Syscall allocate memory for user

Question

I have a structure I am declaring/defining in the linux kernel (3.2), and I am currently trying to allocate one of these structures inside a syscall, and return a pointer to it for the process calling the syscall.

How can I #include this file in a program outside the kernel (the question might be which file should I include)? Currently, I am declaring the structure in include/linux/syscalls.h and defining it in a file I created myself in kernel/mysystemcall.c. If I try and use the structure in a program, I get error: dereferencing pointer to incomplete type.
How can I actually read from this memory, given that if I dereference it, I get a segmentation fault? Currently, I am using kmalloc to allocate the memory; is there a flag I need to turn on to access the memory, or should I be using something else to allocate this memory?

Thanks for any help provided!

Current syscall implementation:

#include <linux/linkage.h>
#include <linux/sched.h>
#include <linux/slab.h>

struct threadinfo_struct {
    int pid;
    int nthreads;
    int *tid;
};

asmlinkage struct threadinfo_struct *sys_threadinfo(void) {
    struct threadinfo_struct *info = kmalloc(sizeof(struct threadinfo_struct), GFP_KERNEL);
    info->pid = current->pid;
    info->nthreads = -1;
    info->tid = NULL;
    return info;
}

Current test code (outsider kernel):

#include <stdio.h>
#include <linux/unistd.h>
#include <sys/syscall.h>
#define sys_threadinfo 349

int main(void) {
    int *ti = (int*) syscall(sys_threadinfo);
    printf("Thread id: %d\n", *ti); // Causes a segfault
    return 0;
}

EDIT: I realize I can have my syscall take a pointer to already allocated memory, and just to fill in the values for the user, but it is preferred (teacher preference) to do it this way for the assignment.

Look at the implementation of the system calls whose purpose is to allocate memory for a user-space process: `brk` and `mmap`. (`mmap` does other things as well, but it should be clear which code path is which.) Both are defined in `mm/mmap.c`. — zwol, Oct 20 '12 at 01:57
@Zack is this the `SYSCALL_DEFINE1(brk, unsigned long, brk)` and `SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)` definitions that you are referring to? I can see the mmap uses `copy_from_user` (not much is clear here - new to linux kernel), is there a way I can give the memory to the user without using something like `copy_to_user`? — Casey Kuball, Oct 20 '12 at 04:16
Those are not the *implementations* of the system calls; those are effectively only interface declarations (despite the DEFINE in the name). The *implementations* are in `mm/mmap.c`, as I said. Look for functions named `sys_brk`, `sys_mmap`, `do_brk`, and `do_mmap`. — zwol, Oct 20 '12 at 14:05
@Zack Ah, I will look at those functions then. (Those SYSCALL_DEFINE1 parts were both in mm/mmap.c) — Casey Kuball, Oct 21 '12 at 17:21

score 2 · Accepted Answer · edited May 23 '17 at 11:49

2

After looking at this answer:

Don't attempt to allocate memory for userspace from the kernel - this is a huge violation of the kernel's abstraction layering.

I decided to go the route of having the user space program allocate the memory itself, after asking the kernel how much memory is necessary.

This means that I can simply copy the structure into both the user and kernel space files, and there is no need to #include a kernel file for the structure definition.

edited May 23 '17 at 11:49

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answered Feb 24 '13 at 18:54

Casey Kuball

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Be sure to point out to your teacher why you altered the specification, including the potential and serious security implications of his preferred approach. – Nik Bougalis Feb 24 '13 at 19:22

Syscall allocate memory for user

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