source: branches/minix3-book/servers/fs/protect.c@ 23

/* This file deals with protection in the file system.  It contains the code
 * for four system calls that relate to protection.
 *
 * The entry points into this file are
 *   do_chmod:  perform the CHMOD system call
 *   do_chown:  perform the CHOWN system call
 *   do_umask:  perform the UMASK system call
 *   do_access: perform the ACCESS system call
 *   forbidden: check to see if a given access is allowed on a given inode
 */

#include "fs.h"
#include <unistd.h>
#include <minix/callnr.h>
#include "buf.h"
#include "file.h"
#include "fproc.h"
#include "inode.h"
#include "param.h"
#include "super.h"

/*===========================================================================*
 *                              do_chmod                                     *
 *===========================================================================*/
PUBLIC int do_chmod()
{
/* Perform the chmod(name, mode) system call. */

  register struct inode *rip;
  register int r;

  /* Temporarily open the file. */
  if (fetch_name(m_in.name, m_in.name_length, M3) != OK) return(err_code);
  if ( (rip = eat_path(user_path)) == NIL_INODE) return(err_code);

  /* Only the owner or the super_user may change the mode of a file.
   * No one may change the mode of a file on a read-only file system.
   */
  if (rip->i_uid != fp->fp_effuid && !super_user)
        r = EPERM;
  else
        r = read_only(rip);

  /* If error, return inode. */
  if (r != OK)  {
        put_inode(rip);
        return(r);
  }

  /* Now make the change. Clear setgid bit if file is not in caller's grp */
  rip->i_mode = (rip->i_mode & ~ALL_MODES) | (m_in.mode & ALL_MODES);
  if (!super_user && rip->i_gid != fp->fp_effgid)rip->i_mode &= ~I_SET_GID_BIT;
  rip->i_update |= CTIME;
  rip->i_dirt = DIRTY;

  put_inode(rip);
  return(OK);
}

/*===========================================================================*
 *                              do_chown                                     *
 *===========================================================================*/
PUBLIC int do_chown()
{
/* Perform the chown(name, owner, group) system call. */

  register struct inode *rip;
  register int r;

  /* Temporarily open the file. */
  if (fetch_name(m_in.name1, m_in.name1_length, M1) != OK) return(err_code);
  if ( (rip = eat_path(user_path)) == NIL_INODE) return(err_code);

  /* Not permitted to change the owner of a file on a read-only file sys. */
  r = read_only(rip);
  if (r == OK) {
        /* FS is R/W.  Whether call is allowed depends on ownership, etc. */
        if (super_user) {
                /* The super user can do anything. */
                rip->i_uid = m_in.owner;        /* others later */
        } else {
                /* Regular users can only change groups of their own files. */
                if (rip->i_uid != fp->fp_effuid) r = EPERM;
                if (rip->i_uid != m_in.owner) r = EPERM;  /* no giving away */
                if (fp->fp_effgid != m_in.group) r = EPERM;
        }
  }
  if (r == OK) {
        rip->i_gid = m_in.group;
        rip->i_mode &= ~(I_SET_UID_BIT | I_SET_GID_BIT);
        rip->i_update |= CTIME;
        rip->i_dirt = DIRTY;
  }

  put_inode(rip);
  return(r);
}

/*===========================================================================*
 *                              do_umask                                     *
 *===========================================================================*/
PUBLIC int do_umask()
{
/* Perform the umask(co_mode) system call. */
  register mode_t r;

  r = ~fp->fp_umask;            /* set 'r' to complement of old mask */
  fp->fp_umask = ~(m_in.co_mode & RWX_MODES);
  return(r);                    /* return complement of old mask */
}

/*===========================================================================*
 *                              do_access                                    *
 *===========================================================================*/
PUBLIC int do_access()
{
/* Perform the access(name, mode) system call. */

  struct inode *rip;
  register int r;

  /* First check to see if the mode is correct. */
  if ( (m_in.mode & ~(R_OK | W_OK | X_OK)) != 0 && m_in.mode != F_OK)
        return(EINVAL);

  /* Temporarily open the file whose access is to be checked. */
  if (fetch_name(m_in.name, m_in.name_length, M3) != OK) return(err_code);
  if ( (rip = eat_path(user_path)) == NIL_INODE) return(err_code);

  /* Now check the permissions. */
  r = forbidden(rip, (mode_t) m_in.mode);
  put_inode(rip);
  return(r);
}

/*===========================================================================*
 *                              forbidden                                    *
 *===========================================================================*/
PUBLIC int forbidden(register struct inode *rip, mode_t access_desired)
{
/* Given a pointer to an inode, 'rip', and the access desired, determine
 * if the access is allowed, and if not why not.  The routine looks up the
 * caller's uid in the 'fproc' table.  If access is allowed, OK is returned
 * if it is forbidden, EACCES is returned.
 */

  register struct inode *old_rip = rip;
  register struct super_block *sp;
  register mode_t bits, perm_bits;
  int r, shift, test_uid, test_gid, type;

  if (rip->i_mount == I_MOUNT)  /* The inode is mounted on. */
        for (sp = &super_block[1]; sp < &super_block[NR_SUPERS]; sp++)
                if (sp->s_imount == rip) {
                        rip = get_inode(sp->s_dev, ROOT_INODE);
                        break;
                } /* if */

  /* Isolate the relevant rwx bits from the mode. */
  bits = rip->i_mode;
  test_uid = (call_nr == ACCESS ? fp->fp_realuid : fp->fp_effuid);
  test_gid = (call_nr == ACCESS ? fp->fp_realgid : fp->fp_effgid);
  if (test_uid == SU_UID) {
        /* Grant read and write permission.  Grant search permission for
         * directories.  Grant execute permission (for non-directories) if
         * and only if one of the 'X' bits is set.
         */
        if ( (bits & I_TYPE) == I_DIRECTORY ||
             bits & ((X_BIT << 6) | (X_BIT << 3) | X_BIT))
                perm_bits = R_BIT | W_BIT | X_BIT;
        else
                perm_bits = R_BIT | W_BIT;
  } else {
        if (test_uid == rip->i_uid) shift = 6;          /* owner */
        else if (test_gid == rip->i_gid ) shift = 3;    /* group */
        else shift = 0;                                 /* other */
        perm_bits = (bits >> shift) & (R_BIT | W_BIT | X_BIT);
  }

  /* If access desired is not a subset of what is allowed, it is refused. */
  r = OK;
  if ((perm_bits | access_desired) != perm_bits) r = EACCES;

  /* Check to see if someone is trying to write on a file system that is
   * mounted read-only.
   */
  type = rip->i_mode & I_TYPE;
  if (r == OK)
        if (access_desired & W_BIT)
                r = read_only(rip);

  if (rip != old_rip) put_inode(rip);

  return(r);
}

/*===========================================================================*
 *                              read_only                                    *
 *===========================================================================*/
PUBLIC int read_only(ip)
struct inode *ip;               /* ptr to inode whose file sys is to be cked */
{
/* Check to see if the file system on which the inode 'ip' resides is mounted
 * read only.  If so, return EROFS, else return OK.
 */

  register struct super_block *sp;

  sp = ip->i_sp;
  return(sp->s_rd_only ? EROFS : OK);
}