Made of Bugs

Tracking Down a Memory Leak in Ruby's EventMachine

At Stripe, we rely heavily on ruby and EventMachine to power various internal and external services. Over the last several months, we've known that one such service suffered from a gradual memory leak, that would cause its memory usage to gradually balloon from a normal ~50MB to multiple gigabytes.

It was easy enough to work around the leak by adding monitoring and restarting the process whenever memory usage grew too large, but we were determined to track down the root cause. Our exploration is a tour through a number of different debugging tools and techniques, so I thought I would share it here.

Checking for ruby-level leaks

One powerful technique for tracking down tough memory leaks is post-mortem analysis. If our program's normal memory footprint is 50MB, and we let it leak until it's using, say, 2GB, 1950/2000 = 97.5% of the program's memory is leaked objects! If we look at a core file (or, even better, a running image in gdb), signs of the leak will be all over the place.

So, we let the program leak, and, when its memory usage got large enough, failed active users over to a secondary server, and attached gdb to the bloated image.

Our first instinct in a situation like this is that our Ruby code is leaking somehow, such as by accidentally keeping a list of every connection it has ever seen. It's easy to investigate this possibility by using gdb, the Ruby C API, and the Ruby internal GC hooks:

(gdb) p rb_eval_string("GC.start")
$1 = 4
(gdb) p rb_eval_string("$gdb_objs = Hash.new 0")
$2 = 991401552
(gdb) p rb_eval_string("ObjectSpace.each_object {|o| $gdb_objs[o.class] += 1}")
$3 = 84435
(gdb) p rb_eval_string("$stderr.puts($gdb_objs.inspect)")
$4 = 4

Calling rb_eval_string lets us inject arbitrary ruby code into the running ruby process, from within gdb. Using that, we first trigger a GC – making sure that any unreferenced objects are cleaned up – and then walk the Ruby ObjectSpace, building up a census of which Ruby objects exist. Looking at the output and filtering for the top objects, we find:

String => 26399
Array  => 8402
Hash   => 2161
Proc   => 608

Just looking at those numbers, my gut instinct is that nothing looks too out of whack. Running some back-of-the-envelope numbers confirms this instinct:

  • In total, that's about 40,000 objects for those object types.
  • We're looking for a gradual leak, so we expect lots of small objects. Let's guess that "small" is around 1 kilobyte.
  • 40,000 1k objects is only 40MB. Our process is multiple GB at this point, so we are nowhere near explaining our memory usage.

It's possible, of course, that one of those Strings has been growing without bound, and is now billions of characters long, but that feels unlikely. A quick survey of String lengths using ObjectSpace would confirm that, but we didn't even bother at this point.

Searching for C object leaks

So, we've mostly ruled out a Ruby-level leak. What now?

Well, as mentioned, 95+% of our program's memory footprint is leaked objects. So if we just take a random sample of bits of memory, we will find leaked objects with very good probability. We generate a core file in gdb:

(gdb) gcore leak.core
Saved corefile leak.core

And then look at a random page (4k block) of the core file:

$ off=$(($RANDOM % ($(stat -c "%s" leak.core)/4096)))
$ dd if=leak.core bs=4096 skip=$off count=1 | xxd
0000000: 0000 0000 0000 0000 4590 c191 3a71 b2aa  ........E...:q..
...

Repeating a few times, we notice that most of the samples include what looks to be a repeating pattern:

