xv6/syscall.c

#include "types.h"
#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "mmu.h"
#include "proc.h"
#include "x86.h"
#include "syscall.h"
#include "syscalltable.h"

// User code makes a system call with INT T_SYSCALL.
// System call number in %eax.
// Arguments on the stack, from the user call to the C
// library system call function. The saved user %esp points
// to a saved program counter, and then the first argument.

// Fetch the int at addr from the current process.
int fetchint(uint addr, int *ip)  {
    struct proc *curproc = myproc();

    if (addr >= curproc->sz || addr + 4 > curproc->sz) {
        return -1;
    }
    *ip = *(int*)(addr);
    return 0;
}

// Fetch the nul-terminated string at addr from the current process.
// Doesn't actually copy the string - just sets *pp to point at it.
// Returns length of string, not including nul.
int fetchstr(uint addr, char **pp) {
    char *s, *ep;
    struct proc *curproc = myproc();

    if (addr >= curproc->sz) {
        return -1;
    }
    *pp = (char*)addr;
    ep = (char*)curproc->sz;
    for (s = *pp; s < ep; s++) {
        if (*s == 0) {
            return s - *pp;
        }
    }
    return -1;
}

// Fetch the nth 32-bit system call argument.
int argint(int n, int *ip) {
    return fetchint((myproc()->tf->esp) + 4 + 4 * n, ip);
}

// Fetch the nth word-sized system call argument as a pointer
// to a block of memory of size bytes.  Check that the pointer
// lies within the process address space.
int argptr(int n, char **pp, int size) {
    int i;
    struct proc *curproc = myproc();

    if (argint(n, &i) < 0) {
        return -1;
    }
    if (size < 0 || (uint)i >= curproc->sz || (uint)i + size > curproc->sz) {
        return -1;
    }
    *pp = (char*)i;
    return 0;
}

// Fetch the nth word-sized system call argument as a string pointer.
// Check that the pointer is valid and the string is nul-terminated.
// (There is no shared writable memory, so the string can't change
// between this check and being used by the kernel.)
int argstr(int n, char **pp) {
    int addr;
    if (argint(n, &addr) < 0) {
        return -1;
    }
    return fetchstr(addr, pp);
}

void syscall(void) {
    int num;
    struct proc *curproc = myproc();

    num = curproc->tf->eax;
    if (num > 0 && num < NELEM(syscalls) && syscalls[num]) {
        curproc->tf->eax = syscalls[num]();
    }
    else {
        cprintf("%d %s: unknown sys call %d\n",
                curproc->pid, curproc->name, num);
        curproc->tf->eax = -1;
    }
}