30 May 2021
The weirdest bug I've ever encountered
I am responsible for the reliability of the firmware updates on our devices and this work never gets boring. The thing about firmware updates is that by definition, every bug is caused by the updater because before installing the affected version, the bug was not present. You could say that updates are the cause and the solution of all software problems. So, a lot of update bug reports that find their way to me have nothing to do with the updater per se, they just manifest themselves immediately, giving the impression that the updater is broken. But I'm getting ahead of myself, so let's start from the beginning.
A new bug comes in: Update failed from version X to version Y, error getting log files from sensor. After that, the user manually rebooted the machine and it worked again.
I connect to the machine and download all the logfiles. In the logs, I see that the update installed successfully. After the reboot which starts the newly installed firmware, there were some minor problems: Some processes took a bit longer to start than expected. Other than that, the log looks clean, no errors that could explain this behaviour.
I try to install the affected Firmware version Y on various devices. The problem does not appear. But as luck has it, a co-worker encountered the problem and had the presence of mind to recognize it and leave the system undisturbed for analysis. Connecting to the system is difficult because it is laggy and slow. The device can barely handle simple commands. Weird. Let's run
60 processes; 646 threads;
CPU states: 16.0% user, 35.0% kernel
CPU 0 Idle: 55.4%
CPU 1 Idle: 42.4%
Memory: 0 total, 543M avail, page size 4K
PID TID PRI STATE HH:MM:SS CPU COMMAND
1 12 10 Run 1:51:21 17.72% kernel
1 5 10 Run 2:13:10 17.04% kernel
602168 1 10 Rdy 3:30:01 13.98% ps
229398 1 21 Rcv 0:09:32 0.67% smb-tunnelcreek
778301 1 10 Rply 0:00:00 0.62% top
253978 2 21 NSlp 0:05:43 0.40% devi-tsc2007
1 6 10 Rcv 2:08:09 0.25% kernel
7 2 21 Rcv 0:01:41 0.07% io-pkt-v4-hc
581675 15 10 CdV 0:00:17 0.07% telescope
8200 9 21 Rcv 0:00:35 0.05% io-usb
Min Max Average
CPU 0 idle: 55% 55% 55%
CPU 1 idle: 42% 42% 42%
Mem Avail: 543MB 543MB 543MB
Processes: 60 60 60
Threads: 646 646 646
Okay, we immediately have some clues here. The QNX kernel is doing a lot of stuff and so is
ps. We see that the CPU load of the two cores is about 50%. But we know that the system has hyperthreading and only has one physical core, so the real load is actually more like 100%. To me, it looks like
ps is saturating the core completely, the kernel load is probably caused by whatever ps is doing.
Who is calling
ps -ef we can see the
PPID, the parent process id, to track down who spawned this process. Ironically, we use
ps to debug this
ps problem. Like this, I find that it was spawned by the shell
sh, which in turn was spawned by one of our proprietary processes. Equipped with the knowledge of which process is affected and analysing the logs to find out where this process got stuck, I quickly find the offending line of code. It looks like this:
int rc = system( "ps -e | grep Foo" ); where
Foo is some program whose presence we need to check. Not the prettiest way to implement it, but it is legacy code that worked for many years. I also see that this code only gets executed on certain old hardwares. This explains why the problem never reproduced on most systems, they have newer hardware. Even on the old hardware, it is exceedingly rare and hard to reproduce. Killing the offending
ps process, which loops infinitely otherwise, unblocks the system and restores normal operation.
We now know a lot about this problem, but what is the root cause? The
ps program causes 100% CPU load... What is this? We use QNX 6.6 and
ps is supplied as a closed source binary. To understand this problem, we need to analyse the
ps utility itself. We can already do that at the assembly level. The QNX Momentics IDE has an option "Attach to Remote Process via QConn" with which I can tap right into the running process. We see that it is stuck in a loop that calls
devctl() over and over again. The return value of this
3 which is
ESRCH: No such process. This error comes straight from
MsgSendv_r which returned
-3. It documents that
The server died while the calling thread was SEND-blocked or REPLY-blocked..
ps gets stuck in an infinite loop. Dare I say it: Do we have a
ps bug on our hands? The QNX ecosystem is generally quite robust. In the past, it almost always ended up being my own mistake when I suspected problems in QNX and its utilities. But how can the infinite loop seen in live assembly debugging be explained with a user error of
At this point, an intermezzo with some QNX history is in order. A bit more than a decade ago, the QNX source code was available to the public. Back then, QNX had a vibrant open source community. People would experiment with the kernel, write various useful utilities and help each other in forums. QNX even had a fully featured Desktop GUI, ran Firefox and was self-hosting, so you could develop for QNX right on QNX itself with full IDE and compiler support. It was beautiful. Then QNX was bought, source code access was revoked and the community largely withered away. Questions were increasingly asked via private support tickets directly to QNX, locked away from the public. QNX know-how becomes harder and harder to acquire, open source software for modern QNX releases is essentially non-existent and the driver situation is a catastrophe. The QNX kernel is the most beautiful and interesting kernel I have ever had the pleasure of working with, but it lies in the shackles of corporate ownership.
