Re: [PATCH v3 8/8] unwind: arm64: Use sframe to unwind interrupt frames.

[Jens Remus <jremus@linux.ibm.com>]

On 4/20/2026 7:56 AM, Dylan Hatch wrote:
> On Fri, Apr 17, 2026 at 8:45 AM Jens Remus <jremus@linux.ibm.com> wrote:

>>> +     case UNWIND_CFA_RULE_REG_OFFSET:
>>> +     case UNWIND_CFA_RULE_REG_OFFSET_DEREF:
>>> +             if (!regs)
>>
>>                 if (!regs || frame.cfa.regnum > 30)
>>
>>> +                     return -EINVAL;
>>> +             cfa = regs->regs[frame.cfa.regnum];
>>
>> In unwind user this is guarded by a topmost frame check, as arbitrary
>> registers are otherwise not available.  Isn't this necessary in the
>> kernel case?
> 
> It is necessary, though as you point out the way I wrote the check is
> not as obvious as it probably should be.
> 
> The saved state->regs is set when the current frame is recovered from
> the saved PC of a struct pt_regs, and then immediately set back to
> NULL after the next frame has been recovered. In other words, the
> state->regs is only ever set when it is relevant to the current frame,
> which occurs when state->source == KUNWIND_SOURCE_REGS_PC. This only
> happens when the topmost frame is recovered from a pt_regs, or when a
> pt_regs is recovered from the stack due to an interrupt.
> 
> I can make this more readable by adding an explicit check for
> KUNWIND_SOURCE_REGS_PC in addition to state->regs != NULL.

Thanks for the explanation!  Maybe just add an explanation to the commit
message and a short comment above the (!regs) test?

/* regs only available in topmost frame */

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
jremus@de.ibm.com / jremus@linux.ibm.com

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