linux-kernel - Re: [PATCH v5 1/2] selinux: add brief info to policydb

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Date:   Wed, 17 May 2017 17:43:03 +0200
From:   Sebastien Buisson <sbuisson.ddn@...il.com>
To:     William Roberts <bill.c.roberts@...il.com>
Cc:     Stephen Smalley <sds@...ho.nsa.gov>,
        "selinux@...ho.nsa.gov" <selinux@...ho.nsa.gov>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-security-module@...r.kernel.org,
        Sebastien Buisson <sbuisson@....com>,
        James Morris <james.l.morris@...cle.com>
Subject: Re: [PATCH v5 1/2] selinux: add brief info to policydb

2017-05-17 17:34 GMT+02:00 William Roberts <bill.c.roberts@...il.com>:
>>>>> Is there a particular reason to not just return policybrief_len here as
>>>>> well, for consistency in the interface?  How do you intend to use this
>>>>> value in the caller?
>>>>
>>>> As called in the other patch to expose policy brief via selinuxfs
>>>> (sel_read_policybrief), the intent is to provide the caller with the
>>>> length of the string returned.
>>>> Or should I set *len to policy brief_len here, and just make the
>>>> caller aware that the returned length is in fact the length of the
>>>> buffer (i.e. including terminating NUL byte)?
>>>
>>> What is the caller supposed to do with length? This interface seemed kind of
>>> odd. If it's guaranteed NUL byte terminated, do they even need length?
>>
>> The length is useful as an input parameter in case the caller provides
>> its own buffer (instead of letting the function allocate one), and as
>
> This is what I don't get, why doesn't the function just always allocate?

For performance reasons mainly. The caller would have a statically
allocated buffer, reused every time it needs to get the policy brief
info.