3--共享内存的实践到内核--撤销共享内存的映射和控制 - 如何从应用程序进入linux内
来源:百度文库 编辑:神马文学网 时间:2024/05/01 04:33:21
case SHMDT:
return sys_shmdt ((char __user *)ptr);
。。。。。。省略
case SHMCTL:
return sys_shmctl (first, second,
(struct shmid_ds __user *) ptr);
也就是看到应用程序与我们内核的系统调用函数参数是对正的,参数的含义我们就不用再重复介绍了,我们分别看一下sys_shmdt()函数和sys_shmctl()函数。
asmlinkage long sys_shmdt(char __user *shmaddr)
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma, *next;
unsigned long addr = (unsigned long)shmaddr;
loff_t size = 0;
int retval = -EINVAL;
if (addr & ~PAGE_MASK)
return retval;
down_write(&mm->mmap_sem);
/*
* This function tries to be smart and unmap shm segments that
* were modified by partial mlock or munmap calls:
* - It first determines the size of the shm segment that should be
* unmapped: It searches for a vma that is backed by shm and that
* started at address shmaddr. It records it's size and then unmaps
* it.
* - Then it unmaps all shm vmas that started at shmaddr and that
* are within the initially determined size.
* Errors from do_munmap are ignored: the function only fails if
* it's called with invalid parameters or if it's called to unmap
* a part of a vma. Both calls in this function are for full vmas,
* the parameters are directly copied from the vma itself and always
* valid - therefore do_munmap cannot fail. (famous last words?)
*/
/*
* If it had been mremap()'d, the starting address would not
* match the usual checks anyway. So assume all vma's are
* above the starting address given.
*/
vma = find_vma(mm, addr);
while (vma) {
next = vma->vm_next;
/*
* Check if the starting address would match, i.e. it's
* a fragment created by mprotect() and/or munmap(), or it
* otherwise it starts at this address with no hassles.
*/
if ((vma->vm_ops == &shm_vm_ops) &&
(vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff) {
size = vma->vm_file->f_path.dentry->d_inode->i_size;
do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
/*
* We discovered the size of the shm segment, so
* break out of here and fall through to the next
* loop that uses the size information to stop
* searching for matching vma's.
*/
retval = 0;
vma = next;
break;
}
vma = next;
}
/*
* We need look no further than the maximum address a fragment
* could possibly have landed at. Also cast things to loff_t to
* prevent overflows and make comparisions vs. equal-width types.
*/
size = PAGE_ALIGN(size);
while (vma && (loff_t)(vma->vm_end - addr) <= size) {
next = vma->vm_next;
/* finding a matching vma now does not alter retval */
if ((vma->vm_ops == &shm_vm_ops) &&
(vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff)
do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
vma = next;
}
up_write(&mm->mmap_sem);
return retval;
}
其实这段代码并不复杂,主要是注释多有点吓人,呵呵,如果有内核基础的朋友相信不难理解这段代码,但是对于其他朋友我们要说明一点,vma结构变量是表示虚拟空间的专用数据结构,这段代码首先是找到共享内存在进程中指定地址处的虚拟空间段,然后利用do_munmap()函数撤销对这个虚拟空间段的所有内存映射。do_munmap()函数我们还是在内存管理那部分分析吧,这里朋友们先暂且知道有这个一个函数,从函数名称上也能理解出他的作用。
我们再看另一个控制共享内存的函数sys_shmctl()
asmlinkage long sys_shmctl(int shmid, int cmd, struct shmid_ds __user *buf)
{
struct shmid_kernel *shp;
int err, version;
struct ipc_namespace *ns;
if (cmd < 0 || shmid < 0) {
err = -EINVAL;
goto out;
}
version = ipc_parse_version(&cmd);
ns = current->nsproxy->ipc_ns;
switch (cmd) { /* replace with proc interface ? */
case IPC_INFO:
{
struct shminfo64 shminfo;
err = security_shm_shmctl(NULL, cmd);
if (err)
return err;
memset(&shminfo,0,sizeof(shminfo));
shminfo.shmmni = shminfo.shmseg = ns->shm_ctlmni;
shminfo.shmmax = ns->shm_ctlmax;
shminfo.shmall = ns->shm_ctlall;
shminfo.shmmin = SHMMIN;
if(copy_shminfo_to_user (buf, &shminfo, version))
return -EFAULT;
down_read(&shm_ids(ns).rw_mutex);
err = ipc_get_maxid(&shm_ids(ns));
up_read(&shm_ids(ns).rw_mutex);
