Hi all,
Ok, I am resending the same question with more details.
I am trying to implement a protocol in linux kernel (2.6.14) for
experimental purposes and I am facing a weird problem.
rt_instance is an instance of a structure in user space.
rt_u is a variable in user space which points to rt_instance.
rt_k is a variable in kernel space which I want to use to read rt_instance,
hence, it should point to rt_instance too.
The following code is a system call which I wrote, which basically passes
the address of rt_instance to the kernel by copying rt_u to rt_u . Then, I
use copy_from_user to copy rt_instance to a local variable (local_rt) in
kernel space. Then, I read the content of local_rt and the code works fine:
n = 0, and local_rt.session_list shows the correct value.
asmlinkage int sys_asimcast(struct routing_table *rt_u) {
rt_k = rt_u;
if (rt_k != NULL) {
struct routing_table local_rt;
int n = copy_from_user(&local_rt, rt_k, sizeof(struct
routing_table));
printk("rt_k.session_list = %d; n = %d.\n",
(int)local_rt.session_list, n);
}
return 2;
}
Now, I inserted the same code into ip_rcv function (/net/IPv4/ip_input.c,
right before the return statement) to read again the rt_instance, using the
same pointer rt_k.
copy_from_user returns 0 (means that it could read the structure properly)
BUT the data which has been read is incorrect.
if (rt_k != NULL) {
struct routing_table local_rt;
int n = copy_from_user(&local_rt, rt_k, sizeof(struct
routing_table)); printk("local_rt.session_list = %d; n = %d.\n",
(int)local_rt.session_list, n);
}
The bottom line is: the same code, written in the system call works, but
doesn't work in the kernel. Whenever I read the structure from the sys call,
it is read correctly, while from the ip_rcv function, it is not.
BTW, I made sure that rt_k is pointing to the same address in both cases (a
print statement showed the same value).
I am wondering if this problem has to do with the fact that copy_from_user
is user context only? I am not sure exactly what this means. Should I change
the context and how?
Best regards,
-- Laurent
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