Lecture 8: System calls

Preparation

• Read xv6 §3, Traps, interrupts, and drivers.
• Do the interrupt exercise.

Recap

• interrupt sources: exceptions, device interrupts, software interrupts (int)
• setup
  • IDT register %idtr: point to the base address of IDT
  • task register: we only use ss0 & esp0
• what does CPU do for kernel → user transition
  • switch stack
  • push data
  • find interrupt handler from IDT
• iret: user → kernel

Exceptions

• rerun div 1 0: user → kernel
  • set a breakpoint at idivl again
  • b vector0
  • x/6x $esp when entering kernel
  • CPU pushes old eip/cs/eflags/esp/ss; no error code
  • kernel pushes more data for struct trapframe
  • Unix: often translate into signals - man signal
• div trap in kernel: kernel → kernel
  • inject bug: append int z = 0; ticks = ticks / z; to idtinit()
  • b vector0
  • x/6x $esp
  • CPU pushes old eip/cs/eflags; no ss/esp for kernel → kernel
• page fault: user → kernel w/ error code
  • inject page fault in div.c: memset something
  • b vector14
  • x/6x $esp
  • CPU pushes one extra error code
  • see error code summary or JOS kern/trap.c

Device interrupts

• hardware
  • Intel 8259A, Figure 3b, or photo
  • Intel 82093AA, Figure 2
• example: timer interrupt
  • yield
  • is this possible: too many timer interrupts come in and overflow the kernel stack?

Software interrupts

• syscall calling convention
  • example: xv6
    • int 0x40 (64)
    • usys.S (user) and syscall.c (kernel)
  • example: Linux x86-32
    • gcc -nostdlib
    • try strace

#include <sys/syscall.h>
#include <unistd.h>
void _start(void)
{
        int fd = 1;
        char buf[] = "hello world!\n";
        size_t count = sizeof(buf) - 1;
        asm volatile ("int $0x80"
                : /* ignore output */
                : "a"(__NR_write), "b"(fd), "c"(buf), "d"(count)
                : "cc", "edi", "esi", "memory"
        );
        asm volatile ("int $0x80"
                : /* no output */
                : "a"(__NR_exit), "b"(0)
        );
}

• sysenter/syscall instructions (lab3 challenge)
• vsyscall/vDSO: e.g., gettimeofday

Discussions

• xv6 question
  • CPU doesn not push error code for int
  • it does for page fault (14)
  • can we directly invoke int $14 from user space to confuse the kernel?
• can we change the behavior for div by zero?
  • for example, resume execution & set result to 42
  • is it possible for applications not to trust the kernel? check SGX
• JOS
  • one kernel stack per processor
  • kernel cannot be interrupted
  • different syscall calling convention