linux-kernel - Re: [PATCH] let ERR_PTR BUILD_BUG_ON when we know its argument is not a valid errno

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Message-Id: <20080512163830.04ef13fd.akpm@linux-foundation.org>
Date:	Mon, 12 May 2008 16:38:30 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] let ERR_PTR BUILD_BUG_ON when we know its argument is
 not a valid errno

On Sun, 11 May 2008 22:12:14 +0200
Marcin Slusarz <marcin.slusarz@...il.com> wrote:

> Signed-off-by: Marcin Slusarz <marcin.slusarz@...il.com>
> Cc: Andrew Morton <akpm@...l.org>
> ---
> allmodconfig compile tested (on x86_64)
> 
> should be applied after:
> net/sunrpc/xprtrdma: fix svc_rdma_create out of memory error path
> jfs: 0 is not valid errno value
> ---
>  include/linux/err.h |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/err.h b/include/linux/err.h
> --- a/include/linux/err.h
> +++ b/include/linux/err.h
> @@ -19,11 +19,13 @@
>  
>  #define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
>  
> -static inline void *ERR_PTR(long error)
> +static inline void *__ERR_PTR(long error)
>  {
>  	return (void *) error;
>  }
>  
> +#define ERR_PTR(error) (BUILD_BUG_ON(!IS_ERR_VALUE(error)), __ERR_PTR(error))
> +
>  static inline long PTR_ERR(const void *ptr)
>  {
>  	return (long) ptr;

Not sure about this one.  BUILD_BUG_ON only makes sense if the value is
a compile-time constant.  I think the code as you have it will take this:

	int e = foo();

	p = ERR_PTR(e);

and will attempt to evaluate sizeof() on a negative-sized array at
runtime.  The conmpile will laugh and throw that all away, but it's a
bit weird.

Plus I'd have thought that the amount of code which does ERR_PTR(-EFOO)
is fairly small, but perhaps that's wrong.

If I _am_ wrong then I do think it'd be saner to only do the
BUILD_BUG_ON() if __builtin_constant_p(error) evaluates true.  And even
then I do think we'd like to see a more lengthy justification of why
the kernel needs this check.  More lengthy than zero, anyway...

(If a compile-time check is needed then why not a runtime one also?)

Thanks.
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