linux-kernel - Re: [PATCH] security, crypto: LLVMLinux: Remove VLAIS from ima

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Date:	Fri, 05 Sep 2014 17:45:03 -0700
From:	Behan Webster <behanw@...verseincode.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	zohar@...ux.vnet.ibm.com, d.kasatkin@...sung.com,
	james.l.morris@...cle.com, linux-ima-devel@...ts.sourceforge.net,
	linux-ima-user@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, serge@...lyn.com,
	torvalds@...ux-foundation.org,
	Mark Charlebois <charlebm@...il.com>,
	Jan-Simon Möller <dl9pf@....de>
Subject: Re: [PATCH] security, crypto: LLVMLinux: Remove VLAIS from ima_crypto.c

On 09/05/14 17:18, Thomas Gleixner wrote:
> On Fri, 5 Sep 2014, behanw@...verseincode.com wrote:
>
>> From: Behan Webster <behanw@...verseincode.com>
>>
>> Replaced the use of a Variable Length Array In Struct (VLAIS) with a C99
>> compliant equivalent. This patch allocates the appropriate amount of memory
>> using an char array.
>>
>> The new code can be compiled with both gcc and clang.
>>
>> struct shash_desc contains a flexible array member member ctx declared with
>> CRYPTO_MINALIGN_ATTR, so sizeof(struct shash_desc) aligns the beginning
>> of the array declared after struct shash_desc with long long.
>>
>> No trailing padding is required because it is not a struct type that can
>> be used in an array.
>>
>> The CRYPTO_MINALIGN_ATTR is required so that desc is aligned with long long
>> as would be the case for a struct containing a member with
>> CRYPTO_MINALIGN_ATTR.
>>
>> Signed-off-by: Behan Webster <behanw@...verseincode.com>
>> Signed-off-by: Mark Charlebois <charlebm@...il.com>
>> Signed-off-by: Jan-Simon Möller <dl9pf@....de>
> This SOB chain is completely ass backwards. See Documentation/...
"The Signed-off-by: tag indicates that the signer was involved in the
development of the patch, or that he/she was in the patch's delivery path."

All three of us were involved. Does that not satisfy this rule?

>> ---
>>   security/integrity/ima/ima_crypto.c | 53 +++++++++++++++++--------------------
>>   1 file changed, 25 insertions(+), 28 deletions(-)
>>
>> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
>> index 0bd7328..a6aa2b0 100644
>> --- a/security/integrity/ima/ima_crypto.c
>> +++ b/security/integrity/ima/ima_crypto.c
>> @@ -380,17 +380,16 @@ static int ima_calc_file_hash_tfm(struct file *file,
>>   	loff_t i_size, offset = 0;
>>   	char *rbuf;
>>   	int rc, read = 0;
>> -	struct {
>> -		struct shash_desc shash;
>> -		char ctx[crypto_shash_descsize(tfm)];
>> -	} desc;
>> +	char desc[sizeof(struct shash_desc) +
>> +		crypto_shash_descsize(tfm)] CRYPTO_MINALIGN_ATTR;
>> +	struct shash_desc *shash = (struct shash_desc *)desc;
> That anon struct should have never happened in the first place.
Sadly this is a design pattern used in many places through out the 
kernel, and appears to be fundamental to the crypto system. I was 
advised *not* to change it, so we haven't.

I agree that it's not a good practice.

>   Not
> your problem, but you are not making it any better. You replace open
> coded crap with even more unreadable crap.
>
> Whats wrong with
>
>        SHASH_DESC_ON_STACK(shash, tfm);
Nothing is wrong with that. I would have actually preferred that. But it 
would have fundamentally changed a lot more code.

I'm not sure making fundamental changes to the crypto code in order to 
make "my favourite compiler happy" is really a better plan. I prefer 
smaller more measured steps.

> or something along that line and hide all the stack allocation, type
> conversion crap and make my favourite compiler happy in a single
> place?
At this point it is less about making a particular compiler happy, and 
more about making the code at least C99 compliant which enables a 
different compiler so that more people can use it to make more 
fundamental changes like you are suggesting.

I appreciate your feedback,

Behan

-- 
Behan Webster
behanw@...verseincode.com

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