linux-kernel - Re: [RFC] PTR_ERR: return 0 if ptr isn't an error value.

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Message-ID: <87y5ajq14w.fsf@rustcorp.com.au>
Date:	Sun, 09 Jun 2013 15:25:11 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:	Thomas Meyer <thomas@...3r.de>, mst@...hat.com,
	grant.likely@...aro.org, rob.herring@...xeda.com,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Julia Lawall <Julia.Lawall@...6.fr>
Subject: Re: [RFC] PTR_ERR: return 0 if ptr isn't an error value.

Uwe Kleine-König <u.kleine-koenig@...gutronix.de> writes:
> Hello Rusty,
>
> [added akpm to Cc: who took the patch back then and Julia for the
> coccinelle part below]
>
> On Mon, Jun 03, 2013 at 11:59:15AM +0930, Rusty Russell wrote:
>> 
>> Back in 2011, Uwe Kleine-König added the nonsensically-named
>> PTR_RET(), providing a means to avoid if() statements in code (commit 
>> fa9ee9c4b9).
>> 
>> Instead, just make PTR_ERR() return 0 if the pointer isn't an error
>> value.  This is harmless, since PTR_ERR() should have never been
>> passed a non-error value.  And GCC is usually smart enough to remove
>> the extra test if IS_ERR() has already been called.
> I wonder in which situations gcc fails to be smart enough. Did you check
> this?

Good q.  I just wrote a simple test to confirm that GCC usually did,
then compared the text sizes.  Since it was bigger, I assume GCC isn't
doing it all the time.

Hmm, first change is in do_debug.  Ah, no surprise here:

	if (notify_die(DIE_DEBUG, "debug", regs, PTR_ERR(&dr6), error_code,
							SIGTRAP) == NOTIFY_STOP)
		goto exit;

Huh?  dr6 is on the stack?  Ancient, fixed.

I'll wade through the other differences after the weekend...

Cheers,
Rusty.
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