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How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

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Re: How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

#21

Post by stefan_ »

jiggawatts wrote: post #8
hdmoore wrote: post #3

Erm, qmail had lots of bugs[1], when compiled for 64-bit processors (lots of integer overflows), but djb pushed back and said 64-bit wasn't supported. If anything, qmail is known as the most annoying MTA to package, since no modifications to the source are permitted, and the application has to be built using a massive patch tree instead. The quirky management daemons required to run qmail were also obnoxious and at o…

Reminds me of the era when dual-core processors started becoming generally available. Suddenly the bugs in multi-threaded software were much more apparent. Vendors replied to complaints with: “We don’t support those processors”. No buddy, you don’t support stable software. It’s buggy even on a single core, it’s just less obvious.

It's a very interesting phenomenon. There are lots of claims along the lines of "a single CPU is so fast it will exercise all kinds of multi-threading patterns very quickly" but in my experience it indeed takes multiple processors to reliably show up various races and faulty implicit orderings.

Re: How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

#22

Post by literalAardvark »

troutwine wrote: post #5
daneel_w wrote: post #2

Does anyone know how qmail has fared since this PDF was written in 2007? Did it make it to 2023 without any bugs surfacing?

It didn't make the transition to 64 bits worth of memory with the record intact. https://lwn.net/Articles/820969/ Although the CVE _is_ from 2005 so perhaps it doesn't count.

I mean... I'd say expecting a program written for a 32 bit OS to just work on 64 is excessive. And the fix is just to add mildly sane memory limits to the processes, which is an OS level task, thus, not necessarily a bug.

Not that the whole thing was very well handled.

Re: How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

#23

Post by jiggawatts »

stefan_ wrote: post #21
jiggawatts wrote: post #8

Earlier quoted context omitted.

Reminds me of the era when dual-core processors started becoming generally available. Suddenly the bugs in multi-threaded software were much more apparent. Vendors replied to complaints with: “We don’t support those processors”. No buddy, you don’t support stable software. It’s buggy even on a single core, it’s just less obvious.

It's a very interesting phenomenon. There are lots of claims along the lines of "a single CPU is so fast it will exercise all kinds of multi-threading patterns very quickly" but in my experience it indeed takes multiple processors to reliably show up various races and faulty implicit orderings.

Single processors interleave threads using very coarse "quantums" of execution, where each thread runs for many milliseconds at a time. Dual processors will run the instruction streams truly in parallel.

From a correctness perspective, they're "identical", because even on a single core a context switch can occur anywhere. In theory, even one core can interleave individual instructions, it's just not done for performance reasons.

So if multiple cores causes a crash, then that crash bug was always there, and could have occurred even on a single core. It's just much less likely to be triggered.

Thankfully now every mainstream computer and practically all phones are multi-core, so software has to be written properly. The early 2000s were a mess though, with a lot of heisenbugs that were maddening to track down.

Re: How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

#24

Post by troutwine »

literalAardvark wrote: post #22
troutwine wrote: post #5

Earlier quoted context omitted.

It didn't make the transition to 64 bits worth of memory with the record intact. https://lwn.net/Articles/820969/ Although the CVE _is_ from 2005 so perhaps it doesn't count.

I mean... I'd say expecting a program written for a 32 bit OS to just work on 64 is excessive. And the fix is just to add mildly sane memory limits to the processes, which is an OS level task, thus, not necessarily a bug. Not that the whole thing was very well handled.

I would argue that the fix is for the software to refuse to function if it can detect that it's in an environment where it won't function safely, to fail-closed rather than fail-open.

Re: How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

#27

Post by jrpelkonen »

kens wrote: post #7

The title of the actual paper is "Some thoughts on security after ten years of qmail 1.0". The post currently has the made-up title "How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]".

It is also somewhat ironic that the editorialized title has an off-by-one bug in it: It has been 26 years since 1997.

Re: How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

#28

Post by emailsysadmin »

commandersaki wrote: post #6
daneel_w wrote: post #2

Does anyone know how qmail has fared since this PDF was written in 2007? Did it make it to 2023 without any bugs surfacing?

https://lwn.net/Articles/820969/

no one in their right mind keeps qmail-verify active for many reasons.

Re: How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

#29

Post by gerdesj »

Karrot_Kream wrote: post #20
gerdesj wrote: post #14

I ran several Qmail systems. Daemontools worked rather well. They had a habit of delivering email reliably. "Life with Qmail" was a very decent primer. I also ran several other mail systems at the same time (and still do). Exchange's smtpd is still a bit of a pain and I never put it on the internet directly. Nowadays (last 15 years) I use Exim for a MTA/proxy - at home and at work.

Any reason you prefer Exim? I've enjoyed running Postfix forever and though the config is a bit baroque, have had no issues with running it in my somewhat weird setup.

Exim has a rather mad ACL etc system which is what I need. Postfix is absolutely ideal for receive mail, pass to AV/spam checker, pass it on (or not).

Exim is hugely configurable. If you use it on Debian/Ubuntu and you have somewhat complex needs then ditch the split config thingie and simply create your own /etc/exim4/exim4.conf from scratch.

You can test a config with "exim -bhc " to simulate a SMTP session from , before you restart Exim to make the config active. So:

  helo me
  mail from: name@example.co.uk
  rcpt to: name@another.example.co.uk 
  data
  etc etc
At each stage you get a debug output of what would happen. You have to be a bit careful with this because a new Exim daemon will use the new config. For small changes do it "inline", for major changes, copy the conf to a new name and specify it with -C and test until it works. There are a lot of options.

Documentation is complete for Exim. If you find a flaw then Jeremy and co will fix it - the Exim devs are very, very driven, for want of a better word. You will get short shrift on the mailing lists if you obviously haven't tried to do any research but you will soon get some extremely knowledgable help if you show willing. No Disqus etc - just old school mailing lists and that is the way it should be.

The big boys are destroying SMTP gradually but it can and does still work, if you are careful.

EDIT:

Re: How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]

#30

Post by bykhun »

kens wrote: post #7

The title of the actual paper is "Some thoughts on security after ten years of qmail 1.0". The post currently has the made-up title "How to Write Software with Zero bugs – 25 years after qmail 1.0 – D. Bernstein [pdf]".

Sorry, that's actually how I jokingly refer to that paper whenever I quote it, I didn't know the title should match the link title exactly. Now I see why it's flagged
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