linux-kernel - Re: [PATCH 01/13] seqnum_ops: Introduce Sequence Number Ops

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Date:   Tue, 10 Nov 2020 16:18:21 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Shuah Khan <skhan@...uxfoundation.org>, corbet@....net,
        peterz@...radead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/13] seqnum_ops: Introduce Sequence Number Ops

On Tue, Nov 10, 2020 at 09:43:02PM +0100, Greg KH wrote:
> On Tue, Nov 10, 2020 at 09:41:40PM +0100, Greg KH wrote:
> > On Tue, Nov 10, 2020 at 12:53:27PM -0700, Shuah Khan wrote:
> > > +Decrement interface
> > > +-------------------
> > > +
> > > +Decrements sequence number and doesn't return the new value. ::
> > > +
> > > +        seqnum32_dec() --> atomic_dec()
> > > +        seqnum64_dec() --> atomic64_dec()
> > 
> > Why would you need to decrement a sequence number?  Shouldn't they just
> > always go up?
> > 
> > I see you use them in your patch 12/13, but I don't think that really is
> > a sequence number there, but rather just some other odd value :)

To that end, they should likely be internally cast to u32 and u64 (and
why is seqnum64 ifdef on CONFIG_64BIT?).

> Note, other than this, I like the idea.  It makes it obvious what these
> atomic variables are being used for, and they can't be abused for other
> things.  Nice work.

Agreed: this is a clear wrapping sequence counter. It's only abuse would
be using it in a place where wrapping actually is _not_ safe. (bikeshed:
can we call it wrap_u32 and wrap_u64?)

-- 
Kees Cook