linux-kernel - Re: [PATCH] cgroup: add explicit cast and comment for return type conversion

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Message-ID: <20150524203528.GB7099@htj.duckdns.org>
Date:	Sun, 24 May 2015 16:35:28 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Nicholas Mc Guire <hofrat@...dl.org>
Cc:	Li Zefan <lizefan@...wei.com>, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: add explicit cast and comment for return type
 conversion

Hello,

On Sun, May 24, 2015 at 03:07:52PM +0200, Nicholas Mc Guire wrote:
> Type-checking coccinelle spatches are being used to locate type mismatches
> between function signatures and return values in this case this produced:
> ./kernel/cgroup.c:2525 WARNING: return of wrong type
> 	ssize_t != size_t, 
> 
> Returning unsigned types converted to a signed type can be problematic
> but in this case the size_t is <= PATH_MAX which is less than ulong/2 so
> the conversion is safe - to make static code checking happy this is 
> resolved by an explicit cast and appropriate comment.
> 
> Patch was compile tested with x86_64_defconfig (implies CONFIG_CGROUPS=y)
> 
> Patch is against 4.1-rc4 (localversion-next is -next-20150522)
> 
> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
> ---
> 
> Not sure if "cleanups" like this are acceptable - in this case I did not
> find any better way to make static code checkers happy though.
> 
>  kernel/cgroup.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index b91177f..04de621 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2523,7 +2523,11 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
>  		sizeof(cgrp->root->release_agent_path));
>  	spin_unlock(&release_agent_path_lock);
>  	cgroup_kn_unlock(of->kn);
> -	return nbytes;
> +
> +	/* the path of the release notifier is <= PATH_MAX
> +	 * so "downsizing" to signed long is safe here
> +	 */
> +	return (ssize_t)nbytes;

idk, does this actually help anything?  This isn't different from any
other implicit type casts.  Are we gonna convert all downward implicit
casts to be explicit?

Thanks.

-- 
tejun
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