Messages in this thread | | | Subject | Re: Bug: power supply - NULL pointer dereference in bq27x00 driver | From | "Dr. H. Nikolaus Schaller" <> | Date | Mon, 18 May 2015 15:30:11 +0200 |
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Hi Krzysztof,
Am 18.05.2015 um 14:32 schrieb Krzysztof Kozlowski <k.kozlowski@samsung.com>:
> 2015-05-18 18:40 GMT+09:00 Dr. H. Nikolaus Schaller <hns@goldelico.com>: >> Hi, >> we tried to upgrade from 4.0 to 4.1-rc3 kernel and ran into a NULL pointer problem within the bq27x00 driver. >> >> It appears to be introduced by your patch 297d716f6260cc9421d971b124ca196b957ee458 >> >> The problem appears to be that bq27x00_powersupply_init() calls power_supply_register_no_ws() and >> sets di->bat *after* return. The old code did pass an uninitialized struct pointer. >> >> Now for reasons I don’t understand, the power_supply_register_no_ws() appears to call >> uevent related stuff which in turn calls bq27x00_battery_get_property() before di->bat >> is properly initialized. >> >> I have checked with printk in bq27x00_battery_get_property() that di>bat == NULL in this case and >> right before we see the segfault. >> >> The old code simply did pass a zeroed struct power_supply and perhaps initialized its components >> during registration. >> >> Returning some -EINVAL if di->bat == NULL would likely solve the NULL pointer dereference but >> I don’t know what it does to the uevent and if it restores previous operation. >> >> It could have been that it was for good purpose that power_supply_register_no_ws() did not return >> by value, but by reference to the di->bat struct: >> >> - ret = power_supply_register_no_ws(di->dev, &di->bat, NULL); >> + di->bat = power_supply_register_no_ws(di->dev, psy_desc, &psy_cfg); >> >> So that code called within the context of power_supply_register_no_ws() could already >> refer to initialized di->bat. > > Indeed this issue was not present in previous design however I think > the architecture was still error-prone. Starting from driver's probe: > - some_driver_probe() > - power_supply_register(&psy) > - device_add() > - kobject_uevent() - notify userspace > - power_supply_uevent() > - power_supply_show_property() > - power_supply_get_property() > - some_driver_get_property() > > So before probe() ends the power supply core calls driver's > get_property(). In that time the driver internal structures may not be > ready yet, because the probe did not end.
Yes, that is indeed a problem. Maybe it did work with the bq27x00 driver earlier because the call to power_supply_register(&psy) was the last activity.
> The get_property() could > access some registers or other core functions which will be ready > after power_supply_register() call. For example the > bq27x00_battery_get_property() accesses delayed work which could be > (by mistake) not yet initialized. > > I looked at other dev_uevent implementations and almost all of them do > not call back the driver. > > Of course this does not change that I introduced the issue and I feel > bad about it. > I got some ideas for resolving it: > 1. Fix bq27x00_battery and maybe others so they won't access the 'psy' > member of its state container. This does not solve the issue from > architectural point of view - still some uninitialised data may be > accessed because probe() is in progress. However this would be the > fastest and the least intrusive.
The problem might be that it fundamentally changes the driver code architecture. For example one call using di->bat is in
bq27x00_battery_status() { … else if (power_supply_am_i_supplied(di->bat)) status = POWER_SUPPLY_STATUS_NOT_CHARGING; … }
> > 2. Remove calls to get_property() (and other functions provided by > driver) from power_supply_uevent(). Unfortunately this may break > userspace which expects that some data will be present in uevent. > > 3. Change the API to the previous convention, which you pointed as a remedy: > ret = power_supply_register_no_ws(di->dev, &di->bat, psy_desc, &psy_cfg); > This also won't solve the issue from 1. that uevent will be called > before probe ends.
Well, we could require that power_supply_register_no_ws() is the last activity to be done in any_driver_probe().
> > 4. Block the power_supply_uevent() from calling the get_property() > functions until device_add() finishes. This would solve this issue but > first uevent messages (from adding device) won't contain all of device > data (just like in solution no 2.) and this won't solve other race - > someone may call sysfs show() on created device nodes and thus access > the get_property() before probe finishes.
> > Any ideas?
5. is it possible to delay the call to kobject_uevent() after some_driver_probe() is finished? E.g. by registering a one-shot delayed work?
There seems to be a shared workqueue (mentioned e.g. in <http://www.makelinux.net/ldd3/chp-7-sect-6>) but that is an area of the kernel I am not familiar with.
BR, Nikolaus
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