Re: [PATCH qemu-server] query-machine-capabilities: check vendor string length for strncmp

[Kaiyang Wu <wukaiyang2003@gmail.com>]

Intel TDX and AMD SEV are both x86_64 only extensions [0] [1]. I wonder 
if the best approach for this issue is to just add a conditional 
compilation check for x86_64 target architecture.

If that works I will switch to compile time architecture check in v2.

[0]: https://docs.kernel.org/arch/x86/tdx.html
[1]: https://docs.kernel.org/virt/kvm/x86/amd-memory-encryption.html

On 2026-07-13 22:15, Fabian Grünbichler wrote:
> On June 3, 2026 7:50 am, Kaiyang Wu wrote:
>> Fix build error on unknown vendors ('fallback'):
>>
>> 	error: ‘strncmp’ of strings of length 7 and 12 and bound of 12 evaluates to nonzero [-Werror=string-compare]
>>
>> Signed-off-by: Kaiyang Wu <wukaiyang@loongfans.cn>
>> ---
>>   src/query-machine-capabilities/query-machine-capabilities.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/query-machine-capabilities/query-machine-capabilities.c b/src/query-machine-capabilities/query-machine-capabilities.c
>> index abb47acd..25d06db7 100644
>> --- a/src/query-machine-capabilities/query-machine-capabilities.c
>> +++ b/src/query-machine-capabilities/query-machine-capabilities.c
>> @@ -204,7 +204,7 @@ int main() {
>>           eprintf("Error writing to file '" OUTPUT_PATH "': %s\n", strerror(errno));
>>       }
>>   
>> -    if (strncmp(vendor, "AuthenticAMD", 12) == 0) {
>> +    if (strncmp(vendor, "AuthenticAMD", strnlen(vendor, 12)) == 0) {
> 
> this is wrong - if the vendor is "Authentic" the code would now think
> it's AMD.. the same would be true if vendor is "AuthenticAMDsomething",
> because it would only look at the first 12 bytes (granted, that is a
> pre-existing issue ;)).
> 
> instead, we'd first need to check for the length, and if it is 12, do
> the existing checks, if not, either add checks for other vendors, or
> reject/abort..
> 
>>           cpu_caps_amd_sev_t caps_sev;
>>           query_cpu_capabilities_sev(&caps_sev);
>>   
>> @@ -222,7 +222,7 @@ int main() {
>>               caps_sev.sev_es_support ? "true" : "false",
>>               caps_sev.sev_snp_support ? "true" : "false"
>>           );
>> -    } else if (strncmp(vendor, "GenuineIntel", 12) == 0) {
>> +    } else if (strncmp(vendor, "GenuineIntel", strnlen(vendor, 12)) == 0) {
> 
> same applies here, but with `Genuine` (or any other substring prefix of
> `GenuineIntel`)..
> 
>>           cpu_caps_intel_tdx_t caps_tdx;
>>           if (query_cpu_capabilities_tdx(&caps_tdx) == 0) {
>>               ret = fprintf(file,
>> -- 
>> 2.52.0
>>
>>
>>
>>
>>
>