# DF-1957 Verification

## Verdict
**SOURCE-CONFIRMED, INCONCLUSIVE-RUNTIME (HW/module gated).**

The cited defect exists in the audited source at `sys/dev/drm/drm_rect.c:53-119`. Reproduction
on the running guest is **not possible** because the affected code path is
gated behind hardware that is not present in the audit QEMU/KVM guest
(no AMD/i915 GPU, no LSI MegaRAID, no MMC/SDHCI controller, no FireWire, no
ATAPI floppy, etc.) and/or lives in a kernel module that is not loaded on the
GENERIC-running guest.

## Mechanism (source-only confirmation)
drm core (used by all DRM drivers, but DRM itself is loaded as module). Source: clip_scaled at L53-65 computes DIV_ROUND_UP/DOWN_ULL(tmp,dst) at L62/64. drm_rect_clip_scaled at L85-92 calls clip_scaled with drm_rect_width(dst)=dst->x2-dst->x1=0 when crtc_w=0. No zero-check on dst → divide by zero. Upstream drm_atomic_plane_check (drm_atomic.c) does NOT validate crtc_w!=0.

## Recommended fix
Add `if (dst == 0) return 0;` at the top of clip_scaled.

The full `git apply`-able diff lives in `fix.diff` in this folder; it was
applied as part of a single combined 41-finding kernel build that compiled
cleanly (rc=0, -Werror clean) — see `../fix_build_summary.txt`.

## Build validation
* `git apply --check` on this fix.diff: **OK**
* Combined kernel build (`X86_64_GENERIC`, INVARIANTS ON) with all 41
  findings' fix.diffs applied: **rc=0**, no warnings, no errors.
* The patched kernel was not booted/run because the affected code path
  requires hardware that the audit guest does not have.