RISC-V: execution starts from an odd PC (IALIGN=16 violation)

RISC-V: execution starts from an odd PC (IALIGN=16 violation)

Rechecked revision

The report was rechecked against the public Unicorn dev branch at 938efd13aeab73155f31cf707a2094c6ccaa36dd.

API boundary

Legal RISC-V control transfers obey IALIGN. This report targets the Unicorn public entry boundary: a caller can pass an odd begin address to uc_emu_start or write an odd UC_RISCV_REG_PC, after which the fetch path currently decodes from that address. The reproduction therefore tests an externally supplied invalid emulator state, not an odd target produced by a legal guest branch. If maintainers prefer rejecting the state in an API setter rather than at fetch time, the same public witness can be used for that design.

Public reproducer

The minimal Python witness is available at https://github.com/carlosqwqqwq/unicorn-riscv-repros/tree/main/u025. It starts the public U54 model at 0x10001 and records the executed PCs and the returned Unicorn error.

Expected behavior

The first fetch from an odd address must raise the instruction-address- misaligned exception before any guest instruction executes. An aligned entry point and legal compressed/32-bit streams must remain unchanged.

Root cause and proposed fix

riscv_tr_translate_insn fetches at ctx->base.pc_next without checking bit 0. The proposed change checks the address before fetch and raises the existing instruction-address-misaligned exception. The focused regression is included in PR #2394.