6.828 Fall 2007 Lab 6: the shell

Handed out Wednesday, November 21, 2007
Tarball Due Thursday, December 6, 2007
Private Demos on December 10 and 11, 2007
In Class Demos on December 12, 2007

Introduction

1. You will generalize the file descriptor support to handle a variety of file types rather than just on-disk files. This will pave the way for pipes and a console device.
2. You will change fork and spawn to share their file descriptors with the child environment. Sharing them across fork and spawn allows the parent to set up i/o redirection before starting the child. This enables i/o redirection of child programs without needing to do anything special in the child.
3. You will implement pipes.
4. You will implement the system call linkage for the keyboard driver. We have provided the hardware driver itself as well as the console device file support.
5. You will implement pipes and redirection in the shell. We have provided a basic shell.

Lab Requirements

To complete the assignment, you will have to demonstrate your shell for us in person the last week of the term. Dates and a sign-up form will be posted soon.

Every group should be prepared to do a 5 minute in-class demo on Wednesday December 12, 2007.

Late policy. If you are working in a pair, you may combine your late days, so if one of you has one day and the other also has one day, you can turn in the assignment up to two days late without penalty. However, the assignment must be turned in by the evening of Friday, December 14, even if you have enough late days (or want to use penalty days) to be able to hand it in later. Labs received after Friday, December 14 will receive an F.

Getting Started

The pingpong and primes test should still work. Check this by runing gmake run-pingpong and gmake run-primes.

The file system tests will not work yet. To facilitate file descriptor sharing and multiple types of files, the implementation of routines like open, close, read, and write have changed. As the first exercise in this lab, you will adapt your solutions from lab 5 to fit in this framework.

Exercise 1: the file system switch

The file support we wrote in the last lab assumed that disk files were the only possible type of file. In this exercise, we will address that shortcoming, making files a generic concept, implemented by the disk file, pipe, and console devices.

The generic file descriptor code is in lib/fd.c. You already encountered this code in lab 5, but now you will now need to understand it in more detail in order to implement other kinds of file descriptors.

As we outlined in lab 5, each environment has a file descriptor table located at virtual address FDTABLE (which happens to be 0xCFC00000). Each page in the table represents a single file descriptor. For example, file descriptor 2 is represented by the page at FDTABLE+2*PGSIZE. If the file descriptor is closed, there is no page mapped there. The file descriptor page contains a struct Fd, declared in inc/fd.h:

	struct Fd
	{
		u_int fd_dev_id_id;	// device id 
		u_int fd_offset;	// offset for read/write
		u_int fd_omode;		// open mode
	};

Each device exports a struct Dev with function pointers:

	struct Dev
	{
		u_int dev_id;
		char *dev_name;
		int (*dev_read)(struct Fd*, void *buf, u_int n);
		int (*dev_write)(struct Fd*, const void *buf, u_int n);
		int (*dev_close)(struct Fd*);
		int (*dev_stat)(struct Fd*, struct Stat*);
	};

As before, each file descriptor has a 4MB region of virtual memory reserved for its own use. The file system device still uses this range to map the file. The pipe device will use it to map the pipe buffer.

To make things concrete, let's consider the implementation of two of the generic functions: write and dup. Write looks like this:

	int
	write(int fdnum, const void *buf, u_int n)
	{
		int r;
		struct Dev *dev;
		struct Fd *fd;
	
		if ((r = fd_lookup(fdnum, &fd)) < 0
		||  (r = dev_lookup(fd->fd_dev_id_id, &dev)) < 0)
		        return r;
		if ((fd->fd_omode & O_ACCMODE) == O_RDONLY)
		        return -E_INVAL;
		r = (*dev->dev_write)(fd, buf, n, fd->fd_offset);
		if (r > 0)
		        fd->fd_offset += r;
		return r;
	}