Back to the bug. So there is old QNX source code lying around. Do you think anyone has modified the source code of the
ps utility throughout the last 15 years? Me neither! Let's dive right into the old code.
ps utility compiles for QNX 6.6 on the first try. Nice! It works just like the closed source binary we have. I install my newly compiled
ps, write some software to automatically test reboots in a loop overnight and am able to reproduce the problem with this custom
ps binary. Perfect! Now we can use the "Attach to Remote Process via QConn" feature again, but this time we have the source code of our
ps. I already read the source code of the old
ps before I did all of this, and I already had my suspicions as to which
devctl() call was being repeated endlessly in the loop, so it comes as no surprise when the debugger points me to the following line:
if (devctl (fd, DCMD_PROC_TIDSTATUS, &tinfo, sizeof (tinfo), 0) == EOK)
The root cause of the bug is the following. For every process in the
ps opens a file descriptor to this process' address space (
as) file to read the process information to be displayed. For each process, it loops through all the threads of this process, and the bug is that this loop has insufficient termination criteria (in other words, in some cases, it loops infinitely). The problem occurs when the process whose threads we want to inspect terminates right before
ps enters this loop. In that case, the
devctl() call fails and as you can see in the following simplified snippet, it will never terminate because the
if-clause is never entered.
if (devctl (fd, DCMD_PROC_TIDSTATUS, &tinfo, sizeof (tinfo), 0) == EOK)
// stop when we have gone through each thread, or when
// the user only want process info
if ((usingThreads == 0) || (tcount == info.num_threads))
I also verify this hypothesis by halting the
ps process in the debugger, killing the process whose threads we are about to inspect and resuming
ps and it hangs in precisely the same way. This is what it looks like in the QNX Momentics IDE:
After this analysis, I'm quite sure that this 15 years old bug is still alive and well in our closed source binary.
Why did this problem suddenly appear in our firmware? We can only speculate, it must have been changes in the scheduling order or timing at boot time. The bug is a race condition, so it can rear its ugly head whenever it wants. The affected code was old and deployed in production for many years.
How did I fix it? I briefly considered shipping our own
ps utility, but I was still unsure about other bugs that might potentially be fixed in the latest closed source binary, those fixes would be lost again if I revert to the old open source version. At the end of the day, I decided to comb through our code base and just eliminate the usage of
ps in non-interactive code, there weren't that many instances. Our
ps utility remains buggy as it is, but it's pretty much impossible to reproduce this bug in an interactive terminal, and our firmware no longer uses it. Needless to say, this particular update problem never occurred again after that.
ps bug still exist in QNX ecosystems more recent than QNX 6.6? Most likely, yes. If it was open source, I would fix it and send a pull request. Because it's not, they have to deal with the issue themselves. Maybe in a decade, an unfortunate soul runs into this bug again. Let's hope this blog post will save them some trouble.
What do we learn?
- No matter how battle-tested and old the code and how reputable the distributor - the code contains bugs.
- Old bugs can manifest themselves seemingly out of nowhere, caused by subtle changes in timing or memory layout.
- Whenever the file system is involved, there is a significant danger that bugs are caused by race conditions.
- Closed source operating systems and ecosystems are a pain to debug. Even old open source releases help.
- Do not use interactive shell utilities in non-interactive code. Avoid
system()whenever possible. Not only is the performance terrible, it can give rise to bugs like this one.
- Make sure your loops terminate. Bounded loop variants that increase or decrease strictly monotonically guarantee loop termination. Don't be too clever with loops!
I used to be able to find good info from your blog articles.
Greetings! This is my 1st comment here so I just wanted to give a
quick shout out and tell you I really enjoy reading your posts.
Can you recommend any other blogs/websites/forums that cover the
same subjects? Appreciate it!
Aw, thіs was an extremely ցood post. Spending some timе and actual effort tο create ɑ geat article… bᥙt what cɑn І sаy… I procrastinate ɑ lot and never manage
to get anytһing done.
Appreciating the commitment you put into your site and in depth information you present.
