inline assembly snippets for pintOS

pintos-uk-copy.md

A sample and extended code snippnet for copying data between kernel and userland. please refer to 3.1.5 Accessing User Memory(https://web.stanford.edu/~ouster/cgi-bin/cs140-spring18/pintos/pintos_3.html#SEC39) for more details.

uk-copy.c

/**
 * @brief Get a user byte 
 * 
 * @param uaddr user address to get byte from 
 * @return char user byte obtained, can be -1 on segfault.
 * 
 * please note that it's impossible to distinguish between real -1 and segment -1,
 * outside of this function's context.
 */
static char
get_user_byte (const uint8_t *uaddr)
{
  int result;
  asm ("movl $1f, %0; movzbl %1, %0; 1:"
       : "=&a" (result) : "m" (*uaddr));
  return result;
}

/**
 * @brief Get a user dword
 * 
 * @param uaddr user address to get byte from 
 * @return int user dword obtained, can be -1 on segfault.
 * 
 * please note that it's impossible to distinguish between real -1 and segment -1,
 * outside of this function's context.
 */
static int
get_user_dword (const uint32_t *uaddr)
{
  int result;
  asm ("movl $1f, %0; movl %1, %0; 1:"
       : "=&a" (result) : "m" (*uaddr));
  return result;
}

/**
 * @brief Get the user string
 * 
 * @param uaddr user address to copy from
 * @param kaddr kernel address to copy to
 * @param length maximum copy length
 * @return true succeeded to get user string
 * @return false segmentation fault occured
 * 
 * please note that `rep` prefixes check for ecx == 0 before they decrement.
 */
static bool 
get_user_str (char *uaddr, char *kaddr, size_t length)
{
  int result;
  asm ("cld; movl $1f, %0; rep movsb; movl $0, %%eax; 1:"
       : "=&a" (result), "+S" (uaddr), "+D" (kaddr), "+c" (length) : : "memory");
  return result == 0;
}

/**
 * @brief Put string to userland
 * 
 * @param uaddr user address to copy to
 * @param kaddr kernel address to copy from
 * @param length maximum copy length
 * @return true succeeded to put string to userland
 * @return false segmentation fault occured
 * 
 * please note that `rep` prefixes check for ecx == 0 before they decrement. 
 */
static bool 
put_user_str (char *uaddr, char *kaddr, size_t length)
{
  int result;
  asm("cld; movl $1f, %0; rep movsb; movl $0, %%eax; 1:"
       : "=&a" (result), "+D" (uaddr), "+S" (kaddr), "+c" (length) : : "memory");
  return result == 0;
}

/**
 * @brief scan user string, finding byte. similar to strchr()
 * 
 * @param uaddr user address to start searching
 * @param byte the byte to search
 * @param maxlen maximum search length
 * @return char* pointer to the byte found. NULL if not found. -1 on segmentation fault
 * 
 * please note that `loop` instructions, unlike `rep` instructions, decrement eax before they check.
 * using scasb will really simplify this into a simple `cld; repne scasb` but scans are designed to work with eax/ax/al. 
 */
static char * 
scan_user_str (char *uaddr, const uint8_t byte, size_t maxlen)
{
  char * result;
  asm("movl $1f, %0; test %%ecx, %%ecx; jz 3f; \n" //if maxlen == 0 then GOTO 3f; fi
      "2: movzbl (%%esi), %%edx; cmp %%edx, %%ebx; jz 4f; inc %%esi; loop 2b;\n" //while(maxlen) if *uaddr == byte then GOTO 4f; else then uaddr++; maxlen--; fi
      "3: xor %%eax, %%eax; jmp 1f; \n" //return 0;
      "4: movl %%esi, %%eax; 1:" //return uaddr;
       : "=&a" (result), "+S" (uaddr), "+c" (maxlen): "b"(byte) : "memory", "edx");
  return result;
}

/**
 * @brief copy string from userland to kernel, stop on meeting byte.
 * 
 * @param uaddr user address to copy from
 * @param kaddr kernel address to copy to
 * @param byte terminator byte
 * @param maxlen maximum size to copy
 * @return size_t size actually copied, including the byte found. -1 on segmentation fault
 * 
 * please note that `loop` instructions, unlike `rep` instructions, decrement eax before they check.
 * using scasb will really simplify this into a simple `cld; repne scasb` but scans are designed to work with eax/ax/al. 
*/
static size_t
scan_copy_user_str (char *uaddr, char *kaddr, const uint8_t byte, size_t maxlen)
{
  size_t result, leftover;
  asm("movl $1f, %0; test %%ecx, %%ecx; jz 1f; \n" //if maxlen == 0 then goto 1f; fi
      "2: movzbl (%%esi), %%edx; movb %%dl, (%%edi); cmp %%edx, %%ebx; jz 3f; \n"
      "inc %%esi; inc %%edi; loop 2b; jmp 1f; \n" // while(maxlen) *kaddr = *uaddr; if *uaddr == byte then GOTO 3f; else then uaddr++; kaddr++; maxlen--; fi GOTO 1f;  
      "3: dec %%ecx; 1:" //dec maxlen;
       : "=&a" (result), "+S" (uaddr), "+D" (kaddr), "=c" (leftover)
       : "c" (maxlen), "b"(byte) : "memory", "edx");
  return result == 0xffffffff ? -1 : maxlen - leftover;
}