# Re: \[RFC PATCH v1 08/16\] unwind\_user: Enable archs that save RA/FP in other registers

- From: Josh Poimboeuf \<jpoimboe@kernel.org\>
- Date: 2025-07-17 07:24:50 
- List: linux-trace-kernel
- Page: https://ratatoskr.run/linux-trace-kernel/2025/07/3138160
- Thread: https://ratatoskr.run/linux-trace-kernel/2025/07/3129870/t.md

```text
On Wed, Jul 16, 2025 at 11:57:51PM -0400, Steven Rostedt wrote:
> On Wed, 16 Jul 2025 19:01:06 -0700
> Josh Poimboeuf [ref] wrote:
> 
> > > +		if (unwind_user_get_reg(&ra, frame->ra.regnum))
> > > +			goto done;
> > > +		break;
> > > +	default:
> > > +		WARN_ON_ONCE(1);
> > > +		goto done;  
> > 
> > The default case will never happen, can we make it a BUG()?
> 
> Is this really serious enough to crash the system? WARN_ON_ONCE() *is* for
> things that will never happen.
> 
> The only time I ever use BUG() is if it's too dangerous to continue (like a
> function graph trampoline that gets corrupted and has no place to return
> to). In general, usage of BUG() should be avoided.

This is an unreachable code path, but __builtin_unreachable() is crap
due to undefined behavior.  IMO, BUG() for unreachable paths is cleaner
than WARN_ON_ONCE(), but it doesn't matter much either way I suppose.

-- 
Josh
```