Josef “Jeff” Sipek

Post Formats in blahgd

After reading What creates a good wikitext dialect depends on how it’s going to be used, I decided to write a short post about how I handled the changing needs that my blahg experienced.

One of the things that I implemented in my blogging software a long time ago was the support for different flavors of markup. This allowed me to switch to a “saner” markup without revisiting all the previous entries. Since every post already has metadata (title, publication time, etc.) it was easy enough to add another field (“fmt”) which in my case is just a simple integer identifying how the post contents should be parsed.

Over the years, there have been four formats:

fmt 0 (removed)
Wordpress compat
fmt 1
“Improved” Wordpress format
fmt 2
raw html
fmt 3
LaTeX-like (my current format of choice)

The formats follow a pretty natural progression.

It all started in January 2009 as an import of about 250 posts from Wordpress. Wordpress stores the posts in a html-esque format. At the very least, it inserts line and paragraph breaks. If I remember correctly, one newline is a line break and two newlines are a paragraph break, but otherwise is passes along HTML more or less unchanged. Suffice to say, I was not in the mood to rewrite all the posts in a different format right away so I implemented something that resembled the Wordpress behavior. I did eventually end up converting these posts to a more modern format and then I removed support for this one.

The next format (fmt 1) was a natural evolution of fmt 0. One thing that drove me nuts about fmt 0 was the handling of line breaks. Since the whole point of blahgd was to allow me to compose entries in a text editor (vim if you must know) and I like my text files to be word wrapped, the transformation of every newline to <br/> was quite annoying. (It would result in jagged lines in the browser!) So, about a month later (February 2009), I made a trivial change to the parsing code to treat a single newline as just whitespace. I called this changed up parser fmt 1. (There are currently 24 posts using this format.)

A couple of months after I added fmt 1, I came to the conclusion that in some cases I just wanted to write raw HTML. And so fmt 2 was born (April 2009). (There are currently 5 posts using this format.)

After using fmt 2 for about a year and a half, I concluded that writing raw HTML is not the way to go. Any time I wanted to change the rendering of a certain thing (e.g., switching between <strong> and <b>), I had to revisit every post. Since I am a big fan of LaTeX, I thought it would be cool to have a LaTeX-like markup. It took a couple of false starts spanning about six months but eventually (February 2011) I got the lex and yacc files working well enough. (There are currently 422 posts using this format.)

While I am reasonably happy with fmt 3, but I do see some limitations that I’d like to address. This will inevitably lead to fmt 4. I am hoping to make a separate post about the fmt 3 limitations and how fmt 4 will address them sometime in the (hopefully) near future.

Inline Assembly & clang

Recently I talked about inline assembly with GCC and clang where I pointed out that LLVM seems to produce rather silly machine code. In a comment, a reader asked if this was LLVM’s IR doing this or if it was the machine code generator being silly. I was going to reply there, but the reply got long enough to deserve its own post.

I’ve dealt with LLVM’s IR for a couple of months during the fall of 2010. It was both interesting and quite painful.

The IR is at the Wikipedia article: single static assignment level. It assumes that stack space is cheap and infinite. Since it is a SSA form, it has no notion of registers. The optimization passes transform the IR quite a bit and at the end there is very little (if any!) useless code. In other words, I think it is the machine code generation that is responsible for the unnecessary stack frame push and pop. With that said, it is time to experiment.

Using the same test program as before, of course:

#define _KERNEL
#define _ASM_INLINES
#include <sys/atomic.h>

void test(uint32_t *x)
{
	atomic_inc_32(x);
}

Emitting LLVM IR

Let’s compile it with clang passing in the -emit-llvm option to have it generate test.ll file with the LLVM IR:

$ clang -S -emit-llvm -Wall -O2 -m64 test.c

There is a fair amount of “stuff” in the file, but the relevant portions are (line-wrapped by me):

; Function Attrs: nounwind
define void @test(i32* %x) #0 {
entry:
  tail call void asm sideeffect "lock; incl $0",
    "=*m,*m,~{dirflag},~{fpsr},~{flags}"(i32* %x, i32* %x) #1, !srcloc !1
  ret void
}

attributes #0 = { nounwind uwtable "less-precise-fpmad"="false"
  "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
  "no-infs-fp-math"="false" "no-nans-fp-math"="false"
  "stack-protector-buffer-size"="8" "unsafe-fp-math"="false"
  "use-soft-float"="false" }

LLVM’s IR happens to be very short and to the point. The function prologue and epilogue are not expressed as part of IR blob that gets passed to the machine code generator. Note the function attribute no-frame-pointer-elim being true (meaning frame pointer elimination will not happen).