It's nice to come across a blog every once in a while that isn't the same unwanted rehashed material.
Great read! I've saved your site and I'm adding your RSS feeds to my Google
I am sure this post has touched all the internet people, its
really really nice piece of writing on building
up new webpage.
Hmm it appears like yօur website ate my fіrst commеnt (it ᴡas super l᧐ng) so
I guess I'll јust ѕum it uup what I submitted and sаy, I'm thorߋughly enjoying your blog.
Ӏ too am an aspiring blog logger Ьut I'm ѕtіll new too tһе wholе thing.
Do you haᴠe any suggestions fօr newbie blog writers? Ι'd ceгtainly aρpreciate іt.
I'm extremely inspired together with your writing abilities and also with the format for your weblog.
Is that this a paid theme or did you modify it yourself?
Anyway keep up the nice high quality writing, it's rare to peer a great weblog like
this one nowadays..
You're so cool! I do not believe I've truly read a single thing like this before.
So wonderful to discover another person with original thoughts on this issue.
Seriously.. many thanks for starting this up.
This site is something that's needed on the internet, someone with a bit of originality!
This is a topic that's close to my heart... Cheers!
Exactly where are your contact details though?
Its like you learn my thoughts! You appear to understand so much about this,
such as you wrote the guide in it or something.
I believe that you just can do with some % to force the message house a bit, but other than that, this is wonderful blog.
An excellent read. I'll certainly be back.
Ꭲһanks for tһe marvelous posting! I seriously enjoyed reading іt, you might
be a great author.I will ensure that I bookmark your blog and will eventually come ƅack very soon. I
want to encoᥙrɑge yourself to continue your great work, have a niϲe evening!
I am not very excellent with English but I get hold this very easygoing to understand.
If you want your betta fish to reside long and healthy lives, an aquarium tank heater is often a necessity to be sure the temperature will consistently stay within the safe range.
I believe this is among the most important information for
me. And i'm glad reading your article. However wanna statemnt on some common things,
The website style is great, the articles is in point
of fact excellent : D. Excellent task, cheers
I know this site gives quality dependent posts and extra information, is there any other web page which
presents these things in quality?
Write more, thats all I have to say. Literally, it seems as though you relied on the video to make your point.
You obviously know what youre talking about, why waste your intelligence on just posting videos to your site when you could be giving us
something informative to read?
Excellent beat ! I would like to apprentice even as you amend your website, how could i subscribe
for a blog website? The account helped me a appropriate deal.
I had been a little bit acquainted of this your broadcast provided vivid clear concept
Hello to all, for the reason that I am really keen of reading this
webpage's post to be updated on a regular basis.
It carries fastidious stuff.
Great ? I should certainly pronounce, impressed with your web site.
I had no trouble navigating through all the tabs and related information ended up being truly simple to do to
access. I recently found what I hoped for before you know it in the least.
Reasonably unusual. Is likely to appreciate it for those who add forums or something, website theme .
a tones way for your client to communicate. Nice task.
Inspiring story there. What occurred after? Take care!
After looking at a few of the blog articles on your web page, I seriously appreciate your way of writing a blog.
I bookmarked it to my bookmark website list and will be checking
back in the near future. Please visit my web site
as well and tell me how you feel.
Wow! This could be one particular of the most useful blogs We've ever arrive across on this subject.
Actually Magnificent. I'm also an expert in this topic therefore I can understand your
OggiⲤrypto è un bellissimo sito per aggiornarsi sulla nicchia delle cripto valute.
Se anche tu vuoi imparare di più su crіptovaluta governo cinese
viita ѕu oggi cripto
Hey there! I've been following your weblog for
a while now and finally got the bravery to go ahead and
give you a shout out from Austin Texas! Just wanted to mention keep
up the excellent job!
You're so cool! I do not suppose I have read through a single thing like this before.
So great to find another person with some unique thoughts on this subject.
Seriously.. thanks for starting this up. This website
is something that is required on the web, someone
with a little originality!
I think this is among the most important info for me.
And i am glad reading your article. But should remark on some general things,
The site style is great, the articles is really great : D. Good job, cheers
I think this is one of the such a lot significant info for me.
And i am satisfied reading your article. But should statement on some
normal things, The site taste is perfect, the articles is in reality great : D.
Just right activity, cheers
Pretty great post. I just stumbled upon your weblog and wanted to mention that I have really enjoyed surfing around your
blog posts. After all I'll be subscribing for your feed and I'm hoping you write again soon!
hi!,I like your writing very much! share we keep in touch extra
about your article on AOL? I require a specialist in this space to unravel my problem.
