linux-kernel - Re: [PATCH] scatterlist: don't BUG when we can trivially return a proper error.

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Date:	Wed, 14 Nov 2012 13:05:00 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Bowler <nbowler@...iptictech.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scatterlist: don't BUG when we can trivially return a
 proper error.

On Thu,  1 Nov 2012 15:03:00 -0400
Nick Bowler <nbowler@...iptictech.com> wrote:

> There is absolutely no reason to crash the kernel when we have a
> perfectly good return value already available to use for conveying
> failure status.

Yes, I suppose that's true.  I don't see a case for BUGging the kernel
here.

> Let's return an error code instead of crashing the kernel: that sounds
> like a much better plan.
> 
> Signed-off-by: Nick Bowler <nbowler@...iptictech.com>
> ---
>  lib/scatterlist.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index 3675452b23ca..11ecaf000696 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -248,7 +248,8 @@ int __sg_alloc_table(struct sg_table *table, unsigned int nents,
>  	unsigned int left;
>  
>  #ifndef ARCH_HAS_SG_CHAIN
> -	BUG_ON(nents > max_ents);
> +	if (WARN_ON_ONCE(nents > max_ents))
> +		return -E2BIG;
>  #endif

OK, pet peeve: if this E2BIG gets returned to userspace, our poor user
will look it up and see "Argument list too long; used when the
arguments passed to a new program being executed with one of the exec
functions occupy too much memory space".  He then gets to spend half a
day reviewing his code's exec() callsites!

See?  Although the error's name sounds like a nice match to the
internal state, it isn't really a match at all and our use of it is
misleading.

Unfortunately there is no EKERNELSCREWEDUP, so we usually use EINVAL.
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