CS 412 HOTNEWS

for CMSC 412

This page contains late breaking announcements from Dr. Hollingsworth and the TAs.
Please check this page a couple of times a week.

Additional steps to load fonts on WAM machines: 

1- Start XFS
    xfs -config /afs/wam.umd.edu/home/wam/s/o/sodre/pub/bochs.xfs &

2- Set XFS in the current display
    xset +fp tcp/you.are.working.machine:7100

3- Rehash it
    xset fp rehash

The submit program is now installed in ~jh41201/BIN/submit.

Please delete the file hd.img before submitting your program.

Buildfat has been added to the list of files on the project #1 web page. It should be a subdirectory off your main project1 directory.

The project should use only one TSS. When you switch to user mode, just update the one TSS to refer to the appropriate LDT.

When you dispatch a user mode process, you will need to load the ldt selector for that process (via the assembler instruction lldt). You will probably need to add code to lowlevel.asm near the label (.restore) and in the function Restore_Thread to do this.

Likewise the various code/data/stack selectors for that process will get pushed onto the stack so that the iret instruction can finish the context switch.

On context switch to use mode, you will also want to to set the TSS ring0 stack to be a stack for the kernel thread. The easiest thing to do is to set it to be the current kthread's stack plus one page.

There is an issue with the gcc compiler trying to use special inlined versions of string functions. The improved solution is to add the compiler flag -fno-builtin to the CFLAGS variable in the kernel Makefile.

Clarification on setting up the ldt descriptor and ldtSelector. The following bit of code shows the sequence required. 

    context->ldtDescriptor = Allocate_Segment_Descriptor();
    Init_LDT_Descriptor( context->ldtDescriptor, context->ldt, NUM_USER_LDT_ENTRIES );
    index = Get_Descriptor_Index(context->ldtDescriptor);
    context->ldtSelector = Selector( KERNEL_PRIVILEGE, TRUE, index );

Tips on debugging:

  • change your .bochsrc file (last line) to be panic: action=fatal. This will cause the simulator to stop on an error. You need bochs 1.2 (in ~hollings/bin/bochs12 for this option to be abvailable).
  • You can use gdb on kernel.exe and use the disas command to find out where in the kernel you code crashed (by looking around the value of the eip register when you get a crash. Don't forget that you also need to double check if you died in user mode or kernel mode (though you should get a gpf that is cleaned up by the kernel when a user space program crashes).
  • The dump will also give you values for all of the registers and the segment registers.

    Bugs in GeekOS We have located a couple of bugs in GeekOS. You should follow the direction at destroyThread.html to fix a problem in kthead.c and replace pfat.c.

    Submission Guidlines: All submissions should be an uncompressed tar file and should not contain a directory name for the top directory. For example, extracting your tar file should produce all of the GeekOS source into the current working directory, not a subdirectory (such as p2). To do this, run tar from the directory your are submitting (i.e. tar cvf ../proj3.tar *).
    Bug found in strcpy (in string.c). There is a bug in strcpy. It was failing to add a null character to the destination string when it gets copied. The correct version is 
    char *strcpy(char *dest, const char *src)
{
    char *ret;

    ret = dest;
    while (*src) {
	*dest++ = *src++;
    }
    *dest = '\0';
    return ret;
}

    4/19/02 - Bug found in bitset.c, please download a new copy. The call to memset had the paramters in the wrong order.

    4/19/02 - The system call function associated with SYS_STAT is Stat(...) not State(...).

    4/20/02 Several bugs found in vfs.c. A mutex has been added around mount table operations in vfs.c to prevent a possible race condition. Also, the read call for PFAT filesystem would always return the first n bytes of a file.

    5/8/02 There were a few things that may not have been clear in the project #5 handout, and one that is out write wrong.

  • The description of setEffectiveUid is wrong it should read
    "If the uid of a process is **0** when this system call is made, the uid is changed to the passed uid."
  • The limit of the number of active message queues is 20.
  • A process needs write access to a file to change it's ACLs.
  • Delete does not require write access to a directory to run, it just needs write access to the file.