Re: [PATCH v1 5/6] x86/hyperv: Implement hypervisor ram collection into vmcore

[Mukesh R <mrathor@linux.microsoft.com>]

On 2/25/26 23:44, Ard Biesheuvel wrote:
> 
> On Wed, 25 Feb 2026, at 23:27, Mukesh R wrote:
>> On 2/21/26 08:43, Ard Biesheuvel wrote:
>>> Just spotted this code in v7.0-rc
>>>
>>> On Wed, 10 Sep 2025, at 02:10, Mukesh Rathor wrote:
>>> ...
>>>
>>>> +static asmlinkage void __noreturn hv_crash_c_entry(void)
>>>
>>> 'asmlinkage' means that the function may be called from another compilation unit written in assembler, but it doesn't actually evaluate to anything in most cases. Combining it with 'static' makes no sense whatsoever.
>>
>> 'static' means scope is limited to the file. Common in cases where function
>> pointers are used, like here in this file way below.
>>
>> Like the comment says:
>>       "This is the C entry point from the asm glue code after...."
>>
>> IOW, called from assembly function (asm == assembly).
>>
> 
> I wasn't asking you to explain what 'static' means. I was explaining to you that asmlinkage means 'external linkage' whereas 'static' means the opposite, and so combining them makes no sense.
> 
> 
>>>
>>>> +{
>>>> +	struct hv_crash_ctxt *ctxt = &hv_crash_ctxt;
>>>> +
>>>> +	/* first thing, restore kernel gdt */
>>>> +	native_load_gdt(&ctxt->gdtr);
>>>> +
>>>> +	asm volatile("movw %%ax, %%ss" : : "a"(ctxt->ss));
>>>> +	asm volatile("movq %0, %%rsp" : : "m"(ctxt->rsp));
>>>> +
>>>
>>> This code is truly very broken. You cannot enter a C function without a stack, and assign RSP half way down the function. Especially after allocating local variables and/or calling other functions - it may happen to work in most cases, but it is very fragile. (Other architectures have the concept of 'naked' functions for this purpose but x86 does not)
>>
>> Local variable refers to static bss struct. IOW,
>>
>>         asm volatile("movq %0, %%rsp" : : "m"(ctxt->rsp));
>>
>> same as:
>>         asm volatile("movq %0, %%rsp" : : "m"(&hv_crash_ctxt.rsp));
>>
>>
> 
> No, it is *not* the same. In practice, the compiler might perform this substitution, but there is no guarantee that this happens.
> 
> 
>>> IOW, this whole function should be written in asm.
>>>> +	asm volatile("movw %%ax, %%ds" : : "a"(ctxt->ds));
>>>> +	asm volatile("movw %%ax, %%es" : : "a"(ctxt->es));
>>>> +	asm volatile("movw %%ax, %%fs" : : "a"(ctxt->fs));
>>>> +	asm volatile("movw %%ax, %%gs" : : "a"(ctxt->gs));
>>>> +
>>>> +	native_wrmsrq(MSR_IA32_CR_PAT, ctxt->pat);
>>>> +	asm volatile("movq %0, %%cr0" : : "r"(ctxt->cr0));
>>>> +
>>>> +	asm volatile("movq %0, %%cr8" : : "r"(ctxt->cr8));
>>>> +	asm volatile("movq %0, %%cr4" : : "r"(ctxt->cr4));
>>>> +	asm volatile("movq %0, %%cr2" : : "r"(ctxt->cr4));
>>>> +
>>>> +	native_load_idt(&ctxt->idtr);
>>>> +	native_wrmsrq(MSR_GS_BASE, ctxt->gsbase);
>>>> +	native_wrmsrq(MSR_EFER, ctxt->efer);
>>>> +
>>>> +	/* restore the original kernel CS now via far return */
>>>> +	asm volatile("movzwq %0, %%rax\n\t"
>>>> +		     "pushq %%rax\n\t"
>>>> +		     "pushq $1f\n\t"
>>>> +		     "lretq\n\t"
>>>> +		     "1:nop\n\t" : : "m"(ctxt->cs) : "rax");
>>>> +
>>>> +	/* We are in asmlinkage without stack frame,
>>>
>>> You just switched to __KERNEL_CS via the stack.
>>
>> compiler doesn't know that.
>>
> 
> So? But does it means to 'be in asmlinkage' in your interpretation? Did you check what 'asmlinkage' actually evaluates to?
> 
> I am not asking you to justify why this broken code works in practice, I am asking you to fix it.


STOP bossing me! I am not your servant nor your slave. And you are not the
only genius around here.

Now, many people looked at this code before it was merged and no one really
thought any self respecting compiler in modern times would create an issue
here. Still, I see the remote possibility of that happening. All you had
to do was to show your concern and suggest using __naked here (which looks
like we all missed, or maybe it came after the code was written), and it
would have been addressed. This is x64 specific code for very special case
of hyperv or kernel-on-hyperv crashing.

In future if you choose to correspond, watch your tone!



>>>> hence make a C function
>>>> +	 * call which will buy stack frame to restore the tss or clear PT
>>>> entry.
>>>> +	 */
>>>
>>> Where does one buy a stack frame?
>>
>> A stack market :).  Callee will create stack frame now that rsp is
>> setup.
>>
> 
> This code is beyond broken. Please propose fixes rather than try to argue why carrying broken code like this is acceptable.