Re: [RFC v2] lis3lv02d: avoid divide by zero due to unchecked registerread

From: Éric Piel
Date: Tue May 17 2011 - 17:46:15 EST

Op 16-05-11 23:36, Christian Lamparter schreef:
On Monday 16 May 2011 13:16:46 Éric Piel wrote:
Op 16-05-11 00:46, Christian Lamparter schreef:
From my POV, it looks like the hardware is not working as expected
and returns a bogus data rate. The driver doesn't check the result
and directly uses it as some sort of divisor in some places:

msleep(lis3->pwron_delay / lis3lv02d_get_odr());

Under this circumstances, this could very well cause the
"divide by zero" exception from above.

However, I'd fix it a bit differently: let lis3lv02d_get_odr() return
the raw data, and create a special function
lis3lv02d_get_pwron_delay_ms() which does the "lis3->pwron_delay /
lis3lv02d_get_odr()" with special handling for 0 (returning a large
value and also sending a printk_once() ).
Do you know how "volatile" this data rate is? If it never changes
[at least it doesn't here?] then why not read it once in init_device
and store it in the device context?
It is not normally changing, normally it is set just at init/unsuspend (where the bios can also interfere sometimes) and when the user changes it. So definitely within the same function it's not going to suddenly change. We could avoid calculating/checking it twice in lis3lv02d_selftest(). Care to do a third version with this little clean up?

Anyway, I updated the code... But instead of returning a "large value"
I went for the -ENXIO to bail-out early, so now we won't continue if
something went bad [after resume for instance?].
Sounds even better than using a conservative value!

As you have noted, we might want to check other parts of the driver to
validate the data from the device. So far, all the code considers that
the device is flawless :-S
CHECK drivers/misc/lis3lv02d/lis3lv02d.c
drivers/misc/lis3lv02d/lis3lv02d.c:170:52: warning: cast to restricted __le16
This is not introduced by your patch, right? So it's fine for now :-)

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