emu/riscv64.c

// https://docs.riscv.org/reference/isa/_attachments/riscv-unprivileged.pdf
// https://riscv.org/wp-content/uploads/2024/12/riscv-calling.pdf
#include <gelf.h>
#include <libelf.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define PARSE_I_INS(ins)                                                       \
  uint8_t rd = (ins >> 7) & 0b11111;                                           \
  uint8_t funct3 = (ins >> 12) & 0b111;                                        \
  uint8_t rs1 = (ins >> 15) & 0b11111;                                         \
  int32_t imm = ((int32_t)ins) >> 20

#define PARSE_R_INS(ins)                                                       \
  uint8_t rd = (ins >> 7) & 0b11111;                                           \
  uint8_t funct3 = (ins >> 12) & 0b111;                                        \
  uint8_t rs1 = (ins >> 15) & 0b11111;                                         \
  uint8_t rs2 = (ins >> 20) & 0b11111;                                         \
  int32_t funct7 = ((int32_t)ins) >> 25

#define PARSE_B_INS(ins)                                                       \
  uint8_t funct3 = (ins >> 12) & 0b111;                                        \
  uint8_t rs1 = (ins >> 15) & 0b11111;                                         \
  uint8_t rs2 = (ins >> 20) & 0b11111;                                         \
  int32_t imm_12 = (ins >> 31) & 0b1;                                          \
  int32_t imm_10_5 = (ins >> 25) & 0b111111;                                   \
  int32_t imm_4_1 = (ins >> 8) & 0b1111;                                       \
  int32_t imm_11 = (ins >> 7) & 0b1;                                           \
  int32_t imm =                                                                \
      (imm_12 << 12) | (imm_11 << 11) | (imm_10_5 << 5) | (imm_4_1 << 1);      \
  imm = (imm << 19) >> 19

#define PARSE_S_INS(ins)                                                       \
  uint8_t funct3 = (ins >> 12) & 0b111;                                        \
  uint8_t rs1 = (ins >> 15) & 0b11111;                                         \
  uint8_t rs2 = (ins >> 20) & 0b11111;                                         \
  int32_t imm_11_5 = (ins >> 25) & 0b1111111;                                  \
  int32_t imm_4_0 = (ins >> 7) & 0b11111;                                      \
  int32_t imm = (imm_11_5 << 5) | imm_4_0;                                     \
  imm = (imm << 20) >> 20

#define PARSE_J_INS(ins)                                                       \
  uint8_t rd = (ins >> 7) & 0b11111;                                           \
  int32_t imm_20 = (ins >> 31) & 0b1;                                          \
  int32_t imm_10_1 = (ins >> 21) & 0b1111111111;                               \
  int32_t imm_11 = (ins >> 20) & 0b1;                                          \
  int32_t imm_19_12 = (ins >> 12) & 0b11111111;                                \
  int32_t imm =                                                                \
      (imm_20 << 20) | (imm_19_12 << 12) | (imm_11 << 11) | (imm_10_1 << 1);   \
  imm = (imm << 11) >> 11

#define PARSE_U_INS(ins)                                                       \
  uint8_t rd = (ins >> 7) & 0b11111;                                           \
  int32_t imm = ins >> 12

static const char *const REGS[32] = {
    "zero", "ra", "sp", "gp", "tp",  "t0",  "t1", "t2", "fp", "s1", "a0",
    "a1",   "a2", "a3", "a4", "a5",  "a6",  "a7", "s2", "s3", "s4", "s5",
    "s6",   "s7", "s8", "s9", "s10", "s11", "t3", "t4", "t5", "t6"};

typedef struct {
  size_t offset;
  size_t size;
} Section;

typedef struct {
  uint8_t *memory;
  size_t pc;
  int64_t regs[32];
  Section code_section;
} RISCV64;

Section riscv64_get_code_section(uint8_t *exe_bytes, size_t exe_size) {
  Elf *elf = elf_memory((char *)exe_bytes, exe_size);
  if (!elf) {
    fprintf(stderr, "elf_begin failed: %s\n", elf_errmsg(-1));
    exit(1);
  }

  size_t str_table_index;
  if (elf_getshdrstrndx(elf, &str_table_index) != 0) {
    fprintf(stderr, "elf_getshdrstrndx failed: %s\n", elf_errmsg(-1));
    exit(1);
  }

