On Tue, Jul 27, 2021 at 07:46:34PM +0200, gregkh@linuxfoundation.org wrote:
On Tue, Jul 27, 2021 at 10:30:36AM -0700, Luis Chamberlain wrote:
On Sat, Jul 24, 2021 at 12:15:10PM +0000, David Laight wrote:
From: Luis Chamberlain
Sent: 22 July 2021 23:19
There is quite a bit of tribal knowledge around proper use of try_module_get() and that it must be used only in a context which can ensure the module won't be gone during the operation. Document this little bit of tribal knowledge.
...
Some typos.
+/**
- try_module_get - yields to module removal and bumps reference count otherwise
- @module: the module we should check for
- This can be used to check if userspace has requested to remove a module,
a module be removed
- and if so let the caller give up. Otherwise it takes a reference count to
- ensure a request from userspace to remove the module cannot happen.
- Care must be taken to ensure the module cannot be removed during
- try_module_get(). This can be done by having another entity other than the
- module itself increment the module reference count, or through some other
- means which gaurantees the module could not be removed during an operation.
guarantees
- An example of this later case is using this call in a sysfs file which the
- module created. The sysfs store / read file operation is ensured to exist
^^^^^^^^^^^^^^^^^^^Not sure what that is supposed to mean.
I'll clarify further. How about:
The sysfs store / read file operations are gauranteed to exist using kernfs's active reference (see kernfs_active()).
But that has nothing to do with module reference counts. kernfs knows nothing about modules.
Yes but we are talking about sysfs files which the module creates. So but inference again, an active reference protects a module.
So there is a potentially horrid race: The module unload is going to do: driver_data->module_ref = 0; and elsewhere there'll be: ref = driver_data->module_ref; if (!ref || !try_module_get(ref)) return -error;
You have to have try_module_get() to allow the module unload function to sleep. But the above code still needs a driver lock to ensure the unload code doesn't race with the try_module_get() and the 'ref' be invalidated before try_module_get() looks at it. (eg if an interrupt defers processing.)
So there can be no 'yielding'.
Oh but there is. Consider access to a random sysfs file 'add_new_device' which takes as input a name, for driver foo, and so foo's add_new_foobar_device(name="bar") is called. Unless sysfs file "yields" by using try_module_get() before trying to add a new foo device called "bar", it will essentially be racing with the exit routine of module foo, and depending on how locking is implemented (most drivers get it wrong), this easily leads to crashes.
In fact, this documentation patch was motivated by my own solution to a possible deadlock when sysfs is used. Using the same example above, if the same sysfs file uses *any* lock, which is *also* used on the exit routine, you can easily trigger a deadlock. This can happen for example by the lock being obtained by the removal routine, then the sysfs file gets called, waits for the lock to complete, then the module's exit routine starts cleaning up and removing sysfs files, but we won't be able to remove the sysfs file (due to kernefs active reference) until the sysfs file complets, but it cannot complete because the lock is already held.
Yes, this is a generic problem. Yes I have proof [0]. Yes, a generic solution has been proposed [1], and because Greg is not convinced and I need to move on with life, I am suggesting a temporary driver specific solution (to which Greg is still NACK'ing, without even proposing any alternatives) [2].
[0] https://lkml.kernel.org/r/20210703004632.621662-5-mcgrof@kernel.org [1] https://lkml.kernel.org/r/20210401235925.GR4332@42.do-not-panic.com [2] https://lkml.kernel.org/r/20210723174919.ka3tzyre432uilf7@garbanzo
My problem with your proposed solution is that it is still racy, you can not increment your own module reference count from 0 -> 1 and expect it to work properly. You need external code to do that somewhere.
You are not providing *any* proof for this. And even so, I believe I have clarified as best as possible how a kernfs active reference implicitly protects the module when we are talking about sysfs files.
Now trying to tie sysfs files to the modules that own them would be nice, but as we have seen, that way lies way too many kernel changes, right?
It's not a one-liner fix. Yes.
Hm, maybe. Did we think about this from the kobj_attribute level? If we use the "wrapper" logic there and the use of the macros we already have for attributes, we might be able to get the module pointer directly "for free".
Did we try that?
That was my hope. I tried that first. Last year in November I determined kernfs is kobject stupid. But more importantly *neither* are struct device specific, so neither of them have semantics for modules or even devices.
this thread has been going on for so long I can't remember anymore...
Please...
Luis