How exec() works

Split the flow into 2 phases.

Overview

Split the flow into 4 phases:

1. Read args.

2. Load program from disk to memory.

3. Allocate stacks.

4. Prepare the new images.

Flow

Find inode by path
Check ELF header
Load program by memory. uvmalloc to allocate new size.
loadseg load file to memory address at ph.vaddr. Note: Exec loads bytes from the ELF file into memory at addresses specified by the ELF file.
Allocate 2 pages. 2nd one is user stack.
Push args to stack. Prepare ustack to save each argument with its address.
Push the array of argv[] pointers (ustack) to stack.
Set sp to a1.
Commit to user image. Free old process’ page table.
Set epc, sp, sz.
return argc which set argc in reg a0. Now we have main(args, args) from entry, a0, a1

Read User Args

uint64
sys_exec(void)
{
  char path[MAXPATH], *argv[MAXARG];
  int I;
  uint64 uargv, uarg;

  if(argstr(0, path, MAXPATH) < 0 || argaddr(1, &uargv) < 0){
    return -1;
  }
  memset(argv, 0, sizeof(argv));
  for(I=0;; I++){
    if(i >= NELEM(argv)){
      goto bad;
    }
    if(fetchaddr(uargv+sizeof(uint64)*I, (uint64*)&uarg) < 0){
      goto bad;
    }
    if(uarg == 0){
      argv[I] = 0;
      break;
    }
    argv[I] = kalloc();
    if(argv[i] == 0)
      panic("sys_exec kalloc");
    if(fetchstr(uarg, argv[i], PGSIZE) < 0){
      goto bad;
    }
  }

  int ret = exec(path, argv);

The argv is a pointer to list of arguments from user space.

Each pointer in RISC-V is 64 bits.

argaddr(1, &uargv) is to get the base pointer. Then, do fetchaddr(uargv+sizeof(uint64)*I, (uint64*)&uarg) in a loop to find each argument’s address.

fetchstr(uarg, argv[i], PGSIZE) is to load the string in argv[I].

Fetch address and string

// Fetch the uint64 at addr from the current process.
int
fetchaddr(uint64 addr, uint64 *ip)
{
  struct proc *p = myproc();
  if(addr >= p->sz || addr+sizeof(uint64) > p->sz)
    return -1;
  if(copyin(p->pagetable, (char *)ip, addr, sizeof(*ip)) != 0)
    return -1;
  return 0;
}

// Fetch the nul-terminated string at addr from the current process.
// Returns length of string, not including nul, or -1 for error.
int
fetchstr(uint64 addr, char *buf, int max)
{
  struct proc *p = myproc();
  int err = copyinstr(p->pagetable, buf, addr, max);
  if(err < 0)
    return err;
  return strlen(buf);
}

copyinstr Copy a null-terminated string from user to kernel.

Load program and Allocate pages

Check ELF header
Load program into memory
Allocate 2 pages, one for user stack. Another for guard page.

Note: this graph has a bug. The following 3 items should not exist: argv, argc, 0xFFFFFF

Prepare the new image

Push argument strings to stack. Prepare rest of stack.

Push the array of argv[] pointers to stack.

Set argv in a1 register.

Set process info, including pagetable, sz

Set epc = elf.entry, initial program counter to main initial stack pointer.

Return argc, in RSIC-V, this sets argc in register a0. So arguments main(argc, argv) is set.

The jump preparation is done. Program will go to main, and get args set properly.

How stack look after pushing args?

Core `exec` code for reference

int
exec(char *path, char **argv)
{
  char *s, *last;
  int i, off;
  uint64 argc, sz, sp, ustack[MAXARG+1], stackbase;
  struct elfhdr elf;
  struct inode *ip;
  struct proghdr ph;
  pagetable_t pagetable = 0, oldpagetable;
  struct proc *p = myproc();

  begin_op(ROOTDEV);

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

  // 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;

  // Load program into memory.
  sz = 0;
  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((sz = uvmalloc(pagetable, sz, ph.vaddr + ph.memsz)) == 0)
      goto bad;
    if(ph.vaddr % PGSIZE != 0)
      goto bad;
    if(loadseg(pagetable, ph.vaddr, ip, ph.off, ph.filesz) < 0)
      goto bad;
  }
  iunlockput(ip);
  end_op(ROOTDEV);
  ip = 0;

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

  // Allocate two pages at the next page boundary.
  // Use the second as the user stack.
  sz = PGROUNDUP(sz);
  if((sz = uvmalloc(pagetable, sz, sz + 2*PGSIZE)) == 0)
    goto bad;
  uvmclear(pagetable, sz-2*PGSIZE);
  sp = sz;
  stackbase = sp - PGSIZE;

  // 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; // riscv sp must be 16-byte aligned
    if(sp < stackbase)
      goto bad;
    if(copyout(pagetable, sp, argv[argc], strlen(argv[argc]) + 1) < 0)
      goto bad;
    ustack[argc] = sp;
  }
  ustack[argc] = 0;

  // 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;

  // arguments to user main(argc, argv)
  // argc is returned via the system call return
  // value, which goes in a0.
  p->tf->a1 = sp;

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

  // Commit to the user image.
  oldpagetable = p->pagetable;
  p->pagetable = pagetable;
  p->sz = sz;