Now, let’s add in the -fomit-frame-pointer option.

$ clang -S -emit-llvm -Wall -O2 -m64 -fomit-frame-pointer test.c

Now, the relevant IR pieces are:

; Function Attrs: nounwind
define void @test(i32* %x) #0 {
entry:
  tail call void asm sideeffect "lock; incl $0",
    "=*m,*m,~{dirflag},~{fpsr},~{flags}"(i32* %x, i32* %x) #1, !srcloc !1
  ret void
}

attributes #0 = { nounwind uwtable "less-precise-fpmad"="false"
  "no-frame-pointer-elim"="false" "no-infs-fp-math"="false"
  "no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
  "unsafe-fp-math"="false" "use-soft-float"="false" }

The no-frame-pointer-elim attribute changed (from true to false), but the IR of the function itself did not change. (The no-frame-pointer-elim-non-leaf attribute disappeared as well, but it really makes sense since -fomit-frame-pointer is a rather large hammer that just forces frame pointer elimination everywhere and so it doesn’t make sense to differentiate between leaf and non-leaf functions.)

So, to answer Steve’s question, the LLVM IR does not include the function prologue and epilogue. This actually makes a lot of sense given that the IR is architecture independent and the exact details of what the prologue has to do are define by the ABIs.

IR to Assembly

We can of course use llc to convert the IR into real 64-bit x86 assembly code.

$ llc --march=x86-64 test.ll
$ gas -o test.o --64 test.s

Here is the disassembly for clang invocation without -fomit-frame-pointer:

test()
    test:     55                 pushq  %rbp
    test+0x1: 48 89 e5           movq   %rsp,%rbp
    test+0x4: f0 ff 07           lock incl (%rdi)
    test+0x7: 5d                 popq   %rbp
    test+0x8: c3                 ret    

And here is the disassembly for clang invocation with -fomit-frame-pointer:

test()
    test:     f0 ff 07           lock incl (%rdi)
    test+0x3: c3                 ret    

Conclusion

So, it turns out that my previous post simply stumbled across the fact that GCC and clang have different set of optimizations for -O2. GCC includes -fomit-frame-pointer by default, while clang does not.

Generating Random Data

Over the years, there have been occasions when I needed to generate random data to feed into whatever system. At times, simply using /dev/random or /dev/urandom was sufficient. At other times, I needed to generate random data at a rate that exceeded what I could get out of /dev/random. This morning, I read Chris’s blog entry about his need for generating lots of random data. I decided that I should write my favorite approach so that others can benefit.

The approach is very simple. There are two phases. First, we set up our own random pool. Second we use the random pool. I am going to use an example throughout the rest of this post. Suppose that we want to make repeated 128 kB writes to a block device and we want the data to be random so that the device can’t do anything clever (e.g., compress or dedup). Say that during this benchmark we want to write out 64 GB total. (In other words, we will issue 524288 writes.)

Setup Phase

During the setup phase, we create a pool of random data. The easiest way is to just read /dev/urandom. Here, we want to read enough data so that the pool is large enough. For our 128kB write example, we’d want at least 1 MB. (I’ll explain the sizing later. I would probably go with something like 8 MB because unless I’m on some sort of limited system, the extra 7 MB of RAM won’t be missed.)

“Using the Pool” Phase

Now that we have the pool, we can use it to generate random buffers. The basic idea is to pick a random offset into the pool and just grab the bytes starting at that location. In our example, we’d pick a random offset between zero and pool size minus 128 kB, and use the 128 kB at that offset.

