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Date:   Thu, 26 Oct 2017 08:34:49 +0200
From:   Martin Schwidefsky <>
To:     Elena Reshetova <>
Subject: Re: [PATCH 1/3] [S390] vmur: convert urdev.ref_count from atomic_t
 to refcount_t

On Fri, 20 Oct 2017 10:47:48 +0300
Elena Reshetova <> wrote:

> atomic_t variables are currently used to implement reference
> counters with the following properties:
>  - counter is initialized to 1 using atomic_set()
>  - a resource is freed upon counter reaching zero
>  - once counter reaches zero, its further
>    increments aren't allowed
>  - counter schema uses basic atomic operations
>    (set, inc, inc_not_zero, dec_and_test, etc.)
> Such atomic variables should be converted to a newly provided
> refcount_t type and API that prevents accidental counter overflows
> and underflows. This is important since overflows and underflows
> can lead to use-after-free situation and be exploitable.
> The variable urdev.ref_count is used as pure reference counter.
> Convert it to refcount_t and fix up the operations.
> Suggested-by: Kees Cook <>
> Reviewed-by: David Windsor <>
> Reviewed-by: Hans Liljestrand <>
> Signed-off-by: Elena Reshetova <>

Added to s390/linux:features for the next merge window. Thanks.

blue skies,

"Reality continues to ruin my life." - Calvin.

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