Dup looks like this:

	int
	dup(int oldfdnum, int newfdnum)
	{
	        int i, r;
	        u_int ova, nva, pte;
	        struct Fd *oldfd, *newfd;
	
	        if ((r = fd_lookup(oldfdnum, &oldfd)) < 0)
	                return r;
	        close(newfdnum);

	        newfd = (struct Fd*)INDEX2FD(newfdnum);
	        sys_mem_map(0, (u_int)oldfd, 0, (u_int)newfd, vpt[VPN(oldfd)]&PTE_USER);

	        ova = fd2data(oldfd);
	        nva = fd2data(newfd);
	        if (vpd[PDX(ova)])
	                for (i=0; i<PDMAP; i+=BY2PG) {
	                        pte = vpt[VPN(ova+i)];
	                        if(pte&PTE_P)
	                                sys_mem_map(0, ova+i, 0, nva+i, pte&PTE_USER);
	                }
	        return newfdnum;
	}

If we needed to, we could add a dev_dup function pointer to struct Dev in order to allow device implementations to run their own code in response to dup, but for our purposes it isn't necessary.

Make sure you understand these code snippets. You may find it useful to consult the rest of lib/fd.c as well as lib/console.c, an example device implementation.

Run gmake run-icode to check that file operations and spawn still work. If you see:

	init: running sh
	init: starting sh
        <probably some error here>

Exercise 2: sharing library code across fork and spawn

We will change both fork and spawn to know that certain regions of memory are used by the "library operating system" and should always be shared. Rather than hard-code a list of regions somewhere, we will set an otherwise-unused bit in the page table entries (just like we did with the PTE_COW bit in fork).

We have defined a new PTE_SHARE bit in inc/lib.h. This bit is one of the three PTE bits that are marked "available for software use" in the Intel and AMD manuals. We will establish the convention that if a page table entry has this bit set, the PTE should be copied directly from parent to child in both fork and spawn. Note that this is different from marking it copy-on-write: as described in the first paragraph, we want to make sure to share updates to the page.

You should see lines that say "fork handles PTE_SHARE right" and "spawn handles PTE_SHARE right".

Exercise 3: pipes

You may wish to read the pipe manual page [1] for a description, the V6 pipe implementation (/usr/sys/ken/pipe.c) for details, and pipe section of Dennis Ritchie's UNIX history paper [2] for interesting history.

 [1] http://www.freebsd.org/cgi/man.cgi?query=pipe&manpath=Unix+Seventh+Edition
 [2] http://cm.bell-labs.com/cm/cs/who/dmr/hist.html#pipes

In your library operating system, a pipe is represented by a single struct Pipe. For sharing purposes, each struct Pipe is on its own page.

	#define PIPEBUFSIZ 32
	struct Pipe {
		u_int p_rpos;		// read position
		u_int p_wpos;		// write position
		u_char p_buf[PIPEBUFSIZ];	// data buffer
	};

Since the pipe buffer is not infinite, byte i is stored in pipe buffer index i%PIPEBUFSIZ.

To read a byte from a pipe, the reader copies p_buf[p_rpos%PIPEBUFSIZ] and increments p_rpos. But the pipe might be empty! First the reader has to yield until p_rpos < p_wpos.

To write to a pipe, the writer stores into p_buf[p_wpos%PIPEBUFSIZ] and increments p_wpos. But the pipe might be full! First the writer must wait until p_wpos - p_rpos < PIPEBUFSIZ.

There is a final catch -- maybe we are trying to read from an empty pipe but all the writers have exited. Then there is no chance that there will ever be more data in the pipe, so waiting is futile. In such a case, Unix signals end-of-file by returning 0. So will we. To detect that there are no writers left, we could put reader and writer counts into the pipe structure and update them every time we fork or spawn and every time an environment exits. This is fragile -- what if the environment doesn't exit cleanly? Instead we can use the kernel's page reference counts, which are guaranteed to be accurate.

Recall that the kernel page structures are mapped in the user environments as pages. The library function pageref(void *ptr) returns the number of page table references to the page containing the virtual address ptr. So, for example, if fd is a pointer to a particular struct Fd, pageref(fd) will tell us how many different references there are to that structure.

Three pages are allocated for each pipe: the struct Fd for the reader descriptor rfd, the struct Fd for the writer descriptor wfd, and the struct Pipe p shared by both. The following equation holds: pageref(rfd) + pageref(wfd) = pageref(p). Therefore, a reader can check whether there are any writers left by computing pageref(wfd) = pageref(p) - pageref(rfd): there are no writers if pageref(p) == pageref(rfd). A writer can check for readers in the same manner.

