CPSC 822 MIDTERM EXAM, SPRING 2015

Transcription

1 CPSC 822 MIDTERM EXAM, SPRING 2015 NAME: 1. (5) In our warm-up C code that was used to configure a NetApp file server,we see the line while(wait(&status)!=pid); The wait suspends us until we receive any signal. Upon receipt of one particular signal, wait will return the pid we seek. Which signal? A. SIGCONT B. SIGSTOP C. SIGURG D. SIGCHLD E. SIGUSR1 F. SIGKILL G. SIGPID 2. (10) In a graphics application that calls on the DMA capabilities of the Kyouko2 driver, we typically have a user-level call of the form unsigned int *cbuf; ioctl(fd,start_dma,&cbuf); Here cbuf pulls double duty: on input it contains a byte count of the size of the current DMA buffer; on output it contains the user space virtual base address of the next memory-mapped DMA buffer. We pass the address of cbuf because we need output returned. As a result, on input, the kernel receives only the address of a variable containing the desired count. Give a line of kernel code that is used to read the value of the desired count from user space into kernel variable int count; 1

2 3. (5) In the driver for the Kyouko2 graphics card, we used two indices, fill and drain into a circular queue of DMA buffers to keep track of the state of the buffer queue. If, upon entry to the DMA completion interrupt handler, we find fill == drain, what is the state of the buffer queue? A. empty B. full C. empty except for the 1 slot that just filled to generate the interrupt D. full except for the 1 slot that just emptied to generate the interrupt E. error condition: a queued buffer was empty F. error condition: a queued buffer had a faulty address G. error condition: SMP race condition detected 4. (10) In the BIND DMA segment of the kyouko2 ioctl method of the Kyouko2 driver, suppose we have stored for the i th DMA buffer: kernel virtual address: physical address: user virtual address: kyouko2.dmabuf[i].kbase kyouko2.dmabuf[i].pbase kyouko2.dmabuf[i].vbase Suppose we want to insert an integer message, 0x , at the beginning of each buffer so that we can later check proper buffer mapping from the user level. Give one line of kernel code for the BIND DMA segment that will insert this integer into the front of the buffer. 5. (10) Now we check from the user level. Suppose the following user-level code segment is used to load 8 DMA buffers with graphics commands and launch them for rendering. int fd, result; unsigned int *cbuf; int bufs_sent=0; fd = open("/dev/kyouko2",o_rdwr); result = ioctl(fd,vmode,graphics_on); ioctl(fd,sync); result = ioctl(fd,bind_dma,&cbuf); while(bufs_sent<8){ loadbuf(cbuf); cbuf=(unsigned int *)FIXED_BUFFER_LOAD_SIZE; result = ioctl(fd,start_dma,&cbuf); bufs_sent++; 2

3 } result = ioctl(fd,vmode,v_off); close(fd); Give a line of user code that will print the integer message from each new buffer before that buffer is loaded with graphics commands. Show where to insert the line so that all 8 messages are seen. 6. (5) The InterruptStatus register of the Kyouko2 card could be read to determine whether a DMA interrupt had just occurred. Which bit having which value indicated an outstanding interrupt? A. bit 0 value 0 B. bit 0 value 1 C. bit 1 value 0 D. bit 1 value 1 E. bit 2 value 0 F. bit 2 value 1 G. bit 4 value 0 H. bit 4 value 1 I. bit 31 value 0 J. bit 31 value 1 7. (10) In our design of the driver for the Kyouko2 card, we used the mmap system call to memory map the device s control registers and framebuffer, and we used the ioctl system call with command BIND DMA to memory map DMA buffers. This led to an asymmetry wherein we had to switch within kyouko2 mmap to accommodate all calling paths. The switch was based on a somewhat artificial offset recovered from vma vm pgoff. Instead, we could leave the driver s mmap function fixed as: int kyouko2_mmap(struct file *fp, struct vm_area_struct *vma) { io_remap_pfn_range(vma,vma->vm_start,vma->vm_pgoff, vma->vm_end - vma->vm_start,vma->vm_page_prot); return 0; } and, instead of calling mmap() from user-level, call ioctl() with a new command, BIND CONTROL, or a new command, BIND FRAMEBUFFER, that memory maps the device s control region or framebuffer to user space and delivers, in the argument, the user-level base address of 3

4 this region. Provide the kernel code for the new implementation of BIND FRAMEBUFFER. The return value should be a binary success/failure indicator. You may NOT change the kyouko2 mmap function from that suggested above or the link to it in kyouko2 fops. 8. (10) Which of these kernel function calls would retrieve the numerical process id in global process namespace of the process identified by struct task struct *p. A. task tgid vnr(p) B. pid vnr(task tgid(p)) C. pid nr(task tgid(p)) D. pid task(find pid(p),pidtype PID) E. pid task(find vpid(p),pidtype PID) F. getpid() G. sys getpid() H. sys getppid() 9. (10) Suppose a target process with process table entry struct task struct *p has received multiple signals that were sent to it via the kill system call. In which kernel variable would the pending signal bits be stored until p calls do signal() to process pending signals? A. p pending.sig[0] B. p pending.sig[1] C. p pending.signal.sig[0] D. p pending.signal.sig[1] E. p signal shared pending.sig[0] F. p signal shared pending.sig[1] G. p signal shared pending.signal.sig[0] H. p signal shared pending.signal.sig[1] 4

5 10. (10) If we attempt to send signal number 0 to a process whose real user id in the global user namespace matches our own effective user id in the global user namespace, what value is returned by sys kill()? A. EINVAL B. ESRCH C. EPERM D. the numerical pid of the target process E. the uid in the global user namespace of the target process F. -1 G. 0 H (5) When a signal number below SIGRTMIN is sent to a process, which kernel function checks whether that signal is already pending for that process and returns a 1 in order to short-circuit the send? A. signalfd notify B. complete signal C. prepare signal D. get signal to deliver E. dequeue signal F. legacy queue G. is si special H. wants signal 12. (10) In send signal(), if we see that is si special(info) evaluates to FALSE, we can conclude that the numerical value of info is: A. SIG SEND NOINFO B. SIG SEND PRIV C. SIG SEND FORCED D. SI KERNEL E. SI USER F. SI QUEUE G. SI TIMER H. SI TKILL I. the address of an auxiliary structure to be used in queueing the signal 5