llama : fix null pointer dereference by checking for rotation buffer in K-cache shifts

[engeldlgado]

Overview

Upstream AUTO-ACTIVATES Hadamard rotation (attn_rot_k) when the K cache is quantized and the head size is divisible by 64... In these cases, rot is not null and DOES NOT crash (with Qwen head-128 it does not reproduce). The actual window:

1. LLAMA_ATTN_ROT_DISABLE=1 (src/llama-kv-cache.cpp ~329) + quantized K + context shift -> crash.

2. Models with head size NOT divisible by 64 (there are: 80, 96, 112) + quantized K + context shift -> same crash, without touching any environment.

Mechanics: build_input_k_rot returns nullptr when attn_rot_k is false (~1413)... build_rope_shift calls llama_mul_mat_hadamard(ctx, tmp, rot) unconditionally on the quantized branch (~1862/1869)... the helper does rot->ne[0] without checking (src/llama-impl.h ~61). The fix: skip rotation when no rotation is configured. With attn_rot_k active, nothing changes.

To reproduce the error, you can run these commands in main and it should crash as it did for me.

LLAMA_ATTN_ROT_DISABLE=1 ./build/bin/llama-server \
  -m Qwen3-0.6B-Q4_0.gguf -ngl 0 -ctk q8_0 -c 512 --context-shift \
  --port 8199 --host 127.0.0.1

curl -s http://127.0.0.1:8199/completion -H "Content-Type: application/json" \
  -d '{"prompt":"Write a very long story about a lighthouse keeper.","n_predict":700,"ignore_eos":true,"temperature":0}'

Without fix: dies upon reaching the "slot context shift" (~450 tokens). With fix: completes the 700.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure:
  Yes, I used assistance to reproduce the error, but the fix and implementation are mine. I had to fix it to implement llama.cpp in the ToshLLM app for Intel Macs.

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