Re: [PATCH v13 1/3] x86/tdx: Add TDX Guest attestation interface driver

[Sathyanarayanan Kuppuswamy]

On 9/9/22 12:41 PM, Dave Hansen wrote:
> On 9/9/22 12:27, Kuppuswamy Sathyanarayanan wrote:
>> +	u8 reserved[7] = {0};
> ...
>> +	if (!req.reportdata || !req.tdreport || req.subtype ||
>> +		req.rpd_len != TDX_REPORTDATA_LEN ||
>> +		req.tdr_len != TDX_REPORT_LEN ||
>> +		memcmp(req.reserved, reserved, 7))
>> +		return -EINVAL;
> 
> Huh, so to look for 0's, you:
> 
> 1. Declare an on-stack structure with a hard coded, magic numbered field
>    that has to be zeroed.
> 2. memcmp() that structure
> 3. Feed memcmp() with another hard coded magic number
> 
> I've gotta ask: did you have any reservations writing this code?  Were
> there any alarm bells going off saying that something might be wrong?
> 
> Using memcmp() itself is probably forgivable.  But, the two magic
> numbers are pretty mortal sins in my book.  What's going to happen the
> first moment someone wants to repurpose a reserved byte?  They're going
> to do:
> 
> -	__u8 reserved[7];
> +	__u8 my_new_byte;
> +	__u8 reserved[6];
> 
> What's going to happen to the code you wrote?  Will it continue to work?
>  Or will the memcmp() silently start doing crazy stuff as it overruns
> the structure into garbage land?
> 
> What's wrong with:
> 
> 	memchr_inv(&req.reserved, sizeof(req.reserved), 0)

I did not consider the hard coding issue. It is a mistake. Your suggestion
looks better. I will use it.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer