linux-kernel - Re: [PATCH 1/2] CAPABILITIES: add cap

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Date:	Fri, 13 Mar 2015 10:02:46 -0400
From:	Paul Moore <pmoore@...hat.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Eric Paris <eparis@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH 1/2] CAPABILITIES: add cap_isequal helper

On Monday, March 09, 2015 09:35:46 PM Mateusz Guzik wrote:
> Can be used to determine whether two given sets have the same
> capabilities.
> 
> Signed-off-by: Mateusz Guzik <mguzik@...hat.com>
> ---
>  include/linux/capability.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index af9f0b9..2fcf941 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -155,6 +155,16 @@ static inline int cap_isclear(const kernel_cap_t a)
>  	return 1;
>  }
> 
> +static inline int cap_isequal(const kernel_cap_t a, const kernel_cap_t b)
> +{
> +	unsigned __capi;
> +	CAP_FOR_EACH_U32(__capi) {
> +		if (a.cap[__capi] != b.cap[__capi])
> +			return 0;
> +	}
> +	return 1;
> +}

I realize it is currently only a two pass loop so probably not that big of a 
deal, but couldn't you accomplish the same with a memcmp()?  I suppose the 
above implementation might be faster than those architectures which use the 
generic memcmp() implementation, but I wonder if the arch-specific memcmp() 
implementations would be faster.

Also, what is the main motivation for this patchset?  Do you have a workload 
that is being hit hard by prepare_creds()?

-- 
paul moore
security @ redhat

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