In pseudo code:

#define BUF_SIZE	(128 * 1024)
#define POOL_SIZE	(1024 * 1024)

static char pool[POOL_SIZE];

char *generate()
{
	return &pool[rand() % (POOL_SIZE - BUF_SIZE)];
}

That’s it! You can of course make it more general and let the caller tell you how many bytes they want:

#define POOL_SIZE	(1024 * 1024)

static char pool[POOL_SIZE];

char *generate(size_t len)
{
	return &pool[rand() % (POOL_SIZE - len)];
}

It takes a pretty simple argument to show that even a modest sized pool will be able to generate lots of different random buffers. Let’s say we’re dealing with the 128 kB buffer and 1 MB pool case. The pool can return 128 kB starting at offset 0, or offset 1, or offset 2, … or offset 9175043 ($1~MB - 128~kB - 1B$). This means that there are 917504 possible outputs. Recall, that in our example we were planning on writing out 64 GB in total which was 524288 writes.

$\frac{524288}{917504} = 0.571$

In other words, we are planning on using less than 58% of the possible outputs from our 1 MB pool to write out 64 GB of random data! (An 8 MB pool would yield 6.3% usage.)

If the length is variable, the math gets more complicated, but in a way we get even better results (i.e., lower usage) because to generate the same buffer we would need have the same offset and length. If the caller supplies random (pseudo-random or based on some distribution) lengths, we’re very unlikely to get the same buffer out of the pool.

Observations

Some of you may have noticed that we traded generating 128 kB (or a user supplied length) of random data for generating a random integer. There are two options there, either you can use a fast pseudo-random number generator (like the Wikipedia article: Mersenne twister), or you can reuse same pool! In other words:

#define POOL_SIZE	(1024 * 1024)

static char pool[POOL_SIZE];
static size_t *ridx = (size_t *) pool;

char *generate(size_t len)
{
	if ((uintptr_t) ridx == (uintptr_t) &pool[POOL_SIZE])
		ridx = (size_t *) pool;

	ridx++;

	return &pool[*ridx % (POOL_SIZE - len)];
}

I leave it as an exercise for the reader to either make it multi-thread safe, or to make the index passed in similarly to how rand_r takes an argument.

rand_r Considered Harmful

Since we’re on the topic of random number generation, I thought I’d mention what is already rather widely known fact — libc’s rand and rand_r implementations are terrible. At one point, I tried using them with this approach to generate random buffers, but it didn’t take very long before I got repeats! Caveat emptor.

Post Preview

One of the blogs I’ve been reading for a few months now just had a post about partial vs. full entries on blog front pages. Since I have some opinions on the subject, I decided to comment. My response turned into something sufficiently content-full that I decided that my blahg would be a better place for it. Sorry, Chris :P

First of all, my blog doesn’t support partial post display because… technical reasons. (The sinking feeling of discovering a design mistake in your code really resonated with me about this exact thing.) With that said, I don’t think that partial display is necessarily bad. I feel like any reasonable (this is of course subjective) blogging software should follow these rules:

  1. if we’re displaying a atom/rss feed, display full post
  2. if we’re displaying a single post, display full post
  3. if the post contains magical marker that denotes where to stop the preview, display everything above the marker
  4. display full post

I really dislike when the feeds give me the first sentence and I have to click a link to read more. At the very least, it is inconvenient, and in extreeme cases it feels outright insulting.

I think the post-by-post-basis Chris suggests is the way to go, but in the absence of a user-defined division point I would display the whole thing.

Do I write many posts where I wish I could use this magical marker? No. If that were the case, I’d make supporting this a higher priority. However, there have been a handful of times where I believe that the rest of the post is uninteresting to…well…just about everyone and it is really long. So long, that you might get bored trying to scroll past it. (If you are reading my blahg, I don’t want you to be bored because you had to scroll for too long to skip over an entry — you are my guest, and I am here to entertain you.) This is the time I believe displaying a partial post is good.

I’m hoping that eventually I’ll wrestle with my blogging software sufficiently to eliminate the technical reasons preventing me from introducing and processing this special marker. Not that you’ll really notice anything different. :)

SMTP

This is a (long overdue) reply to Ilya’s post: SMPT – Time to chuck it.

I’m going to quote it here, and reply to everything in it. Whenever I say "you," I mean Ilya. So, with that said, let’s get started.

