linux-kernel - Re: [PATCH net v2] net: pse-pd: tps23881: Fix boolean evaluation for bitmask checks

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Message-ID: <20241002170047.2b28e740@kmaincent-XPS-13-7390>
Date: Wed, 2 Oct 2024 17:00:47 +0200
From: Kory Maincent <kory.maincent@...tlin.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Kyle Swenson
 <kyle.swenson@....tech>, Simon Horman <horms@...nel.org>, Oleksij Rempel
 <o.rempel@...gutronix.de>, thomas.petazzoni@...tlin.com, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>
Subject: Re: [PATCH net v2] net: pse-pd: tps23881: Fix boolean evaluation
 for bitmask checks

On Wed, 2 Oct 2024 07:31:56 -0700
Jakub Kicinski <kuba@...nel.org> wrote:

> On Wed, 2 Oct 2024 14:53:02 +0200 Kory Maincent wrote:
> > On Wed, 2 Oct 2024 05:27:32 -0700
> > Jakub Kicinski <kuba@...nel.org> wrote:
> >   
> > > On Wed, 2 Oct 2024 05:24:31 -0700 Jakub Kicinski wrote:    
>  [...]  
>  [...]  
>  [...]  
> > > 
> > > Reading the discussion on v1 it seems you're doing this to be safe,
> > > because there was a problem with x &= val & MASK; elsewhere.
> > > If that's the case, please resend to net-next and make it clear it's
> > > not a fix.    
> > 
> > Indeed it fixes this issue.  
> 
> Is "this" here the &= issue or the sentence from the commit message?
> 
> > Why do you prefer to have it on net-next instead of a net? We agreed with
> > Oleksij that it's where it should land. Do we have missed something?  
> 
> The patch is a noop, AFAICT. Are you saying it changes how the code
> behaves? 
> 
> The patch only coverts cases which are 
> 
> 	ena = val & MASK;
> 
> the automatic type conversion will turn this into:
> 
> 	ena = bool(val & MASK);
> which is the same as:
> 	ena = !!(val & MASK);
> 
> The problem you were seeing earlier was that:
> 
> 	ena &= val & MASK;
> 
> will be converted to:
> 
> 	ena = ena & (val & MASK);
> 
> and that is:
> 
> 	ena = bool(int(ena) & (val & MASK));
>                    ^^^
> 
> IOW ena gets promoted to int for the & operation.
> This problem does not occur with simple assignment.

Indeed you are totally right! It is a noop! Thanks!
Should I drop it?

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com