// User-memory access helpers — see include/uaccess.h for the rules.
//
// Everything here validates against the *current* process's page table
// (myproc()->pgdir), which is also the live CR3 while a syscall runs, so a
// range that passes access_ok() can be dereferenced directly by kernel
// code for the remainder of the syscall (single-threaded processes can't
// unmap it out from under us mid-call).

#include "types.h"
#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "mmu.h"
#include "proc.h"
#include "errno.h"
#include "uaccess.h"

int
access_ok(addr_t uaddr, uint64 size)
{
  addr_t a, end;

  if(size == 0)
    return 1;
  end = uaddr + size;
  if(end < uaddr || end > KERNBASE)
    return 0;
  for(a = PGROUNDDOWN(uaddr); a < end; a += PGSIZE)
    if(uva2ka(myproc()->pgdir, (char*)a) == 0)
      return 0;
  return 1;
}

void*
user_ptr(addr_t uaddr, uint64 size)
{
  if(uaddr == 0 || !access_ok(uaddr, size))
    return 0;
  return (void*)uaddr;
}

int
copy_from_user(void *dst, addr_t uaddr, uint64 n)
{
  if(uaddr + n < uaddr || uaddr + n > KERNBASE)
    return -EFAULT;
  if(copyin(myproc()->pgdir, uaddr, dst, n) < 0)
    return -EFAULT;
  return 0;
}

int
copy_to_user(addr_t uaddr, const void *src, uint64 n)
{
  if(uaddr + n < uaddr || uaddr + n > KERNBASE)
    return -EFAULT;
  if(copyout(myproc()->pgdir, uaddr, (void*)src, n) < 0)
    return -EFAULT;
  return 0;
}

long
strlen_user(addr_t uaddr)
{
  addr_t a = uaddr;

  while(a - uaddr < USER_STR_MAX){
    // Scan through the per-page kernel alias so an unmapped page is a
    // clean failure instead of a fault.
    char *ka = uva2ka(myproc()->pgdir, (char*)a);
    if(ka == 0)
      return -EFAULT;
    addr_t pgend = PGROUNDDOWN(a) + PGSIZE;
    for(; a < pgend; a++, ka++)
      if(*ka == 0)
        return a - uaddr;
  }
  return -EFAULT;
}