00000f0: b05e 9b0a 0000 0000 0000 0000 0000 0000  .^..............
0000100: 0000 0000 0000 0000 0100 0000 dcfa 1939  ...............9
0000110: 0000 0000 0000 0000 0a05 0000 0000 0000  ................
0000120: 0000 0000 0000 0000 d03b a51f 0000 0000  .........;......
0000130: 8000 0000 0000 0000 8100 0000 0000 0000  ................
0000140: 00a8 5853 1b7f 0000 0000 0000 0000 0000  ..XS............
0000150: 0000 0000 0000 0000 0100 0000 0100 0000  ................
0000160: 0000 0000 0000 0000 ffff ffff 0000 0000  ................
0000170: 40ef e145 0000 0000 0000 0000 0000 0000  @..E............
0000180: 0000 0000 0000 0000 0100 0000 0000 0000  ................
0000190: 0000 0000 0000 0000 0a05 0000 0000 0000  ................
00001a0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00001b0: 8000 0000 0000 0000 8100 0000 0000 0000  ................
00001c0: 00a8 5853 1b7f 0000 0000 0000 0000 0000  ..XS............
00001d0: 0000 0000 0000 0000 0100 0000 0100 0000  ................
00001e0: 0000 0000 0000 0000 ffff ffff 0000 0000  ................
00001f0: 4062 1047 0000 0000 0000 0000 0000 0000  @b.G............
0000200: 0000 0000 0000 0000 0100 0000 1b7f 0000  ................
0000210: 0000 0000 0000 0000 0a05 0000 0000 0000  ................
0000220: 0000 0000 0000 0000 e103 0000 0000 0000  ................
0000230: 8000 0000 0000 0000 9100 0000 0000 0000  ................
0000240: 00a8 5853 1b7f 0000 0000 0000 0000 0000  ..XS............
0000250: 0000 0000 0000 0000 0100 0000 0100 0000  ................
0000260: 0000 0000 0000 0000 ffff ffff 0000 0000  ................
0000270: 50b0 350b 0000 0000 0000 0000 0000 0000  P.5.............
0000280: 0000 0000 0000 0000 0100 0000 0000 0000  ................
0000290: 0000 0000 0000 0000 0a05 0000 0000 0000  ................
00002a0: 0000 0000 0000 0000 30de 4027 0000 0000  ........0.@'....
00002b0: 0077 7108 0000 0000 0060 7b97 b4d2 f111  .wq......`{.....
00002c0: 9000 0000 0000 0000 8100 0000 0000 0000  ................
00002d0: 00a8 5853 1b7f 0000 0000 0000 0000 0000  ..XS............
00002e0: 0000 0000 0000 0000 0100 0000 0100 0000  ................

Those ffff ffff blocks, repeated every 128 bytes, leap out at me, and 4 out of 5 samples of the core file reveal a similar pattern. It seems probable that we're leaking 128-byte objects of some sort, some field of which is -1 as a signed 32-bit integer, i.e. ffff ffff in hex.

Looking further, we also notice a repeated 00a8 5853 1b7f 0000, two lines before each ffff ffff. If you've stared at too many Linux coredumps, as I have, that number looks suspicious. Interpreted in little-endian, that is 0x00007f1b5358a800, which points near the top of the userspace portion of the address space on an amd64 Linux machine.

In fewer words: It's most likely a pointer.

The presence of an identical pointer in every leaked object suggests that the pointer most likely points to some kind of "type" object or tag, containing information about type of the leaked object. For instance, if we were leaking Ruby String objects, every one would have an identical pointer to the Ruby object that represents the `String' class. So, let's take a look:

(gdb) x/16gx 0x00007f1b5358a800
0x7f1b5358a800: 0x0000000000000401  0x00007f1b53340f24
0x7f1b5358a810: 0x00007f1b532c7780  0x00007f1b532c7690
0x7f1b5358a820: 0x00007f1b532c7880  0x00007f1b532c78c0
0x7f1b5358a830: 0x00007f1b532c74f0  0x00007f1b532c74c0
0x7f1b5358a840: 0x00007f1b532c7470  0x0000000000000000
0x7f1b5358a850: 0x0000000000000000  0x0000000000000000
0x7f1b5358a860: 0x0000000000000406  0x00007f1b5332a549
0x7f1b5358a870: 0x00007f1b532c7a40  0x00007f1b532c7a30

The first field, 0x401, contains only two bits set, suggesting some kind of flag field. After that, there are a whole bunch of pointers. Let's chase the first one:

(gdb) x/s 0x00007f1b53340f24
0x00007f1b53340f24: "memory buffer"

Great. So we are leaking … memory buffers. Thanks.

But this is actually fantastically informative, especially coupled with the one other piece of information we have: /proc/<pid>/maps for the target program, which tells us which files are mapped into our program at which addresses. Searching that for the target address, we find:

7f1b53206000-7f1b5336c000 r-xp 00000000 08:01 16697      /lib/libcrypto.so.0.9.8

0x7f1b532060000x7f1b5358a800 < 7f1b5336c000, so this mapping contains our "type" object. libcrypto is the library containing OpenSSL's cryptographic routines, so we are leaking some sort of OpenSSL buffer object. This is real progress.

