Some notes about the Linux file system
Friends who work on kernel development might be familiar with the code snippet below.
Here is a typical structure used in Linux kernel drivers:
[cpp] view plain copy
1. static const struct file_operations xxx_fops = {
2. .owner = THIS_MODULE,
3. .llseek = no_llseek,
4. .write = xxx_write,
5. .unlocked_ioctl = xxx_ioctl,
6. .open = xxx_open,
7. .release = xxx_release,
8. };
When a driver opens a device, it often allocates memory for private data using something like this:
[cpp] view plain copy
1. file->private_data = kmalloc(sizeof(struct xxx), GFP_KERNEL);
Then, during read, write, or ioctl operations, the driver accesses this private data through file->private_data.
Finally, when the device is closed, the driver frees this memory in the release function.
If these operations are properly synchronized and only accessed within the standard file system framework, there's generally no problem.
However, in more complex scenarios, especially when asynchronous processes such as timers, interrupts, or inter-process communication (IPC) access the same data, things can get tricky.
These asynchronous processes bypass the standard file system framework, which can lead to race conditions or invalid memory accesses.
To understand why this happens, let’s look at how the kernel handles file operations and what guarantees it provides.
The process of opening a file involves several steps, starting from the user-space open() call down to the kernel’s internal functions. These include sys_open, do_sys_open, do_filp_open, fd_install, and __fd_install.
In __fd_install, the kernel ensures that the file table is locked, preventing concurrent modifications. This lock ensures that once an fd is installed, no other thread can interfere until the operation completes.
For read and write operations, the kernel uses functions like vfs_write and do_sync_write. These functions check if the file is opened for writing, validate the buffer, and then proceed with the actual I/O operation.
Similarly, the ioctl function follows a similar pattern, ensuring that the file is valid before calling the driver’s unlocked_ioctl method.
When a file is closed, the kernel calls __close_fd, which removes the file descriptor from the file table and triggers the release function via filp_close.
This release function is where the driver typically frees any allocated memory. However, because the release is deferred via rcu-based mechanisms, there may be a delay before the actual cleanup occurs.
So, what happens if an asynchronous process accesses the private data after the file has been closed but before the release function is called?
In such cases, the driver must ensure that the data is properly protected, perhaps by using reference counting, locks, or flags to indicate whether the data is still valid.
One common approach is to mark the data as closed in the release function, and have asynchronous processes check this flag before accessing the data. If the data is marked as closed, the process can gracefully handle the situation without causing a crash or data corruption.
By understanding the kernel’s internal synchronization mechanisms and implementing proper safeguards, developers can avoid many of the pitfalls associated with asynchronous access to private data in device drivers.
Rain Test Chamber,Tightness Testing Test Box,Waterproof Rating Test Chamber,Environmental Test Chamber
Wuxi Juxingyao Trading Co., Ltd , https://www.juxingyao.com