[PATCH] Proposed removal of IS_ERR_OR_NULL() (was: Re: [PATCH 1/4] gpiolib: introduce descriptor-based GPIO interface)

[Nicolas Pitre]

On Wed, 9 Jan 2013, Russell King - ARM Linux wrote:

> So, it seems there's some concensus building here, and it seems that
> I've become the chosen victi^wvolunteer for this.  So, here's a patch.
> It's missing a Guns-supplied-by: tag though.

Guns-supplied-by: NRA (obviously)

> From: Russell King <rmk+kernel at arm.linux.org.uk>
> Subject: Mark IS_ERR_OR_NULL() deprecated
> 
> IS_ERR_OR_NULL() attracts a lot of abuse: people use it without much
> thought about it's effects.  Common errors include:
>  1. checking the returned pointer for functions defined as only
>     returning errno-pointer values, rather than using IS_ERR().
>     This leads to: ptr = foo(); if (IS_ERR_OR_NULL(ptr)) return
>      PTR_ERR(ptr);
>  2. using it to check functions which only ever return NULL on error,
>     thereby leading to another zero-error value return.
> In the case of debugfs functions, these return errno-pointer values when
> debugfs is configured out, which means code which blindly checks using 
> IS_ERR_OR_NULL() ends up returning errors, which is rather perverse for
> something that's not implemented.
> 
> Therefore, let's schedule it for removal in a few releases.
> 
> Nicolas Pitre comments:
> > I do agree with Russell here.  Despite the original intentions behind
> > IS_ERR_OR_NULL() which were certainly legitimate, the end result in
> > practice is less reliable code with increased maintenance costs.
> > Unlike other convenience macros in the kernel, this one is giving a
> > false sense of correctness with too many people falling in the trap
> > of using it just because it is available.
> > 
> > I strongly think this macro should simply be removed from the source
> > tree entirely and the code reverted to explicit tests against NULL
> > when appropriate.
> 
> Suggested-by: David Howells <dhowells at redhat.com>
> Tape-measuring-service-offered-by: Will Deacon <will.deacon at arm.com>
> Victim-for-firing-sqad: Russell King <rmk+kernel at arm.linux.org.uk>
> Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>

Acked-by: Nicolas Pitre <nico at linaro.org>

Anyone with good coccinelle skills around to deal with the users?


> ---
> Ok, so I'm in the firing line for suggesting this, but it appears
> several people wish this to happen.
> 
> I'm not intending to push this patch forwards _just_ yet: we need to
> sort out the existing users _first_ to prevent the kernel turning into
> one hell of a mess of warnings.
> 
>  include/linux/err.h |   17 ++++++++++++++++-
>  1 files changed, 16 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/err.h b/include/linux/err.h
> index f2edce2..d5a85df 100644
> --- a/include/linux/err.h
> +++ b/include/linux/err.h
> @@ -34,7 +34,22 @@ static inline long __must_check IS_ERR(const void *ptr)
>  	return IS_ERR_VALUE((unsigned long)ptr);
>  }
>  
> -static inline long __must_check IS_ERR_OR_NULL(const void *ptr)
> +/*
> + * IS_ERR_OR_NULL() attracts a lot of abuse: people use it without much
> + * thought about it's effects.  Common errors include:
> + *  1. checking the returned pointer for functions defined as only returning
> + *     errno-pointer values, rather than using IS_ERR().
> + *     This leads to: ptr = foo(); if (IS_ERR_OR_NULL(ptr)) return PTR_ERR(ptr);
> + *  2. using it to check functions which only ever return NULL on error,
> + *     thereby leading to another zero-error value return.
> + * In the case of debugfs functions, these return errno-pointer values when
> + * debugfs is configured out, which means code which blindly checks using
> + * IS_ERR_OR_NULL() ends up returning errors, which is rather perverse for
> + * something that's not implemented.
> + *
> + * Therefore, let's schedule it for removal in a few releases.
> + */
> +static inline long __must_check __deprecated IS_ERR_OR_NULL(const void *ptr)
>  {
>  	return !ptr || IS_ERR_VALUE((unsigned long)ptr);
>  }
>