Re: [PATCH v5 2/3] x86/cpu: Check if feature string is non-zero

[Sohil Mehta]

On 2/13/2026 2:02 AM, Maciej Wieczor-Retman wrote:
> On 2026-02-12 at 16:28:10 -0800, Sohil Mehta wrote:
>> On 2/12/2026 7:35 AM, Maciej Wieczor-Retman wrote:
>>
>>>
>>> +const char *x86_cap_name(unsigned int bit)
>>> +{
>>> +	unsigned int word = bit >> 5;
>>> +	static char undef_buf[16];
>>> +	const char *name = NULL;
>>> +
>>> +	if (likely(word < NCAPINTS))
>>> +		name = x86_cap_flags[bit];
>>> +	else if (likely(word < NCAPINTS + NBUGINTS))
>>> +		name = x86_bug_flags[bit - 32 * NCAPINTS];
>>> +
>>> +	if (name)
>>> +		return name;
>>> +
>>> +	snprintf(undef_buf, sizeof(undef_buf), "%u:%u", word, bit & 31);
>>> +	return undef_buf;
>>> +}
>>> +
>>
>> Isn't it unsafe to return undef_buf because the pointer is local to this
>> function even though it is marked static?
>>
>> For example, the consecutive calls below would overwrite the value
>> stored at that address.
> 
> I just rechecked and after I caused that pr_warn_once() to be executed the
> strings are printed correctly.
> 

The issue would only happen if both d->feature and d->depends don't have
anything in x86_cap_flags[]. Was that true for your test?

> Looking a bit further down, the pointers of the returned const char* are two
> distinct values. So I assume because there are two calls to x86_cap_name() in
> the pr_warn_once() the two static undef_buf char arrays are statically allocated
> and there is no overwriting, but please correct me if that assumption is wrong.
> 

I don't think it works that way. As the variable is defined as "static",
only a single undef_buf is allocated. So the same pointer should be
returned after every call. I added the below dependency and it prints
the same feature twice.

"x86 CPU feature dependency check failure: CPU0 has '12:19' enabled but
'12:19' disabled. Kernel..."

diff --git a/arch/x86/kernel/cpu/cpuid-deps.c
b/arch/x86/kernel/cpu/cpuid-deps.c
index 1106a5476dca..51c482c18a2c 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -93,6 +93,7 @@ static const struct cpuid_dep cpuid_deps[] = {
        { X86_FEATURE_FRED,                     X86_FEATURE_LKGS      },
        { X86_FEATURE_SPEC_CTRL_SSBD,           X86_FEATURE_SPEC_CTRL },
        { X86_FEATURE_LASS,                     X86_FEATURE_SMAP      },
+       { X86_FEATURE_WRMSRNS,                  X86_FEATURE_FZRM},
        {}
 };


>>>  			pr_warn_once("x86 CPU feature dependency check failure: CPU%d has '%s' enabled but '%s' disabled. Kernel might be fine, but no guarantees.\n",
>>>  				     smp_processor_id(),
>>> -				     x86_feature_name(d->feature, feature_buf),
>>> -				     x86_feature_name(d->depends, depends_buf));
>>> +				     x86_cap_name(d->feature),
>>> +				     x86_cap_name(d->depends));
>>>  		}
>>>  	}
>>>  }
>