Bound SV2 extended-channel extranonce negotiation

[Distortions81]

Summary

• Restrict extended-channel extranonce negotiation to the safe 2-32 byte range.
• Require exact success-message framing and validate parser outputs.
• Add defensive bounds and conversion checks in job creation and share submission.
• Test 0 through 65535, including truncation and trailing data.

Root cause and impact

The pool supplies extranonce_size as 16 bits. It was accepted without a bound, narrowed to eight bits, and later used with fixed 32-byte stack arrays. Values above 32 could cause out-of-bounds access, while values such as 256 changed meaning during narrowing.

Valid 2-32 byte extended channels are unchanged. No authentication or network-policy change is added.

Related work

• Add Stratum V2 (SV2) protocol support #1553 introduced the SV2 extended-channel path hardened here.
• fix: reduce sv2 minimum extranonce size from 6->2 bytes #1799 reduced the valid minimum extranonce to two bytes; this preserves that minimum while enforcing existing 32-byte storage.

Validation

• ESP-IDF 6.0.2 firmware and test builds passed
• Exact ESP32-S3 QEMU environment: 76 passed, 0 failed
• Focused parser and builder boundaries
• git diff --check
• Current GitHub checks are green

An encrypted SV2 pool smoke test remains desirable.

[github-actions]

Test Results

  2 files    2 suites   1s ⏱️
124 tests 124 ✅ 0 💤 0 ❌
126 runs  126 ✅ 0 💤 0 ❌

Results for commit 215077d.

[0xf0xx0]

code ack