E-mail, in particular SMTP (Simple Mail Transfer Protocol) has become an integral part of our lives, people routinely rely on it to send files, and messages. At the inception of SMTP the Internet was only accessible to a relatively small, close nit community; and as a result the architects of SMTP did not envision problems such as SPAM and sender-spoofing. Today, as the Internet has become more accessible, scrupulous people are making use of flaws in SMTP for their profit at the expense of the average Internet user.

Alright, this is pretty much the only thing I agree with.

There have been several attempts to bring this ancient protocol in-line with the current society but the problem of spam keeps creeping in. At first people had implemented simple filters to get rid of SPAM but as the sheer volume of SPAM increased mere filtering became impractical, and so we saw the advent of adaptive SPAM filters which automatically learned to identify and differentiate legitimate email from SPAM. Soon enough the spammers caught on and started embedding their ads into images where they could not be easily parsed by spam filters.

A history lesson…still fine.

AOL (America On Line) flirted with other ideas to control spam, imposing email tax on all email which would be delivered to its user. It seems like such a system might work but it stands in the way of the open principles which have been so important to the flourishing of the internet.

AOL (I believe Microsoft had a similar idea) really managed to think of something truly repulsive. The postal system in the USA didn’t always work the way it does today. A long time ago, the recipient paid for the delivery. AOL’s idea seems a lot like that.

There are two apparent problems at the root of the SMTP protocol which allow for easy manipulation: lack of authentication and sender validation, and lack of user interaction. It would not be difficult to design a more flexible protocol which would allow for us to enjoy the functionality that we are familiar with all the while address some, if not all of the problems within SMTP.

To allow for greater flexibility in the protocol, it would first be broken from a server-server model into a client-server model.

This is first point I 100% disagree with…

That is, traditionally when one would send mail, it would be sent to a local SMTP server which would then relay the message onto the next server until the email reached its destination. This approach allowed for email caching and delayed-send (when a (receiving) mail server was off-line for hours (or even days) on end, messages could still trickle through as the sending server would try to periodically resend the messages.) Todays mail servers have very high up times and many are redundant so caching email for delayed delivery is not very important.

"Delayed delivery is not very important"?! What? What happened to the whole "better late than never" idiom?

It is not just about uptime of the server. There are other variables one must consider when thinking about the whole system of delivering email. Here’s a short list; I’m sure I’m forgetting something:

  • server uptime
  • server reliability
  • network connection (all the routers between the server and the "source") uptime
  • network connection reliability

It does little to no good if the network connection is flakey. Ilya is arguing that that’s rarely the case, and while I must agree that it isn’t as bad as it used to be back in the 80’s, I also know from experience that networks are very fragile and it doesn’t take much to break them.

A couple of times over the past few years, I noticed that my ISP’s routing tables got screwed up. Within two hours of such a screwup, things returned to normal, but that’s 2 hours of "downtime."

Another instance of a network going haywire: one day, at Stony Brook University, the internet connection stopped working. Apparently, a compromised machine on the university campus caused a campus edge device to become overwhelmed. This eventually lead to a complete failure of the device. It took almost a day until the compromised machine got disconnected, the failed device reset, and the backlog of all the traffic on both sides of the router settled down.

Failures happen. Network failures happen frequently. More frequently that I would like them to, more frequently than the network admins would like them to. Failures happen near the user, far away from the user. One can hope that dynamic routing tables keep the internet as a whole functioning, but even those can fail. Want an example? Sure. Not that long ago, the well know video repository YouTube disappeared off the face of the Earth…well, to some degree. As this RIPE NCC RIS case study shows, on February 24, 2008, Pakistan Telecom decided to announce BGP routes for YouTube’s IP range. The result was, that if you tried to access any of YouTube’s servers on the 208.65.152.0/22 subnet, your packets were directed to Pakistan. For about an hour and twenty minutes that was the case. Then YouTube started announcing more granular subnets, diverting some of the traffic back to itself. Eleven minutes later, YouTube announced even more granular subnets, diverting large bulk of the traffic back to itself. Few dozen minutes later, PCCW Global (Pakistan Telecom’s provider responsible for forwarding the "offending" BGP announcements to the rest of the world) stopped forwarding the incorrect routing information.

