On Wed, 9 Jul 2025 12:01:14 +0200
Jens Remus <jre...@linux.ibm.com> wrote:
> > static int unwind_user_next(struct unwind_user_state *state)
> > {
> > - /* no implementation yet */
> > + struct unwind_user_frame *frame;
> > + unsigned long cfa = 0, fp, ra = 0;
> > + unsigned int shift;
> > +
> > + if (state->done)
> > + return -EINVAL;
> > +
> > + if (fp_state(state))
> > + frame = &fp_frame;
> > + else
> > + goto done;
> > +
> > + if (frame->use_fp) {
> > + if (state->fp < state->sp)
>
> if (state->fp <= state->sp)
>
> I meanwhile came to the conclusion that for architectures, such as s390,
> where SP at function entry == SP at call site, the FP may be equal to
> the SP. At least for the brief period where the FP has been setup and
> stack allocation did not yet take place. For most architectures this
> can probably only occur in the topmost frame. For s390 the FP is setup
> after static stack allocation, so --fno-omit-frame-pointer would enforce
> FP==SP in any frame that does not perform dynamic stack allocation.
From your latest email, I take it I can ignore the above?
>
> > + goto done;
> > + cfa = state->fp;
> > + } else {
> > + cfa = state->sp;
> > + }
> > +
> > + /* Get the Canonical Frame Address (CFA) */
> > + cfa += frame->cfa_off;
> > +
> > + /* stack going in wrong direction? */
> > + if (cfa <= state->sp)
> > + goto done;
> > +
> > + /* Make sure that the address is word aligned */
> > + shift = sizeof(long) == 4 ? 2 : 3;
> > + if ((cfa + frame->ra_off) & ((1 << shift) - 1))
> > + goto done;
>
> Do all architectures/ABI mandate register stack save slots to be aligned?
> s390 does.
I believe so.
>
> > +
> > + /* Find the Return Address (RA) */
> > + if (get_user(ra, (unsigned long *)(cfa + frame->ra_off)))
> > + goto done;
> > +
>
> Why not validate the FP stack save slot address as well?
You mean to validate cfa + frame->fp_off?
Isn't cfa the only real variable here? That is, if cfa + frame->ra_off
works, wouldn't the same go for frame->fp_off, as both frame->ra_off
and frame->fp_off are constants set by the architecture, and should be
word aligned.
-- Steve
>
> > + if (frame->fp_off && get_user(fp, (unsigned long __user *)(cfa +
> > frame->fp_off)))
> > + goto done;
> > +