  Elf_Scn *section = NULL;
  while ((section = elf_nextscn(elf, section)) != NULL) {
    GElf_Shdr header;
    if (gelf_getshdr(section, &header) != &header)
      continue;

    const char *name = elf_strptr(elf, str_table_index, header.sh_name);
    if (name && strcmp(name, ".text") == 0) {
      elf_end(elf);
      return (Section){.offset = header.sh_offset, .size = header.sh_size};
    }
  }

  fprintf(stderr, "Failed to locate .text\n");
  exit(1);
}

RISCV64 riscv64_load(uint8_t *exe_bytes, size_t exe_size) {
  Section section = riscv64_get_code_section(exe_bytes, exe_size);

  return (RISCV64){
      .memory = exe_bytes,
      .pc = section.offset,
      .regs = {0},
      .code_section = section,
  };
}

void riscv64_dump(const RISCV64 *r) {
  printf("REGS:");
  for (size_t i = 0; i < 32; i++) {
    printf(" %lu", r->regs[i]);
  }
  printf("\n");
}

void riscv64_disassemble_one(const RISCV64 *r) {
  uint32_t ins;
  memcpy(&ins, r->memory + r->pc, 4);

  uint16_t opcode = ins & 0b1111111;

  // https://stackoverflow.com/questions/62939410/how-can-i-find-out-the-instruction-format-of-a-risc-v-instruction
  switch (opcode) {
  case 0b1100011: {
    PARSE_B_INS(ins);

    if (funct3 == 0b100) {
      printf("blt %s, %s, %d\n", REGS[rs1], REGS[rs2], imm);
    } else if (funct3 == 0b101) {
      printf("bge %s, %s, %d\n", REGS[rs1], REGS[rs2], imm);
      break;
    } else {
      fprintf(stderr, "B-type: unrecognized funct3: %03b\n", funct3);
      exit(1);
    }
  }; break;
  case 0b0010011: {
    PARSE_I_INS(ins);

    if (funct3 == 0b000) {
      printf("addi %s, %s, %d\n", REGS[rd], REGS[rs1], imm);
    } else {
      fprintf(stderr, "I-type 1: unrecognized funct3: %03b\n", funct3);
      exit(1);
    }
  }; break;
  case 0b0000011: {
    PARSE_I_INS(ins);

    if (funct3 == 0b000) {
      printf("lb %s, %d(%s)\n", REGS[rd], imm, REGS[rs1]);
    } else {
      fprintf(stderr, "I-type 2: unrecognized funct3: %03b\n", funct3);
      exit(1);
    }
  }; break;
  case 0b1100111: {
    PARSE_I_INS(ins);
    printf("jalr %s, %s, %d\n", REGS[rd], REGS[rs1], imm);
  }; break;
  case 0b1110011: {
    PARSE_I_INS(ins);

    if (funct3 == 0b000) {
      if (imm == 0) {
        printf("ecall\n");
      } else {
        fprintf(stderr, "I-type 4: funct3=000 unrecognized imm: %b\n", imm);
        exit(1);
      }
      break;
    } else {
      fprintf(stderr, "I-type 4: unrecognized funct3: %03b\n", funct3);
      exit(1);
    }
  }; break;
  case 0b1101111: {
    PARSE_J_INS(ins);
    printf("jal %s, %d\n", REGS[rd], imm);
  }; break;
  case 0b0110011: {
    PARSE_R_INS(ins);
    if (funct3 == 0b000) {
      if (funct7 == 0) {
        printf("add %s, %s, %s\n", REGS[rd], REGS[rs1], REGS[rs2]);
      } else {
        fprintf(stderr, "R-type 1: funct3=0b000: unrecognized funct7: %b\n",
                funct7);
        exit(1);
      }
    } else {
      fprintf(stderr, "R-type 1: unrecognized funct3: %03b\n", funct3);
      exit(1);
    }
  }; break;
  case 0b0101111:
    fprintf(stderr, "R-type 2: unimplemented\n");
    exit(1);
  case 0b0111011:
    fprintf(stderr, "R-type 3: unimplemented\n");
    exit(1);
  case 0b0100011: {
    PARSE_S_INS(ins);

    if (funct3 == 0b000) {
      printf("sb %s, %d(%s)\n", REGS[rs2], imm, REGS[rs1]);
    } else {
      fprintf(stderr, "S-type: unrecognized funct3: %03b\n", funct3);
      exit(1);
    }
  }; break;
  case 0b0110111:
    fprintf(stderr, "U-type 1: unimplemented\n");
    exit(1);
    break;
  case 0b0010111: {
    PARSE_U_INS(ins);
    printf("auipc %s, %d\n", REGS[rd], imm);
  }; break;
  default:
    fprintf(stderr, "Unrecognized opcode: %07b\n", opcode);
    exit(1);
  }
}

void riscv64_execute(RISCV64 *r) {
  r->pc = r->code_section.offset;

  r->regs[2] = (20 * 1024 * 1024) - 1; // set the stack pointer

  while (r->pc < r->code_section.offset + r->code_section.size) {
    r->regs[0] = 0; // clear the zero register

    uint32_t ins;
    memcpy(&ins, r->memory + r->pc, 4);

    uint16_t opcode = ins & 0b1111111;

