About self-defined syscall

[glassesmouse]

I just try to define a new syscall set_limit(), which is used for set a limitation for syscall read() or write(). This syscall should retain two value max_read_bytes and max_write_bytes in each process. I try to add these two new fields in task_struct. But how can I initialize these two values and make them available in each process, so that read/write can get these values?

[probe]

Ok, this is instruction how to add new system call in kernel 2.6 from web address

http://courses.cs.vt.edu/~cs3204/spr...inux_rev00.pdf

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1. Download the latest version of the 2.6 Linux kernel from The Linux Kernel Archives.
2. Unzip and untar the kernel directory into /usr/src/.
3. In /usr/src/Linux-x.x.x/kernel/, Create a new file myservice.c to define your
system call.
#include <Linux/linkage.h> //for linking a system call
#include <Linux/kernel.h> //for the printk
asmlinkage int sys_myservice (int arg1, char* arg2) {
printk(KERN_EMERG “my service is running”);
//kernel messages logged to /var/log/kernel/warnings
return(1);
}
4. In /usr/src/Linux-x.x.x/include/asm/unistd.h, define an index for your system call.
Your index should be the number after the last system call defined in the list.
#define __NR_myservice 274
5. Also, you should increment the system call count.
#define __NR_syscalls 275
6. In /usr/src/Linux-x.x.x/arch/i386/kernel/entry.S, you should define a pointer to
hold a reference to your system call routine. It is important that your data entry
placement corresponds to the index you assigned to your system call.
.long sys_myservice
7. Add your system call to the Makefile in /usr/src/Linux-x.x.x/kernel/Makefile.
Add your object after the other kernel objects have been declared.
obj-y += myservice.o
8. Make your system from /usr/src/Linux- x.x.x
make xconfig //save the defaults
make dep //make dependency list
make bzImage //build your kernel
9. Add a new boot image to Lilo, by editing /etc/lilo.conf. Your lilo configuration
will vary slightly. After saving, run lilo –v to install your settings. Don’t just
modify an existing lilo entry; you may need it if your new kernel has bugs.
image=/usr/src/Linux-x.x.x/arch/i386/boot/bzImage
label=”Linux-test”
root=/dev/hda5
read-only
10. Making a user test file. You also need to copy your edited unistd.h from
/usr/src/Linux- x.x.x/include/asm/ to /usr/include/kernel/ because it contains your
system call’s index.
#include <Linux/errno.h>
#include<sys/syscall.h>
#include <Linux/unistd.h>
long errno; //this is the return code from the system call
//this is a macro defined in unistd.h to help prototype sys calls
_syscall2(int, myservice, int, arg1, char*, arg2);
main() {
myservice(1, "hi");
}
11. Reboot into your new kernel and compile your user test program to try out
your system call. You will know if it worked if you see a kernel message in
/var/log/kernel/warnings announcing that your service is running.
------------------------------------------------------------------

So this is how you create new syscall [I supose it works, didn't try it yet, but will when I get home tonight ]

Did you figure out where syscalls read() or write() hold their values for max_write_bytes and max_read_bytes. Or does these limitations even exist explicitly in kernel or they have some physical limitations ?

If you add two new fields to task_struct, each process will by default have access to them, since you represent each process with task_struct. So now, you have to look how communication goes between process and read() and write() syscalls, and there set these values. And to do this I think you will have to modify task_struct and read() and write() syscalls accordingly. Maybe even more kernel code to change for this to work.

regards

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