Exercise 3b: races, races everywhere

User-space programs are preemptively scheduled: if an environment is the middle of something important and the clock interrupt comes along, too bad -- it has to stop and pick up again later. (This is transparent to the environment, since the kernel saves and restores its registers as though nothing had happened.)

Preemptive scheduling isn't a problem as long as all the environments are completely isolated from each other -- if they can't interfere with one another, the task switches aren't likely to be a problem.

We've seen that sometimes it's useful for different environments to "interfere constructively" with each other, in the form of IPC and shared memory pages. Unfortunately, preemptive scheduling and shared mutable state is a recipe for trouble. We'd been lucky so far, but our luck just ran out.

There are a couple of race conditions in the implementation of pipes. They center around the test for whether a pipe is closed.

Recall that in _pipeisclosed, we simply check whether "pageref(fd structure) == pageref(pipe structure)", where pageref uses the VPT to get the physical page number holding the given pointer and then looks up the kernel-maintained reference count for that page in the pages array.

_pipeisclosed compares two reference counts which typically change together. If, under some conditions, this comparison gives the wrong answer, we might think the other end of pipe is closed even when it isn't.

If we think of the result of _pipeisclosed as being stored across these two words in memory (the two reference counts), then _pipeisclosed might fail when the writing of the two words is not done atomically and also might fail when the reading of the two words is not done atomically. It turns out that both cases can happen. We fix the first by being careful about ordering the writes. We fix the second by implementing support for a limited form of restartable atomic sequences (RAS).

Remember that anything we do inside the kernel runs without interruption (unless we explicitly yield the processor). From user space, any such changes are atomic -- the user environment sees the whole change or none of it. Our problems happen because the reads and writes of the data are done in user space, and thus not atomic.

The update race

	pipe(p);
	if(fork() == 0){
		close(p[1]);
		read(p[0], buf, sizeof buf);
	}else{
		close(p[0]);
		write(p[1], msg, strlen(msg));
	}

• The child runs first after the fork. It closes p[1] but a clock interrupt comes along before the read, switching execution to the parent.
• The parent starts to close(p[0]), which will unmap the shared pipe structure and then unmap the fd page for p[0]. A clock interrupt comes along between the two unmappings, switching execution back to the child.
• Now the reference count on p[0] is 2 (both parent and child have it mapped) and the reference count on p[1] is 1 (only the parent has it mapped). But the reference count for the pipe structure is 2: the child has it mapped for p[0], and the parent has it mapped for p[1] but not p[0], which is still in the middle of being closed. The invariant no longer holds.
• The child runs. Read sees the buffer is empty and then decides the pipe is closed because pageref(p[0]) and pageref(pipe structure) are both 2. Oops.

The same problem can happen with dup, if an environment is interrupted between mapping the file descriptor page and mapping the pipe data page.

Run gmake run-testpiperace to see the race in action. When the race actually happens, the test code will print RACE: pipe appears closed. (You should read user/testpiperace.c and make sure you understand what's going on.)

Since each system call can only map or unmap a single page, the two mappings cannot be updated atomically, so neither can the reference counts. Since two reference counts cannot be updated atomically, the invariant cannot be maintained -- it is temporarily broken when one reference count is updated but not the other. One way to fix the problem would be to add new system calls to map or unmap two pages at a time. Then the two mappings (and thus the two reference counts) could be updated in one system call, making them atomic from the user-space point of view. We won't do this.

Instead, we can relax the invariant, since _pipeisclosed doesn't actually need all of it. What _pipeisclosed really needs is the following conditions:

• pageref(p[0]) == pageref(pipe structure) if and only if all the writers are closed.
• pageref(p[1]) == pageref(pipe structure) if and only if all the readers are closed.

The race happens because while a pipe is half-mapped or half-unmapped there is an fd reference but not a pipe structure reference, so pageref(p[0])+pageref(p[1]) > pageref(pipe structure). As a consequence, pageref(p[0]) might equal pageref(pipe structure) even though pageref(p[1]) is non-zero.

Suppose instead we arrange that while pipes are half-mapped or half-unmapped, pageref(p[0])+pageref(p[1]) < pageref(pipe structure). Then the conditions would be satisfied, and _pipeisclosed would not give incorrect answers about intermediate states.