    // https://stackoverflow.com/questions/62939410/how-can-i-find-out-the-instruction-format-of-a-risc-v-instruction
    switch (opcode) {
    case 0b1100011: {
      PARSE_B_INS(ins);

      if (funct3 == 0b101) { // bge
        if (r->regs[rs1] >= r->regs[rs2]) {
          r->pc += imm;
          continue;
        }
      } else if (funct3 == 0b100) { // blt
        if (r->regs[rs1] < r->regs[rs2]) {
          r->pc += imm;
          continue;
        }
      } else {
        fprintf(stderr, "B-type: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b0010011: {
      PARSE_I_INS(ins);

      if (funct3 == 0b000) { // addi
        r->regs[rd] = r->regs[rs1] + imm;
      } else {
        fprintf(stderr, "I-type 1: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b0000011: {
      PARSE_I_INS(ins);

      if (funct3 == 0b000) { // lb
        r->regs[rd] = r->memory[r->regs[rs1] + imm];
      } else {
        fprintf(stderr, "I-type 2: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b1100111: { // jalr
      PARSE_I_INS(ins);
      uint32_t target = (r->regs[rs1] + imm) & ~1;
      r->regs[rd] = r->pc + 4;
      r->pc = target;
      continue;
    }; break;
    case 0b1110011: {
      PARSE_I_INS(ins);

      if (funct3 == 0b000) {
        if (imm == 0) { // ecall
          // https://jborza.com/post/2021-05-11-riscv-linux-syscalls/
          switch (r->regs[17]) {
          case 63: { // read
            if (r->regs[10] != 0) {
              fprintf(stderr, "read syscall implemented only for stdin.\n");
              exit(1);
            }

            size_t start = r->regs[11];
            size_t count = r->regs[12];
            size_t bytes_read = 0;

            for (size_t i = 0; i < count; i++) {
              int c = getchar();
              if (c == EOF)
                break;
              r->memory[start + i] = (uint8_t)c;
              bytes_read++;
            }

            r->regs[10] = bytes_read;
          }; break;
          case 64: { // write
            if (r->regs[10] != 1) {
              fprintf(stderr, "write syscall implemented only for stdout.\n");
              exit(1);
            }

            size_t start = r->regs[11];
            size_t end = r->regs[11] + r->regs[12];

            for (size_t i = start; i < end; i++) {
              putchar(r->memory[i]);
            }
          }; break;
          case 93: { // exit
            printf("Program exited with code %lu.\n", r->regs[10]);
            return;
          }; break;
          default:
            fprintf(stderr, "Unimplemented syscall: %lu\n", r->regs[17]);
            exit(1);
          }
          break;
        } else {
          fprintf(stderr, "I-type 4: funct3=000 unrecognized imm: %b\n", imm);
          exit(1);
        };
        break;
      } else {
        fprintf(stderr, "I-type 4: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b1101111: { // jal
      PARSE_J_INS(ins);
      r->regs[rd] = r->pc + 4;
      r->pc += imm;
      continue;
    }; break;
    case 0b0110011: {
      PARSE_R_INS(ins);
      if (funct3 == 0b000) {
        if (funct7 == 0) { // add
          r->regs[rd] = r->regs[rs1] + r->regs[rs2];
          break;
        } else {
          fprintf(stderr, "R-type 1: funct3=0b000: unrecognized funct7: %b\n",
                  funct7);
          exit(1);
        }
      } else {
        fprintf(stderr, "R-type 1: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b0100011: {
      PARSE_S_INS(ins);

      if (funct3 == 0b000) { // sb
        size_t addr = r->regs[rs1] + imm;
        r->memory[addr] = r->regs[rs2];
      } else {
        fprintf(stderr, "S-type: unrecognized funct3: %03b\n", funct3);
        exit(1);
      }
    }; break;
    case 0b0010111: { // auipc
      PARSE_U_INS(ins);
      r->regs[rd] = r->pc + (imm >> 12);
    }; break;
    default:
      fprintf(stderr, "Unrecognized opcode: %07b\n", opcode);
      exit(1);
    }

    r->pc += 4;
  }
}

int main(int argc, char *argv[]) {
  if (elf_version(EV_CURRENT) == EV_NONE) {
    fprintf(stderr, "Failed to initialize libelf: %s\n", elf_errmsg(-1));
    return 1;
  }

  const char *path = NULL;
  for (int i = 1; i < argc; i++) {
    path = argv[i];
  }

  if (path == NULL) {
    fprintf(stderr, "Usage: %s <path>\n", argv[0]);
    return 1;
  }

  FILE *file = fopen(path, "rb");
  if (!file) {
    fprintf(stderr, "Failed to open %s\n", path);
    return 1;
  }

  // probably enough for anything we can handle
  uint8_t *exe_bytes = malloc(20 * 1024 * 1024);
  size_t exe_size = fread(exe_bytes, 1, 20 * 1024 * 1024, file);
  fclose(file);

  RISCV64 r = riscv64_load(exe_bytes, exe_size);

  for (r.pc = r.code_section.offset;
       r.pc < r.code_section.offset + r.code_section.size; r.pc += 4) {
    riscv64_disassemble_one(&r);
  }
  printf("END DISASSEMBLY\n");

  riscv64_execute(&r);
}