Messages in this thread | | | Subject | Re: [PATCH v2 04/11] drivers/base/devcoredump: convert devcd_count to counter_atomic32 | From | Johannes Berg <> | Date | Wed, 07 Oct 2020 21:38:47 +0200 |
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On Wed, 2020-10-07 at 13:33 -0600, Shuah Khan wrote: > On 10/7/20 12:15 PM, Kees Cook wrote: > > On Tue, Oct 06, 2020 at 02:44:35PM -0600, Shuah Khan wrote: > > > counter_atomic* is introduced to be used when a variable is used as > > > a simple counter and doesn't guard object lifetimes. This clearly > > > differentiates atomic_t usages that guard object lifetimes. > > > > > > counter_atomic* variables will wrap around to 0 when it overflows and > > > should not be used to guard resource lifetimes, device usage and > > > open counts that control state changes, and pm states. > > > > > > devcd_count is used to track dev_coredumpm device count and used in > > > device name string. It doesn't guard object lifetimes, device usage > > > counts, device open counts, and pm states. There is very little chance > > > of this counter overflowing. Convert it to use counter_atomic32. > > > > > > This conversion doesn't change the overflow wrap around behavior. > > > > > > Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > > > Signed-off-by: Shuah Khan <skhan@linuxfoundation.org> > > > > I actually wonder if this should use refcount_t just because it is > > designed to be an alway-unique value. It is hard to imagine ever causing > > this to overflow, but why not let it be protected? > > > > This is one of the cases where devcd_count doesn't guard lifetimes, > however if it ever overflows, refcount_t is a better choice. > > If we decide refcount_t is a better choice, I can drop this patch > and send refcount_t conversion patch instead. > > Greg! Any thoughts on refcount_t for this being a better choice?
I'm not Greg, but ... there's a 5 minute timeout. So in order to cause a clash you'd have to manage to overflow the counter within a 5 minute interval, otherwise you can actually reuse the numbers starting again from 0 without any ill effect.
And even if you *do* manage to overflow it quickly enough it'll just fail device_add() and error out, and nothing happens.
So I think it's fairly much pointless to think about protecting against some kind of overflows. It's just trying to get a "temporarily unique ID" here, could be doing anything else instead, but most other things would require bigger data structures and/or (higher level) locking.
OTOH, if you *do* somehow create that many core dumps (huge uptimes and extremely frequent crashes?) it seems like refcount_t would be a bad choice because it saturates, and then you can only do one more dump per 5 minutes? Or maybe that's a good thing in these ill cases ...
I don't think it'll really happen either way :)
johannes
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