
>> freed. More subtly, the module could also be unloaded at any point while
>> executing try_module_get() before incrementing the refount with
>> atomic_inc_not_zero().
>>
>> Neglecting the precondition that the module must exist and be live can
>> cause unexpected race conditions that can lead to crashes. However,
>> ensuring that the precondition is met may require additional locking
>> that increases the complexity of the code and can make it more
>> error-prone.
>>
>> This patch adds a slower yet safer implementation of try_module_get()
>> that checks if the module is valid by looking into the mod_tree before
>> taking the module's refcount. This new function can be safely called on
>> stale and invalid module pointers, relieving developers from the burden
>> of ensuring that the module exists and is live before attempting to take
>> it.
>>
>> The tree lookup and refcount increment are executed after taking the
>> module_mutex to prevent the module from being unloaded after looking up
>> the tree.
>>
>> Signed-off-by: Marco Pagani <marpagan@redhat.com>
> 
> It very much sounds like there is a desire to have this but without a
> user, there is no justification.

I was working on a set of patches to fix an issue in the fpga subsystem
when I came across your commit 557aafac1153 ("kernel/module: add
documentation for try_module_get()") that made me realize we also had a
safety problem. 

To solve this problem for the fpga manager, we had to add a mutex to
ensure the low-level module still exists before calling
try_module_get(). However, having a safer version of try_module_get()
would have simplified the code and made it more robust against changes.

https://lore.kernel.org/linux-fpga/20240111160242.149265-1-marpagan@redhat.com/

I suspect there may be other cases where try_module_get() is
inadvertently called without ensuring that the module still exists
that may benefit from a safer implementation.

>> +bool try_module_get_safe(struct module *module)
>> +{
>> +	struct module *mod;
>> +	bool ret = true;
>> +
>> +	if (!module)
>> +		goto out;
>> +
>> +	mutex_lock(&module_mutex);
> 
> If a user comes around then this should be mutex_lock_interruptible(),
> and add might_sleep()

Would it be okay to return false if it gets interrupted, or should I
change the return type to int to propagate -EINTR? My concern with
changing the signature is that it would be less straightforward to
use the function in place of try_module_get().

>> +
>> +	/*
>> +	 * Check if the address points to a valid live module and take
>> +	 * the refcount only if it points to the module struct.
>> +	 */
>> +	mod = __module_address((unsigned long)module);
>> +	if (mod && mod == module && module_is_live(mod))
>> +		__module_get(mod);
>> +	else
>> +		ret = false;
>> +
>> +	mutex_unlock(&module_mutex);
>> +
>> +out:
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL(try_module_get_safe);
> 
> And EXPORT_SYMBOL_GPL() would need to be used.

Okay, I initially used EXPORT_SYMBOL() to be compatible with
try_module_get().

> 
> I'd also expect selftests to be expanded for this case, but again,
> without a user, this is just trying to resolve a problem which does not
> exist.

I can add selftests in the next versions.
Thanks,
Marco