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User Mode Linux & adding systemcall

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User Mode Linux is a port of Linux to run as a

program inside Linux

We can modify and test kernel without crashingentire system

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Hardware (Disk, CPUetc)

Architecture Layer Drivers

General Kernel

Applications User modearch Drivers

General Kernel

Applications

User Mode Linux As a process of host system

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All UML devices are virtual

constructed from the abstractions provided by the hostkernel.

The UML devices: Consoles and serial linesMain console, virtual console and serial lines.

Block devices Access to anything on the host which can be mounted, e.g.CDROM, disk partition

Network devicesNetwork access between UML to host OS/UML and

between UML to host in outside network.

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Hardware platforms provide a built-in

mechanism for switching between Kernel modeand user mode

UML use ptrace system call tracing mechanismThe transition from user mode to kernel mode isdone by the tracing thread.

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Virtualization of system call

Virtual kernel stack Process stack Tracing thread

Int 0x80Enter kernelNotify parent

Execute system callSignal self when done

Nullify system callSave process stateForce process ontoKernel stack

Continue After system call

Restore process stateSet system callReturn value

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For arch/x86/syscalls/syscall 64.tbl. 311 64 process_vm_writev sys_process_vm_writev 312 64 kcmp sys_kcmp 313 common mysyscall sys_mysyscall

// Acts as a library call that makes the actual system call int mysyscall() {

int ret;

// initializing the system call register with the system call number 313 __asm__("mov$313, %rax"); // executing the TRAP instruction for x86 64 bit architectures __asm__("syscall"); return ret;

} int main() {

printf("Making a system call...\n"); mysyscall(); return 0;

}

asmlinkage long sys_mysyscall(void) {

printk("My custom system call!\n"); // the kernel's version of printf! But not host kernel printk!! return 0;

}

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After TRAP instruction, kernel doesIDT (interrupt descriptor table)

..system call 0x80....

..313 . mysyscall..

System call table

(Call mysyscall handler)

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System Call is generated by user process through

TRAP ( software generated interrupt )

Add new system call number and function nameto system call table and header file

arch/x86/syscalls/syscall 64.tbl (system call table)arch/x86/include/generated/asm/unistd 64.h

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Add implementation of new system call to

Add asmlinkage long sys mysyscall(void) toinclude/linux/syscalls.h Add the implementation below to kernel/sys.c:

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Download Linux kernel and build with

make ARCH=um ( target architecture is User Mode)

Need a root file system for UMLEmulated as a file in a host file system.

Running UMLlinux ubd0=./Debian-Squeeze-AMD64-root_fsmem=128M

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Host file system can be mounted to UML

directory mount none /mnt -t hostfs -o fullPathToYourHostDirectory

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dont forget rebuild Linux kernel to test newsystem call function