exec.c

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "riscv.h"
#include "spinlock.h"
#include "proc.h"
#include "defs.h"
#include "elf.h"

static int loadseg(pde_t *, uint64, struct inode *, uint, uint);

int flags2perm(int flags)
{
    int perm = 0;
    if(flags & 0x1)
      perm = PTE_X;
    if(flags & 0x2)
      perm |= PTE_W;
    return perm;
}

int
exec(char *path, char **argv)
{
  char *s, *last;
  int i, off;
  uint64 argc, sz = 0, sp, ustack[MAXARG], stackbase;
  struct elfhdr elf;
  struct inode *ip;
  struct proghdr ph;
  pagetable_t pagetable = 0, oldpagetable;
  struct proc *p = myproc();

  begin_op();

  if((ip = namei(path)) == 0){
    end_op();
    return -1;
  }
  ilock(ip);

TIP

Check ELF header

  if(readi(ip, 0, (uint64)&elf, 0, sizeof(elf)) != sizeof(elf))
    goto bad;

  if(elf.magic != ELF_MAGIC)
    goto bad;

  if((pagetable = proc_pagetable(p)) == 0)
    goto bad;

TIP

Load program into memory.

  for(i=0, off=elf.phoff; i<elf.phnum; i++, off+=sizeof(ph)){
    if(readi(ip, 0, (uint64)&ph, off, sizeof(ph)) != sizeof(ph))
      goto bad;
    if(ph.type != ELF_PROG_LOAD)
      continue;
    if(ph.memsz < ph.filesz)
      goto bad;
    if(ph.vaddr + ph.memsz < ph.vaddr)
      goto bad;
    if(ph.vaddr % PGSIZE != 0)
      goto bad;
    uint64 sz1;
    if((sz1 = uvmalloc(pagetable, sz, ph.vaddr + ph.memsz, flags2perm(ph.flags))) == 0)
      goto bad;
    sz = sz1;
    if(loadseg(pagetable, ph.vaddr, ip, ph.off, ph.filesz) < 0)
      goto bad;
  }
  iunlockput(ip);
  end_op();
  ip = 0;

  p = myproc();
  uint64 oldsz = p->sz;

TIP

Allocate some pages at the next page boundary. Make the first inaccessible as a stack guard. Use the rest as the user stack.

  sz = PGROUNDUP(sz);
  uint64 sz1;
  if((sz1 = uvmalloc(pagetable, sz, sz + (USERSTACK+1)*PGSIZE, PTE_W)) == 0)
    goto bad;
  sz = sz1;
  uvmclear(pagetable, sz-(USERSTACK+1)*PGSIZE);
  sp = sz;
  stackbase = sp - USERSTACK*PGSIZE;

TIP

Push argument strings, prepare rest of stack in ustack.

  for(argc = 0; argv[argc]; argc++) {
    if(argc >= MAXARG)
      goto bad;
    sp -= strlen(argv[argc]) + 1;
    sp -= sp % 16;
    if(sp < stackbase)
      goto bad;
    if(copyout(pagetable, sp, argv[argc], strlen(argv[argc]) + 1) < 0)
      goto bad;
    ustack[argc] = sp;
  }
  ustack[argc] = 0;

TIP

push the array of argv[] pointers.

  sp -= (argc+1) * sizeof(uint64);
  sp -= sp % 16;
  if(sp < stackbase)
    goto bad;
  if(copyout(pagetable, sp, (char *)ustack, (argc+1)*sizeof(uint64)) < 0)
    goto bad;

TIP

arguments to user main(argc, argv) argc is returned via the system call return value, which goes in a0.

  p->trapframe->a1 = sp;

TIP

Save program name for debugging.

  for(last=s=path; *s; s++)
    if(*s == '/')
      last = s+1;
  safestrcpy(p->name, last, sizeof(p->name));
    

TIP

Commit to the user image.

  oldpagetable = p->pagetable;
  p->pagetable = pagetable;
  p->sz = sz;
  p->trapframe->epc = elf.entry;
  p->trapframe->sp = sp;
  proc_freepagetable(oldpagetable, oldsz);

  return argc;

 bad:
  if(pagetable)
    proc_freepagetable(pagetable, sz);
  if(ip){
    iunlockput(ip);
    end_op();
  }
  return -1;
}

TIP

Load a program segment into pagetable at virtual address va. va must be page-aligned and the pages from va to va+sz must already be mapped. Returns 0 on success, -1 on failure.

static int
loadseg(pagetable_t pagetable, uint64 va, struct inode *ip, uint offset, uint sz)
{
  uint i, n;
  uint64 pa;

  for(i = 0; i < sz; i += PGSIZE){
    pa = walkaddr(pagetable, va + i);
    if(pa == 0)
      panic("loadseg: address should exist");
    if(sz - i < PGSIZE)
      n = sz - i;
    else
      n = PGSIZE;
    if(readi(ip, 0, (uint64)pa, offset+i, n) != n)
      return -1;
  }
  
  return 0;
}