Friends who are involved in kernel development might be familiar with the code snippet below.
In typical scenarios, we allocate memory within the `xxx_open` function, as shown in the example below:
```cpp
file->private_data = kmalloc(sizeof(struct xxx), GFP_KERNEL);
```
This allows us to access the associated data for the file through `file->private_data` during subsequent read, write, or ioctl operations. Finally, in `xxx_release`, we release the memory pointed to by `file->private_data`.
If only these standard operations access the data, there is generally no problem because the kernel's file system framework handles it properly. For bursty access, we can use locks and other synchronization mechanisms to ensure safe access.
However, in practice, we often access the data referenced by `file->private_data` from asynchronous processes such as timers, interrupts, or inter-process communication (IPC). These processes do not go through the kernel’s file system framework, which can lead to race conditions or undefined behavior.
Let’s look into some of the kernel’s internal implementation details to understand how this can happen and what we can do to avoid issues. Our analysis is based on the Linux kernel version 3.10.102.
To get an fd, a user typically calls the C library function `open`. Before `open` returns, other threads cannot obtain the fd, so they won’t perform any operations on it. Once the fd is obtained, the open operation is complete.
However, more complex scenarios can still occur. For instance, due to programming errors or deliberate code construction, other threads may attempt to use the fd before `open` completes. While such cases are rare, they are worth exploring if you're interested in diving deeper into the kernel source.
Now, let's look at the main functions involved in the file open operation: `sys_open`, `do_sys_open`, `do_filp_open`, `fd_install`, and `__fd_install`. The installation of the fd involves locking the file table to ensure atomicity and consistency across all file descriptors.
Here's a simplified version of the `__fd_install` function:
```cpp
void __fd_install(struct files_struct *files, unsigned int fd, struct file *file)
{
struct fdtable *fdt;
spin_lock(&files->file_lock);
fdt = files_fdtable(files);
BUG_ON(fdt->fd[fd] != NULL);
rcu_assign_pointer(fdt->fd[fd], file);
spin_unlock(&files->file_lock);
}
```
For read and write operations, the code structure is similar. Here's an excerpt from the `write` system call:
```cpp
SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf, size_t, count)
{
struct fd f = fdget(fd);
ssize_t ret = -EBADF;
if (f.file) {
loff_t pos = file_pos_read(f.file);
ret = vfs_write(f.file, buf, count, &pos);
file_pos_write(f.file, pos);
fdput(f);
}
return ret;
}
```
The `vfs_write` function checks the file mode and ensures that the appropriate operations are available. It then proceeds with the actual write operation, ensuring proper synchronization and handling of the file state.
Similarly, the `ioctl` system call uses `fdget` and `fdput` to retrieve and release the file pointer, ensuring that the file is valid and accessible during the operation.
Finally, when a file is closed via the `close` system call, the kernel performs several steps to safely remove the file descriptor from the process's file table. This includes locking the file table and updating the file reference count. The `filp_close` function is called, which eventually triggers the `release` method of the `file_operations` structure.
One key point to note is that asynchronous processes, such as those triggered by timers or interrupts, may access `file->private_data` after the file has been closed. If the `release` function does not properly handle this scenario, it could lead to accessing invalid memory.
To prevent this, one approach is to mark the file as "closed" in the `release` function and have asynchronous processes check this status before accessing the data. If the file is closed, the process can gracefully handle the situation and release any resources accordingly.
By understanding the kernel's internal mechanisms and implementing proper synchronization and state management, developers can ensure safe and reliable access to shared resources in the context of asynchronous operations.
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