Re: [PATCH v2 11/13] x86/entry/fred: encode frame pointer on entry

[Xin Li]

> On May 22, 2026, at 3:25 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> 
> On May 20, 2026 3:24:57 PM PDT, David Stevens <stevensd@xxxxxxxxxx> wrote:
>> Setting aside the rest of the RFC, should this patch be merged? If
>> someone tries to use CONFIG_UNWINDER_FRAME_POINTER on a system with
>> FRED, they'll get a "kernel stack regs ... has bad 'bp' value" warning
>> in the logs the first time the unwinder runs.
>> 
>> Thanks,
>> David
>> 
>> On Fri, Apr 24, 2026 at 12:17 PM David Stevens <stevensd@xxxxxxxxxx> wrote:
>>> 
>>> Add missing ENCODE_FRAME_POINTER macro invocation into FRED_ENTER macro,
>>> to prevent the unwinder from encountering a NULL stack frame pointer
>>> when CONFIG_UNWINDER_FRAME_POINTER is enabled
>>> 
>>> Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
>>> Signed-off-by: David Stevens <stevensd@xxxxxxxxxx>
>>> ---
>>> arch/x86/entry/entry_64_fred.S | 2 ++
>>> 1 file changed, 2 insertions(+)
>>> 
>>> diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
>>> index 894f7f16eb80..119b8214748e 100644
>>> --- a/arch/x86/entry/entry_64_fred.S
>>> +++ b/arch/x86/entry/entry_64_fred.S
>>> @@ -7,6 +7,7 @@
>>> #include <linux/kvm_types.h>
>>> 
>>> #include <asm/asm.h>
>>> +#include <asm/frame.h>
>>> #include <asm/fred.h>
>>> #include <asm/segment.h>
>>> 
>>> @@ -19,6 +20,7 @@
>>>        UNWIND_HINT_END_OF_STACK
>>>        ANNOTATE_NOENDBR
>>>        PUSH_AND_CLEAR_REGS
>>> +       ENCODE_FRAME_POINTER
>>>        movq    %rsp, %rdi      /* %rdi -> pt_regs */
>>> .endm
>>> 
>>> --
>>> 2.54.0.rc2.544.gc7ae2d5bb8-goog
>>> 
>> 
> 
> Yes, it should.
> 
> The macroization is weird and I'm somewhat confused about how it is done but I also don't really care because it isn't like frame pointer use is common on x86-64.

I have the same patch in my private branch, but didn’t get motivated to send it.