On Thu, Jul 17, 2025 at 01:28:25PM +0200, Jens Remus wrote:
> >> }
> >>
> >> /* Get the Frame Pointer (FP) */
> >> - if (frame->fp_off && unwind_get_user_long(fp, cfa + frame->fp_off,
> >> state))
> >> + switch (frame->fp.loc) {
> >> + case UNWIND_USER_LOC_NONE:
> >> + break;
> >
> > The UNWIND_USER_LOC_NONE behavior is different here compared to above.
>
> See my comments below.
>
> > Do we also need UNWIND_USER_LOC_PT_REGS?
>
> Sorry, I cannot follow. Do you suggest to rename UNWIND_USER_LOC_REG to
> UNWIND_USER_LOC_PT_REGS?
I think I completely misunderstood the meaning of UNWIND_USER_LOC_NONE.
Never mind :-)
> >> + case UNWIND_USER_LOC_STACK:
> >> + if (!frame->fp.frame_off)
> >> + goto done;
> >> + if (unwind_get_user_long(fp, cfa + frame->fp.frame_off, state))
> >> + goto done;
> >> + break;
> >> + case UNWIND_USER_LOC_REG:
> >> + if (!IS_ENABLED(CONFIG_HAVE_UNWIND_USER_LOC_REG) || !topmost)
> >> + goto done;
> >
> > The topmost checking is *really* getting cumbersome, I do hope we can
> > get rid of that.
>
> Restoring from arbitrary registers is only valid in the topmost frame,
> as their values (i.e. task_pt_regs(current)) are only available there.
> For other frames only SP, FP, and RA register values are available.
>
> I think this test makes sense. Is this test really that expensive?
ra_off=0 (UNWIND_USER_LOC_NONE) on a !topmost frame should never happen
unless the sframe entry is bad. But 0 is *far* from the only potential
bad RA offset value. In the absolute worst case of a 4 byte offset,
there are 4+ billion other possible bad values that can still go
undetected.
So I question the usefulness of those !topmost tests. And they do add
complexity to the code.
--
Josh