if(err<0)
err = 0;
goto out;
}
case SHM_INFO:
{
struct shm_info shm_info;
err = security_shm_shmctl(NULL, cmd);
if (err)
return err;
memset(&shm_info,0,sizeof(shm_info));
down_read(&shm_ids(ns).rw_mutex);
shm_info.used_ids = shm_ids(ns).in_use;
shm_get_stat (ns, &shm_info.shm_rss, &shm_info.shm_swp);
shm_info.shm_tot = ns->shm_tot;
shm_info.swap_attempts = 0;
shm_info.swap_successes = 0;
err = ipc_get_maxid(&shm_ids(ns));
up_read(&shm_ids(ns).rw_mutex);
if(copy_to_user (buf, &shm_info, sizeof(shm_info))) {
err = -EFAULT;
goto out;
}
err = err < 0 ? 0 : err;
goto out;
}
case SHM_STAT:
case IPC_STAT:
{
struct shmid64_ds tbuf;
int result;
if (!buf) {
err = -EFAULT;
goto out;
}
if (cmd == SHM_STAT) {
shp = shm_lock(ns, shmid);
if (IS_ERR(shp)) {
err = PTR_ERR(shp);
goto out;
}
result = shp->shm_perm.id;
} else {
shp = shm_lock_check(ns, shmid);
if (IS_ERR(shp)) {
err = PTR_ERR(shp);
goto out;
}
result = 0;
}
err=-EACCES;
if (ipcperms (&shp->shm_perm, S_IRUGO))
goto out_unlock;
err = security_shm_shmctl(shp, cmd);
if (err)
goto out_unlock;
memset(&tbuf, 0, sizeof(tbuf));
kernel_to_ipc64_perm(&shp->shm_perm, &tbuf.shm_perm);
tbuf.shm_segsz = shp->shm_segsz;
tbuf.shm_atime = shp->shm_atim;
tbuf.shm_dtime = shp->shm_dtim;
tbuf.shm_ctime = shp->shm_ctim;
tbuf.shm_cpid = shp->shm_cprid;
tbuf.shm_lpid = shp->shm_lprid;
tbuf.shm_nattch = shp->shm_nattch;
shm_unlock(shp);
if(copy_shmid_to_user (buf, &tbuf, version))
err = -EFAULT;
else
err = result;
goto out;
}
case SHM_LOCK:
case SHM_UNLOCK:
{
shp = shm_lock_check(ns, shmid);
if (IS_ERR(shp)) {
err = PTR_ERR(shp);
goto out;
}
err = audit_ipc_obj(&(shp->shm_perm));
if (err)
goto out_unlock;
if (!capable(CAP_IPC_LOCK)) {
err = -EPERM;
if (current->euid != shp->shm_perm.uid &&
current->euid != shp->shm_perm.cuid)
goto out_unlock;
if (cmd == SHM_LOCK &&
!current->signal->rlim[RLIMIT_MEMLOCK].rlim_cur)
goto out_unlock;
}
err = security_shm_shmctl(shp, cmd);
if (err)
goto out_unlock;
if(cmd==SHM_LOCK) {
struct user_struct * user = current->user;
if (!is_file_hugepages(shp->shm_file)) {
err = shmem_lock(shp->shm_file, 1, user);
if (!err && !(shp->shm_perm.mode & SHM_LOCKED)){
shp->shm_perm.mode |= SHM_LOCKED;
shp->mlock_user = user;
}
}
} else if (!is_file_hugepages(shp->shm_file)) {
shmem_lock(shp->shm_file, 0, shp->mlock_user);
shp->shm_perm.mode &= ~SHM_LOCKED;
shp->mlock_user = NULL;
}
shm_unlock(shp);
goto out;
}
case IPC_RMID:
case IPC_SET:
err = shmctl_down(ns, shmid, cmd, buf, version);
return err;
default:
return -EINVAL;
}
out_unlock:
shm_unlock(shp);
out:
return err;
}
static int shmctl_down(struct ipc_namespace *ns, int shmid, int cmd,
struct shmid_ds __user *buf, int version)
{
struct kern_ipc_perm *ipcp;
struct shmid64_ds shmid64;
struct shmid_kernel *shp;
int err;
if (cmd == IPC_SET) {
if (copy_shmid_from_user(&shmid64, buf, version))
return -EFAULT;
}
ipcp = ipcctl_pre_down(&shm_ids(ns), shmid, cmd, &shmid64.shm_perm, 0);
if (IS_ERR(ipcp))
return PTR_ERR(ipcp);
shp = container_of(ipcp, struct shmid_kernel, shm_perm);
err = security_shm_shmctl(shp, cmd);
if (err)
goto out_unlock;
switch (cmd) {
case IPC_RMID:
do_shm_rmid(ns, ipcp);
goto out_up;
case IPC_SET:
ipc_update_perm(&shmid64.shm_perm, ipcp);
shp->shm_ctim = get_seconds();
break;
default:
err = -EINVAL;
}
out_unlock:
shm_unlock(shp);
out_up:
up_write(&shm_ids(ns).rw_mutex);
return err;
}
在上面的代码中将会执行重要的函数do_shm_rmid()
static void do_shm_rmid(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
{
struct shmid_kernel *shp;
shp = container_of(ipcp, struct shmid_kernel, shm_perm);
if (shp->shm_nattch){
shp->shm_perm.mode |= SHM_DEST;
/* Do not find it any more */
shp->shm_perm.key = IPC_PRIVATE;
shm_unlock(shp);
} else
shm_destroy(ns, shp);
}