We can do this by making sure that the fd structure is never mapped without the pipe structure also being mapped. This leads to a pair of rules:

• always map the pipe structure, then the fd structure.
• always unmap the fd structure, then the pipe structure.

Test your code by running gmake run-testpiperace. If the race is gone, you won't see "RACE: pipe appears closed" and should see race didn't happen.

The read race

	pipe(p);
	if(fork() == 0){
		close(p[1]);
		read(p[0], buf, sizeof buf);
	}else{
		close(p[0]);
		write(p[1], msg, strlen(msg));
	}

1. Suppose the child runs first after the fork. It closes p[1] and then tries to read from p[0]. The pipe is empty, so read checks to see whether the pipe is closed before yielding. Inside _pipeisclosed, pageref(fd) returns 2 (both the parent and the child have p[0] open), but then a clock interrupt happens.
2. Now the kernel chooses to run the parent for a little while. The parent closes p[0] and writes msg into the pipe. Msg is very long, so the write yields halfway to let a reader (the child) empty the pipe.
3. Back in the child, _pipeisclosed continues. It calls pageref(p), which returns 2 (the child has a reference associated with p[0], and the parent has a reference associated with p[1]). The counts match, so _pipeisclosed reports that the pipe is closed. Oops.

   (If the child checked again, pageref(fd) would now return 1, but _pipeisclosed is holding onto the 2 from before, unaware that something might have happened in the interim to change the count.)

Run "gmake run-testpiperace2" to see this race in action. Like before, you should see "RACE: pipe appears closed" when the race occurs.

This race is a little simpler to fix. Comparing the counts can only be incorrect if another environment ran between when we looked up the first count and when we looked up the second count. In other words, we need to make sure that _pipeisclosed executes atomically.

Since _pipeisclosed does not change any variables (it only reads them), it is safe to run multiple times. If we had some way to check whether _pipeisclosed got interrupted, we could repeatedly run it until it a run completed without being interrupted. Since _pipeisclosed is so short, it will usually not be interrupted.

To tell whether it is being interrupted, we will use the env_runs variable in the environment structure. Each time the kernel context switches back to an environment, it will increment env_runs. Thus, user code can record env->env_runs, do its computation, and then look at env->env_runs again. If env_runs didn't change, then the environment was not interrupted. Conversely, if env_runs did change, then the environment was interrupted.

Effectively, the env_runs counter enables support for restarting read-only atomic sequences from user space.

Real-world example: reading 64-bit interrupt counters on a 32-bit machine

If the counter runs slowly enough that it takes a significant amount of time to go up by 2^32, then instead of protecting the value with a lock, the reader can use the high 32 bits as a way to see whether an interrupt happened between the two word accesses:

	// Copy 64-bit counter src into dst, being careful about read races.
	struct split64 {
		// assumes little-endian memory
		uint lo;
		uint hi;
	};
	void
	readcounter(uint64 *src64, uint64 *dst64)
	{
		struct split64 *src;
		struct split64 *dst;

		src = (struct split64*)src64;
		dst = (struct split64*)dst64;

     		do {
			dst->hi = src->hi;	// read high bits
			dst->lo = src->lo;	// read low bits
		} while (dst->hi != src->hi);	// do over if high bits changed
	}

On the other hand, suppose src->hi is the same before and after the read of src->lo. Then perhaps no interrupt happened. In this case, the copy is good, so we can stop. But perhaps an the interrupt occurred that changed only the bottom 32 bits. If the read of src->lo happened before the interrupt, then dst has the pre-interrupt value. If the read of src->lo happened after the interrupt, then dst has the post-interrupt value. Either way, the read executed atomically, so we can stop.

Exercise 4: the keyboard interface

Test your code by running gmake xrun-testkbd and type a few lines. The system should echo your lines back to you as you finish them. Make sure you type into the X window bochs brings up, not the console.

Exercise 5: the shell

	echo hello world | cat
	cat lorem >out
	cat out
	cat lorem |num
	cat lorem |num |num |num |num |num
	lsfd
	cat script
	sh <script

Run gmake run-testshell to test your shell. Testshell simply feeds the above commands (also found in fs/testshell.sh) into the shell and then checks that the output matches fs/testshell.key.