I am not overly familiar with libssl/libcrypto, so let's go to the source to learn more:

$ apt-get source libssl0.9.8
$ cd openssl*
$ grep -r "memory buffer" .
./crypto/err/err_str.c:{ERR_PACK(ERR_LIB_BUF,0,0)       ,"memory buffer routines"},
./crypto/asn1/asn1.h: * be inserted in the memory buffer
./crypto/bio/bss_mem.c: "memory buffer",
./README:        sockets, socket accept, socket connect, memory buffer, buffering, SSL
./doc/ssleay.txt:-  BIO_s_mem()  memory buffer - a read/write byte array that
./test/times:talks both sides of the SSL protocol via a non-blocking memory buffer

Only one of those is a string constant, so we go browse ./crypto/bio/bss_mem.c and read the docs (bio(3) and buffer(3)) a bit. Sparing you all the details, we learn:

  • OpenSSL uses the BIO structure as a generic abstraction around any kind of source or sink of data that can be read or written to.
  • A BIO has a pointer to a BIO_METHOD, which essentially contains a small amount of metadata and a vtable, describing what specific kind of BIO this is, and how to interact with it. The second field in a BIO_METHOD is a char * pointing at a string holding the name of this type.
  • One of the common types of BIOs is the mem BIO, backed directly by an in-memory buffer (a BUF_MEM). The BIO_METHOD for memory BIOs has the type tag "memory buffer".

So, it appears we are leaking BIO objects. Interestingly, we don't actually seem to be leaking the underlying memory buffers, just the BIO struct that contains the metadata about the buffer.

Tracing the source

This kind of leak has to be in some C code somewhere. Clearly nothing in pure ruby code should be able to do this. The server in question contains no C extensions we wrote, so it's probably in some third-party library we use.

A leak in OpenSSL itself is certainly possible, but OpenSSL is very widely used and quite mature, so let's assume (hope) that our leak is not there, for now.

That leaves EventMachine as the most likely culprit. Our program uses SSL heavily via the EventMachine APIs, and we do know that EventMachine contains a bunch of C/C++, which, to be frank, does not have a sterling reputation.

So, we pull up an EventMachine checkout. There are a number of ways to construct a new BIO, but the most basic and common seems to be BIO_new, sensibly enough. So let's look for that:

$ git grep BIO_new
ext/rubymain.cpp:       out = BIO_new(BIO_s_mem());
ext/ssl.cpp:    BIO *bio = BIO_new_mem_buf (PrivateMaterials, -1);
ext/ssl.cpp:    pbioRead = BIO_new (BIO_s_mem());
ext/ssl.cpp:    pbioWrite = BIO_new (BIO_s_mem());
ext/ssl.cpp:    out = BIO_new(BIO_s_mem());

Great: there are some calls (so it could be one of them!), but not too many (so we can reasonably audit them all).

Starting from the top, we find, in ext/rubymain.cpp:

static VALUE t_get_peer_cert (VALUE self, VALUE signature)
{
    VALUE ret = Qnil;
    X509 *cert = NULL;
    BUF_MEM *buf;
    BIO *out;

    cert = evma_get_peer_cert (NUM2ULONG (signature));

    if (cert != NULL) {
        out = BIO_new(BIO_s_mem());
        PEM_write_bio_X509(out, cert);
        BIO_get_mem_ptr(out, &buf);
        ret = rb_str_new(buf->data, buf->length);
        X509_free(cert);
    BUF_MEM_free(buf);
    }

    return ret;
}

The OpenSSL APIs are less than perfectly self-descriptive, but it's not too hard to puzzle out what's going on here:

We first construct a new BIO backed by a memory buffer:

out = BIO_new(BIO_s_mem());

We write the certificate text into that BIO:

PEM_write_bio_X509(out, cert);

Get a pointer to the underlying BUF_MEM:

BIO_get_mem_ptr(out, &buf);

Convert it to a Ruby string:

ret = rb_str_new(buf->data, buf->length);

And then free the memory:

BUF_MEM_free(buf);

But we've called the wrong free function! We're freeing buf, which is the underlying BUF_MEM, but we've leaked out, which is the BIO itself we also allocated. This is exactly the kind of leak we saw in our core dump!

Continuing our audit, we find the exact same bug in ssl_verify_wrapper in ssl.cpp. Reading code, we learn that t_get_peer_cert is called by the Ruby function get_peer_cert, used to retrieve the peer certificate from a TLS connection, and that ssl_verify_wrapper is called if you pass :verify_peer => true to start_tls, to convert the certificate into a Ruby string for passing to the ssl_verify_peer hook.

So, any time you make a TLS or SSL connection with EventMachine and verify the peer certificate, EventMachine was leaking 128 bytes! Since it's presumably pretty rare to make a large number of SSL connections from a single Ruby process, it's not totally surprising that no one had caught this before.

Having found the issue, the fix was simple, and was promptly merged upstream.