Re: [PATCH v8 08/21] x86/virt/seamldr: Allocate and populate a module update request

[Dave Hansen]

On 5/7/26 06:19, Chao Gao wrote:
...
>>> +	/*
>>> +	 * Don't care about user passing the wrong file, but protect
>>> +	 * kernel ABI by preventing accepting garbage.
>>> +	 */
>>> +	if (memcmp(blob->signature, "TDX-BLOB", 8))
>>> +		return ERR_PTR(-EINVAL);
>>
>> Is there really no helper in the kernel anywhere that can safely do the
>> 8-byte compare against two known-to-the-compiler 8-byte-wide fields
>> without hard-coding the 8?
> 
> I couldn't find a helper that automatically derives the comparison
> length from the operands.  'strcmp()' is not suitable here because
> 'blob->signature' is not NUL-terminated.
> 
> Do you mean just avoiding the hard-coded 8, e.g.
> 
> 	if (memcmp(blob->signature, "TDX-BLOB", sizeof(blob->signature)))
> 		return ERR_PTR(-EINVAL);
> 
> or define the 'u8 signature[8]' as a u64 and compare it with a constant, like
> 
> /* Little-endian encoding of "TDX-BLOB" string */
> #define TDX_IMAGE_SIGNATURE	0x424f4c422d584454ULL
> 
> 	if (blob->signature != TDX_IMAGE_SIGNATURE)
> 		return ERR_PTR(-EINVAL);

Either one of those is fine with me. I'd probably do the sizeof()
variant, but no strong preference.

>>> +	struct seamldr_params *params;
>>> +	int module_pg_cnt, sig_pg_cnt;
>>> +	const u8 *sig, *module;
>>> +	int i;
>>> +
>>> +	params = (struct seamldr_params *)get_zeroed_page(GFP_KERNEL);
>>> +	if (!params)
>>> +		return ERR_PTR(-ENOMEM);
>>
>> kzmalloc(PAGE_SIZE, GFP_KERNEL) will save you a cast.
> 
> I noticed that 'kzalloc_obj()' can be used here, which avoids spelling out
> the size and GFP flags explicitly.  So I ended up with:
> 
>     params = kzalloc_obj(*params);

That's fine too.