So, networks are fragile, which is why having an email transfer protocol that allows for retransmission a good idea.

Instead, having direct communication between the sender-client and the receiver-server has many advantages: opens up the possibility for CAPTCHA systems, makes the send-portion of the protocol easier to upgrade, and allows for new functionality in the protocol.

Wow. So much to disagree with!

  • CAPTCHA doesn’t work
  • What about mailing lists? How does the mailing list server answer the CAPTCHAs?
  • How does eliminating server-to-server communication make the protocol easier to upgrade?
  • New functionality is a nice thing in theory, but what do you want from your mail transfer protocol? I, personally, want it to transfer my email between where I send it from and where it is supposed to be delivered to.
  • If anything eliminating the server-to-server communication would cause the MUAs to be "in charge" of the protocols. This means that at first there would be many competing protocols, until one takes over - not necessarily the better one (Betamax vs. VHS comes to mind).
  • What happens in the case of overzealous firewall admins? What if I really want to send email to bob@example.com, but the firewall (for whatever reason) is blocking all traffic to example.com?

Spam is driven by profit, the spammers make use of the fact that it is cheap to send email. Even the smallest returns on spam amount to good money. By making it more expensive to send spam, it would be phased out as the returns become negative. Charging money like AOL tried, would work; but it is not a good approach, not only does it not allow for senders anonymity but also it rewards mail-administrators for doing a bad job (the more spam we deliver the more money we make).

Yes, it is unfortunately true, money complicates things. Money tends to be the reason why superior design fails to take hold, and instead something inferior wins - think Betamax vs. VHS. This is why I think something similar would happen with competing mail transfer protocols - the one with most corporate backing would win, not the one that’s best for people.

Another approach is to make the sender interact with the recipient mail server by some kind of challenge authentication which is hard to compute for a machine but easy for a human, a Turing test. For example the recipient can ask the senders client to verify what is written on an obfuscated image (CAPTCHA) or what is being said on a audio clip, or both so as to minimize the effect on people with handicaps.

Nice thought about the handicapped, but you are forgetting that only 800-900 million people speak English (see Wikipedia article: Wikipedia). That is something on the order of 13-15 percent. Sorry, but "listening comprehension" tests are simply not viable.

Obfuscated image Wikipedia article: CAPTCHAs are "less" of a problem, but then again, one should consider the blind. I am not blind, and as a matter of fact my vision is still rather good (even after years of staring at computer screens), but at times I’m not sure what those "distorted text" CAPTCHAs are even displaying. I can’t even begin to imagine what it must be like for anyone with poor vision.

You seem to be making the assumption that most if not all legitimate email comes from humans. While that may be true for your average home user, let’s not forget that email is used by more technical people as well. These people will, and do, use email in creative ways. For example, take me…I receive lots of emails that are generated by all sorts of scripts that I wrote over time. These emails give me status of a number of systems I care about, and reminders about upcoming events. All in all, you could say that I live inside email. You can’t do a CAPTCHA for the process sending the automated email (there’s no human sending it), and if you do the CAPTCHA for the receiving, you’re just adding a "click here to display the message" wart to the mail client software user interface.

Just keep in mind that all those automated emails you get from "root" or even yourself were sent without a human answering a CAPTCHA.

It would be essential to also white list senders so that they do not have to preform a user-interactive challenge to send the email, such that mail from legitimate automated mass senders would get through (and for that current implementation of sieve scripts could be used). In this system, if users were to make wide use of filters, we would soon see a problem. If nearly everyone has a white list entry for Bank Of America what is to prevent a spammer to try to impersonate that bank?

White listing is really annoying, and as you point out, it doesn’t work.

And so this brings us to the next point, authentication, how do you know that the email actually did, originate from the sender. This is one of the largest problems with SMTP as it is so easy to fake ones outgoing email address. The white list has to rely on a verifiable and consistent flag in the email. A sample implementation of such a control could work similar to the current hack to the email system, SPF, in which a special entry is made in the DNS entry which says where the mail can originate from. While this approach is quite effective in a sever-server architecture it would not work in a client-server architecture. Part of the protocol could require the sending client to send a cryptographic-hash of the email to his own receiving mail server, so that the receiving party’s mail server could verify the authenticity of the source of the email. In essence this creates a 3 way handshake between the senders client, the senders (receiving) mail server and the receiver’s mail server.

