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c - copy_to_user: how to read a struct type value of kernel space in the user space?


c - copy_to_user: how to read a struct type value of kernel space in the user space?

By : user2951646
Date : November 18 2020, 11:13 AM
hop of those help? You can add the common struct and the dev_name in to a header and include in the driver and the user space program.
Like: data.h
code :
#ifndef DATA_H
#define DATA_H    

typedef struct data {
    int val_delta;
    int val_btn_status;
}data_t;

#define dev_name "/dev/sample"

#endif
read(fd, &str, 1);
read(fd, &str, sizeof(data_t));


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is it essential to copy a returned struct from kernel space to user space?

is it essential to copy a returned struct from kernel space to user space?


By : Vinh Quang
Date : March 29 2020, 07:55 AM
may help you . The documentation of gethostbyaddr reads (see Notes section):
Why do you have to use copy_to_user()/copy_from_user() to access user space from the kernel?

Why do you have to use copy_to_user()/copy_from_user() to access user space from the kernel?


By : Lauriston
Date : March 29 2020, 07:55 AM
This might help you These functions do a few things:
They check if the supplied userspace block is entirely within the user portion of the address space (access_ok()) - this prevents userspace applications from asking the kernel to read/write kernel addresses; They return an error if any of the addresses are inaccessible, allowing the error to be returned to userspace (EFAULT) instead of crashing the kernel (this is implemented by special co-operation with the page fault handler, which specifically can detect when a fault occurs in one of the user memory access functions); They allow architecture-specific magic, for example to ensure consistency on architectures with virtually-tagged caches, to disable protections like SMAP or to switch address spaces on architectures with separate user/kernel address spaces like S/390.
what situations when to read data out of kernel space to user space?

what situations when to read data out of kernel space to user space?


By : Andrew Nazarov
Date : March 29 2020, 07:55 AM
I wish did fix the issue. The operating system's job is allow a lot of components, both hardware and software, to play nice with each other. In general, userland programs can't directly manipulate peripherals nor interfere with each other. I'm not familiar with the specific setup that you're citing, but it doesn't sound unusual.
The USB camera notifies the operating system that it has a new frame. When the kernel (driver) notices this it, will copy the frame with I/O commands into RAM. Since this RAM was allocated by the driver, the userland programs won't be able to see or read it due to virtual memory. To summarise it quickly, the address &0x1000 in the kernel and the address &0x1000 in a program are actually physically distinct locations in RAM. The kernel will then copy the frame into the memory of any process that is expecting input from the camera and then notify it (in this case catusb).
Read from a socket without the associated memcpy from kernel space to user space

Read from a socket without the associated memcpy from kernel space to user space


By : Matthias
Date : March 29 2020, 07:55 AM
I wish this help you Initially I thought AF_PACKET option to socket family can be of help, but it cannot.
Nevertheless it is possible technically, as there is nothing that prevents you from implementing kernel module handling system call that returns user mapped pointer to kernel data (even if it is not very safe).
How does a system call travels from user space to kernel space and back to user space?

How does a system call travels from user space to kernel space and back to user space?


By : Mariano Umbides II
Date : March 29 2020, 07:55 AM
should help you out Software interrupt exception is used to invoke system calls in case of ARM. It will execute function whose address is stored at physical address 0x08.
Syscall wrapper library function lands to architecture specific implementation of SYSCALL(Check sysdeps/unix directory of libc source code). In our case, syscall from sysdeps/unix/sysv/linux/arm/syscall.S file will be executed. In this function it will store the syscall number in R7 and R0-R6 are used to send arguments to the syscall.
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