SkewRows accepts skew values that make output width non-positive

[msg7086]

> Analysis attribution: This issue analysis was created by Codex GPT.
> The GitHub user who submits this issue does not claim authorship of the analysis.

Finding

• Finding ID: B11-skew-rows-output-width
• Status: Reproduced defect
• Severity: High
• Affected component: SkewRows
• Source location: avs_core/filters/misc.cpp:196-209

Summary

The constructor changes vi.width by skew and immediately uses it in height arithmetic and division without validating the resulting width.

Affected Code

SkewRows::SkewRows(PClip _child, int skew, IScriptEnvironment* env)
 : GenericVideoFilter(_child)
{
  if ((vi.NumComponents() > 1) && vi.IsPlanar())
    env->ThrowError("SkewRows: requires non-planar or greyscale input");

  if (vi.IsYUY2() && skew&1)
    env->ThrowError("SkewRows: For YUY2 skew must be even");

  vi.height *= vi.width;
  vi.width  += skew;
  vi.height += vi.width-1; // Ceiling
  vi.height /= vi.width;
}

Correct Behavior

Reject width + skew <= 0 before modifying VideoInfo or performing geometry arithmetic.

Reproduction

Use Y8 width=8 with skew=-9.

Observed Result

observed: construction succeeds
expected: AvisynthError

Impact

Invalid geometry reaches arithmetic and can cause division by zero or invalid VideoInfo.

Validation Criteria

Non-positive output widths fail; valid positive widths preserve geometry.

Version and Environment

• AviSynthPlus revision: 815780226951f95dfc1fc74f1df90ef98824fcf5
• Platform and compiler: Linux x86_64 / Clang 22 debug
• CPU features used: host-default build.