File I/O in a Linux kernel module

Question

I\'m writing a Linux kernel module that needs to open and read files. What\'s the best way to accomplish that?

Answer 1

All of the kernel developers say that file I/O from kernel space is bad (especially if you're referring to these files by their paths) but the mainstream kernel does this when you load firmware. If you just need to read from files, use the

kernel_read_file_from_path(const char *path, void **buf, loff_t *size, loff_t max_size, enum kernel_read_file_id id)

function, which is what the firmware loader code uses, declared in include/linux/fs.h. This function returns a negative value on error.

I'm not really sure of the point of the id variable at the end, if you look at the code it's not really used, so just put something like READING_FIRMWARE there (no quotes).

buf is not null terminated, instead refer to its size in size. If you need it to be null terminated, create a string size + 1 bytes long and copy it over or rewrite the kernel_read_file() function (used by kernel_read_file_from_path(), defined in fs/exec.c) and add one to i_size where memory is allocated. (If you want to do this, you can redefine the kernel_read_file() function in your module with a different function name to avoid modifying the whole kernel.)

If you need to write to files, there is a kernel_write() function (analogous to kernel_read(), which is used by kernel_read_file() and therefore also by kernel_read_file_from_path()), but there is no kernel_write_file() or kernel_write_file_from_path() function. You can look at the code in the fs/exec.c file in the Linux kernel source tree where kernel_read_file() and kernel_read_file_from_path() are defined to write your own kernel_write_file() and kernel_write_file_from_path() functions that you can include in your module.

And as always, you can store a file's contents in a char pointer instead of a void pointer with this function by casting it.