How I fixed libuv

A deep dive into the foundation of Node

It was another lazy, rainy winter afternoon, so once again I was looking to make the world of node.js a better place. Browsing the issues, I was struck by the bizarreness of #4291. The user anseki had written a simple script to read the next line the user types into the terminal. If you resized the window running the program while it was waiting for input, the program crashed with a segmentation fault.

Segmentation faults can happen in C when a program makes an invalid memory access. So somehow, resizing the window triggered an invalid C memory access from this Javascript program. Most untoward!

Here is a simplified version of the script, with some added annotations:

var fs = require('fs'); // node.js's module for interacting with the filesystem (including the terminal)
var buffer = new Buffer(1024); // instantiate a Buffer object, which fs writes data into

process.stdout; // this line seems to do nothing, but the segmentation fault only happens when it's present

var readSize = fs.readSync(fs.openSync('/dev/tty', 'r'), buffer, 0, bufferSize); // receive some input from the user and write it into buffer
var chunk = buffer.toString('utf8', 0, readSize); // turn the buffer into a string to get a pretty log message

console.log('INPUT: ' + chunk);

libuv: The C part of Node

Node.js features several handy libraries for building server applications, such as the fs and http modules. These modules are implemented by calling functions from a library called libuv. libuv is an input/output handling module that deals directly with the operating system. It is written in C, so it is the place to look if you encounter any segmentation faults using node.js. The node.js collaborator evanlucas did some basic profiling to find the line where the segmentation fault happened: apparently it was a null pointer access on line 275 of libuv/unix/fs.c:

result = read(req->file, req->bufs[0].base, req->bufs[0].len);

When the window was resized while our program was running, that line executed with req->bufs having the value NULL. It was up to me to figure out how that happened.

The search

First, I figured out what the req structure is: it’s a uv_req_t, a container that libuv uses to encapsulate data about a filesystem request. req->file is the file descriptor for the file that the request is accessing. A file descriptor is a number that the operating system provides to programs that perform operations on files. If a program performs an operation on a file descriptor, the operating system performs the operation on the appropriate file. req->file in this case is the file descriptor for /dev/tty (the terminal input), since that’s the file that the Javascript code is attempting to read from.

Next, req->bufs is an array of uv_buf_t objects. uv_buf_t is libuv’s structure for buffering data. A uv_buf_t object has a byte array called base and a property called len representing the number of bytes in the array. req->bufs is an array of uv_buf_ts that store the data that the request is interested in. Putting it all together, our read command is supposed to take req->bufs[0].len bytes from the terminal input and place them in req->bufs[0]. But as we’ve seen, in my script req->bufs is NULL rather than a valid array of uv_buf_ts after resizing the terminal. So the attempt to access the base and len of the first element of NULL triggers a segmentation fault.

I had to find out how req->bufs was being NULLified. I searched through fs.c for the string “req->bufs =” to find all the places where req->bufs gets assigned any value. I found a very suspsicious one at the end of a function called uv__fs_buf_iter:

req->bufs = NULL;

That seemed likely to be the place where req->bufs was being set to NULL. Tracing through the code further, I found that the API function uv_fs_read calls a function uv__fs_work, which calls uv__fs_buf_iter. uv__fs_buf_iter in turn calls uv__fs_read (note the double underscore after uv, so this is different from the API function). uv__fs_read finally does the actual reading work, including the call to C’s read function that we saw on line 275 above. When uv__fs_read is finished, uv__fs_buf_iter cleans up the resources that it used, including nullifying req->bufs. So if uv__fs_buf_iter were called twice on the same req, then req->bufs would be NULL during the second call, causing a segmentation fault.

Read, interrupted

So I looked for a way that uv__fs_buf_iter might be called twice on the same req. I found it in uv__fs_work. The body of uv__fs_work is a while loop that makes calls to various uv__ functions, including uv__fs_buf_iter. The loop condition is while (r == -1 && errno == EINTR && retry_on_eintr). If this condition were to hold after a call to uv__fs_buf_iter, we’d immediately make another call to uv__fs_buf_iter with the same req object.

Here’s a diagram of the chain of events I just described, leading to two calls to uv__fs_buf_iter on the same req:

libuv read catastrophe

So I had to see if there was any way (r == -1 && errno == EINTR && retry_on_eintr) could hold after a call to uv__fs_buf_iter. First I had to figure out what it meant!

The first statement in the condition, r == -1, refers to an int r declared earlier in uv__fs_work. uv__fs_work sets r to the return value of whatever uv__ function it calls. uv__fs_read returns the number of bytes read, or -1 if there was an error during the read. So r == -1 checks whether there was an error during uv__fs_read.

The next statement, errno == EINTR, refers to a global variable in C called errno and its potential value, EINTR. Whenever a call from C to the operating system completes, errno is set to one of C’s error codes if there was an error during the operation. (If there was no error, errno is completely undefined.)

EINTR is an errno value that means “Interrupted Function”. It indicates that the read was interrupted by a signal. Signals in C are messages that the operating system can send to a program. When a program receives a signal from the operating system, it stops whatever it is doing to handle the signal. If a read is underway but hasn’t received any data yet, as in the case of my program awaiting user input, then receiving a signal causes that read to fail and set errno to EINTR.

The last part of our condition, retry_on_eintr, is always true for reads in libuv, because receiving a signal is usually orthogonal to a program’s read activity. For instance, if libuv didn’t retry reads on EINTR, then our original Javascript program would terminate when you resize the window. While that’s a little better than segmentation faulting, it is still not the desired behavior, which is for the program to just keep waiting for input.

With some further research, I found that resizing the terminal window causes the operating system to send a window size change signal to the program! Receiving this signal causes read to fail and set errno to EINTR, triggering the disastrous chain of events diagrammed above.

The fix

I decided that the dysfunctional link in this chain was when  uv__fs_buf_iter unconditionally freed the resources in the req. Before freeing the resources, it should check errno to determine whether the read is going to be retried: if so, it should keep the req object valid.

So I cobbled together a pull request adding such a check to the end of uv__fs_buf_iter. The libuv maintainer indutny said he had been working on just such a change too, but since I’d beaten him to the punch he’d code-review my pull request. We worked out the optimal place for the check to go, and he gave me some helpful tips for writing a unit test for the bug fix. Once I had the unit test working right, he and another maintainer saghul suggested a few stylistic tweaks to get my new code to look like the rest of libuv’s source. After I made those, they approved it, and my commit was in! That’s the story of how I fixed libuv.

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