May be that's you! Looking forward to peer you.
Hi there, I discovered your blog via Google even as
looking for a comparable matter, your website came up, it seems to be
good. I have bookmarked it in my google bookmarks.
Hi there, simply become alert to your weblog through Google, and found that it's really informative.
I'm gonna watch out for brussels. I will appreciate in the event
you continue this in future. A lot of folks will likely be benefited out of your writing.
This is my first time visit at here and i
am truly pleassant to read all at single place.
Thank you for another informative website. The place else
could I get that type of info written in such a perfect method?
I've a mission that I am simply now running on, and I've been at the look out for such information.
Thank you, I've recently been looking for information about this topic for
ages and yours is the greatest I've discovered
so far. But, what about the conclusion? Are you certain concerning
Hi are using Wordpress for your blog platform?
I'm new to the blog world but I'm trying to get started and set up my
own. Do you need any coding knowledge to make your own blog?
Any help would be really appreciated!
Definitely believe that which you said. Your favorite reason appeared to be on the web the easiest thing
to be aware of. I say to you, I definitely get annoyed while people think about
worries that they just do not know about.
You managed to hit the nail upon the top and also defined out the whole thing without having
side effect , people could take a signal.
Will likely be back to get more. Thanks
Terrific work! This is the type of info that should be shared across the net.
Disgrace on Google for no longer positioning this post upper!
Come on over and discuss with my web site .
Thank you =)
Pretty great post. I jᥙst stumbled սpon your webⅼog and wished
to say that I've гeally loved browsіng your Ƅlog posts.
In ɑny case I ѡill be subscribing for үour rss feed and
I hoρe you write again very soon!
Very good post. I definitely appreciate this site.
Continue the good work!
If some one wants expert view concerning blogging afterward i propose him/her
to go to see this webpage, Keep up the fastidious job.
I am extremely impressed with your writing skills as well as with the layout on your weblog.
Is this a paid theme or did you customize it yourself?
Anyway keep up the excellent quality writing, it's rare to see a nice blog like this one
Howdy would you mind letting me know which web host you're using?
I've loaded your blog in 3 completely different browsers and I must say this blog loads a lot quicker then most.
Can you suggest a good web hosting provider at a honest price?
Thank you, I appreciate it!
Its like you read my mind! You seem to know so much about this, like
you wrote the book in it or something. I think that you can do with a few pics to drive the message home a little bit,
but other than that, this is wonderful blog. An excellent read.
I will definitely be back.
There is definately a lot to know about this issue. I really like all
the points you've made.
Does your blog have a contact page? I'm having trouble locating it but, I'd like to shoot you an email.
I've got some creative ideas for your blog you might be interested in hearing.
Either way, great site and I look forward to seeing
it grow over time.
It's going to be finish of mine day, but before end I am
reading this enormous article to increase my experience.
Hi there! Someone in my Myspace group shared this site with us
so I came to give it a look. I'm definitely loving the information. I'm bookmarking and will
be tweeting this to my followers! Great blog and outstanding design.
Hi, I do think this is an excellent site. I stumbledupon it ;) I may come back yet again since I bookmarked it.
Money and freedom is the best way to change, may you be rich and continue to guide others.
When I initially commented I clicked the "Notify me when new comments are added" checkbox and now each time
a comment is added I get three emails with the same comment.
Is there any way you can remove people from that service?
I needed to thank you for this good read!! I
definitely enjoyed every bit of it. I have you saved as a favorite to look at new stuff you post?
Wonderful beat ! I wish to apprentice while you amend your website,
how could i subscribe for a blog site? The account helped me a acceptable deal.
I were tiny bit acquainted of this your broadcast offered bright transparent concept.
Hello my loved one! I want to say that this post is amazing, nice written and come with approximately all vital infos.
I'd like to see extra posts like this .
It's going to be end of mine day, except before finish I am reading this impressive piece of
writing to improve my know-how.
I do ϲonsider all of the ideas ʏou һave offered on your post.
Tһey aгe very convincing аnd wiⅼl cеrtainly woгk.
Still, the posts ɑrｅ toο short for starters.
Ꮇay you pleaѕe lengthen them a little from
next time? Tһank yօu foг the post.
Sweet blog! I found it while searching on Yahoo News.
Do you have any tips on how to get listed in Yahoo News?
I've been trying for a while but I never seem to get there!
I don't know whether it's just me or if everyone else
encountering issues with your website. It appears as if some
of the written text in your content are running off the screen.
Can somebody else please provide feedback and let me know
if this is happening to them too? This might be a issue with my internet browser because I've had this happen before.
Many such cases! Sad!