I tend to stay away from making custom authentication protocols.

In this scheme, what guarantees you that the client and his "home server" aren’t both trying to convince the receiving server that the email is really from whom they say it is? In kerberos, you have a key for each system, and a password for each user. The kerberos server knows it all, and this central authority is why things work. With SSL certificates, you rely on the strength of the crypto used, as well as blind faith in the certificate authority.

At first it might seem that this process uses up more bandwidth and increases the delay of sending mail but one has to remember that in usual configuration of sending email using IMAP or POP for mail storage one undergoes a similar process,

Umm…while possible, I believe that very very large majority of email is sent via SMTP (and I’m not even counting all the spam).

first email is sent for storage (over IMAP or POP) to the senders mail server and then it is sent over SMTP to the senders email for redirection to the receivers mail server. It is even feasible to implement hooks in the IMAP and POP stacks to talk to the mail sending daemon directly eliminating an additional socket connection by the client.

Why would you want to stick with IMAP and POP? They do share certain ideas with SMTP.

For legitimate mass mail this process would not encumber the sending procedure as for this case the sending server would be located on the same machine as the senders receiving mail server (which would store the hash for authentication), and they could even be streamlined into one monolithic process.

Not necessarily. There are entire businesses that specialize in mailing list maintenance. You pay them, and they give you an account with software that maintains your mailing list. Actually, it’s amusing how similar it is to what spammers do. The major difference is that in the legitimate case, the customer supplies their own list of email address to mail. Anyway, my point is, in these cases (and they are more common than you think) the mailing sender is on a different computer than the "from" domain’s MX record.

Some might argue that phasing out SMTP is a extremely radical idea, it has been an essential part of the internet for 25 years.

Radical? Sure. But my problem is that there is no replacement. All the ideas you have listed have multiple problems - all of which have been identified by others. And so here we are, no closer to the solution.

But then, when is the right time to phase out this archaic and obsolete protocol, or do we commit to use it for the foreseeable future. Then longer we wait to try to phase it out the longer it will take to adopt something new. This protocol should be designed with a way to coexist with SMTP to get over the adoption curve, id est, make it possible for client to check for recipients functionality, if it can accept email by this new protocol then send it by it rather than SMTP.

Sounds great! What is this protocol that would replace SMTP? Oh, right there isn’t one.

The implementation of such a protocol would take very little time, the biggest problem would be with adoption.

Sad, but true.

The best approach for this problem is to entice several large mail providers (such as Gmail or Yahoo) to switch over. Since these providers handle a large fraction of all mail the smaller guys (like myself) would have to follow suit.

You mentioned Gmail…well, last I heard, Gmail’s servers were acting as open proxies. Congratulations! One of your example "if they switch things will be better" email providers is allowing the current spam problem to go on. I guess that makes you right. If Gmail were to use a protocol that didn’t allow for spam to exist, then things would be better.

There is even an incentive for mail providers to re-implement mail protocol, it would save them many CPU-cycles since Bayesian-spam-filters would no longer be that important.

What about generating all those CAPTCHAs you suggested? What about hashing all those emails? Neither activity is free.

By creating this new protocol we would dramatically improve an end users experience online, as there would be fewer annoyances to deal with. Hopefully alleviation of these annoyances will bring faster adoption of the protocol.

I really can’t help but read that as "If we use this magical protocol that will make things better, things will get better!" Sorry, but unless I see some protocol which would be a good candidate, I will remain sceptical.

As a side note, over the past  90 days, I received about 164MB of spam that SpamAssassin caught and procmail promptly shoved into the spam mail box. Do I care? Not enough to jump on the "let’s reinvent the email system" bandwagon. Sure, it eats up some of my servers clock cycles, and some bandwidth, the spam that gets to me is the few pieces that manage to get through, and show up in my inbox. Would I be happy if I didn’t have to use a spam filter, and not have to delete the few random spams by hand? Sure, but at least for the moment, I don’t see a viable alternative.

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