Threads for sinic
-
Where’s the critical-severity vulnerability they announced a week ago?
-
-
Is every possible sequence of digits found somewhere in π?
-
-
We don’t know for sure: pi is believed to be a normal number. Normal numbers have the pattern that all single digits are equally likely to appear, all pairs are equally likely, all triplets, etc. If this holds, any sequence of digits of length n, represented in base b will appear with probability
1/b^n.An example of a normal number formed by construction is Champernowne’s constant. In base 10, it is
0.12345678910111213(...)- a concatenation of all natural numbers. As you can see, any natural number you can name is guaranteed to appear inside this constant.Normal numbers are interesting because it’s been proven that almost all real numbers are normal; however, outside of numbers explicitly constructed to be normal, very few reals have been proven to be normal.
e: u/sinic is actually more correct than I; all normals are disjunctive but not all disjunctives are normal.
-
Thanks to this thread I understand today’s SMNC! https://www.smbc-comics.com/comic/normal
-
-
That’s an open mathematical question. No one has proven it, but I think the consensus guess is yes.
-
-
I only skimmed the article, but the first animation, while looking nice, doesn’t seem correct to me.
The new process (
ls, in this example) is described as sending its output to the shell, but in reality the forked process sends its output directly to the inherited TTY. The shell never sees any of it, which is also the reason it can’t do anything about background jobs messing up the output of the foreground job.-
It is oversimplifying, which is fair given the introductory nature and all that; it is whatever is mapped to the file descriptor slots for STDIN/STDOUT/STDERR in the new child. That can be be some PTY, other files, or “in the olden days” even empty (not as in /dev/null, as in not allocated).
It was a fun trick to mess with SUID binaries in this way, as the spare allocation requirement means that if you say, close(2); exec(); the next open() will also become STDERR, so find a suid binary where you had partial control of fprintf(stderr, “%s went wrong”, “exploit goes here”) there were some privilege escalation opportunities. For that reason, kernel or libc these days tend to make sure /dev/null go there unless set. Even non-maliciously it cause some terrible bugs (whatever you logged also corrupting the file you were working with etc.).
The sparse allocation requirement for open() absolutely sucks.So in reality it can be both, some or neither. It is a chain of trust thing - the shell can run new jobs in whichever way it seems fit (close/dup/open other things for stdin/stdout/stderr), including nesting new terminal emulators (tmux, screen and so on).
This is really a big pain-point: the shell isn’t certain of what it is running and feeding it instructions. The terminal emulator isn’t certain of what the shell is running. The thing that is running isn’t certain of what it is supposed to be, interactive or in a pipeline – and have few options for being both. It can only guess (isatty() style shenanigans testing various “benign” pty- dependent ioctls and checking for failure).
Now with this premise, try and get buffering, synchronization and signal propagation “right”…
-
Yea I also question this part:
The user types in text which is buffered in the PTY’s STDIN line buffer. The user presses and the pty’s STDIN line buffer is sent to the shell
How would tab completion work? I suspect some really old terminals and shells would buffer the entire line, but modern tooling likely works on a per-character basis. Another example is Fish Shell, which has native type-ahead history/search that appears as you type (I think there are extension that can do this in Zsh too?)
-
The article discusses non-canonical input processing (aka “raw mode”) and how it differs from the default processing that occurs for a tty where the line discipline (usually in the kernel) provides extremely basic line editing capabilities. Indeed, for better control over the display and for advanced features like tab completion, a modern shell will have the tty in raw mode all the time and will switch it back to canonical mode when starting a process at the request of the user.
-
-
-
Historical question:
a user on the network we ran together found out that he could make
rakaur‘s IRC bot runrm -rf --no-preserve-root /, and did so.When did
--no-preserve-rootget added torm? I thought it was reasonably recent, but I couldn’t find it in theNEWSfile of coreutils. -
-
A nice property of Netpbm is that you can pretty much dump raw image data into an Emacs buffer and have it rendered directly: https://nullprogram.com/blog/2012/09/14
-
While we don’t have any specific evidence for this, a possible explanation is that the user database of master.php.net has been leaked, although it is unclear why the attacker would need to guess usernames in that case.
To me, guessing usernames is an indication for some kind of credential stuffing.
-
[the
mandirectory] contains the generatedmanpages. Because it’s adjacent tobin(which is on my path) themanprogram automatically finds themanpages as expected.Whoa! I just tried this: my $PATH contains
~/dotfiles/bin, so I creating a simple text file~/dotfiles/man/man1/mytest.1, and typedman mytest, and it worked! Finally I can easily create & install man pages, rather than implementing a response to a-h/--helpflag.-
-
So, I actually also tried, yesterday, to find the docs for the ‘$PATH -> man path’ auto-mapping in the manpage; but unlike you, I couldn’t find it. If you have an occasion, could you perhaps post the man page / excerpt you found, so I can know what I overlooked? No need to reply if you don’t have an occasion.
I ran
man 1 man 1 manpath 5 manpathand read through those manpages; and I read through the comments in/etc/manpath.config; but the closest I could find was set of directives likeMANPATH_MAP /bin /usr/share/man, which only map specifies specific man dirs to specific PATH dirs. I couldn’t find anything documenting the rewriting rule ‘for any directory in $PATH, look in a sibling directory calledman.’ Pointers welcome!-
Linux man pages do not describe this, but freebsd manpages do.
-
On Linux, old man (1.6g) documents this as well, only modern man-db does not.
-
-
-
-
-
-
sinic
edited
2 years ago
|
link
| on:
Is it appropriate to keep submitting stories of my own blog entries?
For what it’s worth, unless you authored more than half of your submitted stories, you won’t be labelled a heavy self promoter: https://github.com/lobsters/lobsters/blob/9c0e2c03e2c14a2fba39298e76258cc460f6f003/app/models/user.rb#L478
-
-
This is likely what you want:
If you want to go even deeper:
And as others have said the cryptopals exercises are great, and I’ve heard the Schneier book recommend elsewhere too.
-
Dan Boneh, one of the authors of the latter book, offers an online course I have fond memories of.
-
-
So my mental model of commands in vi is that “nC” is the same as executing “C” n times.
There are other commands that invalidate this model:
3dd, when positioned on the second line of a buffer with three lines, deletes only two of them;dddddddeletes all three.What is odd is that
3dddoes nothing at all when positioned on the last line. This appears to be a long-standing issue in Vim (present all the way back to 1991, as far as I can see) and also in Neovim, but not in Evil.
-
0-indexing is better because 0 is a natural number, dangit
Yeah I know lots of people find it unintuitive, but so much math works out so much better when you start counting from 0. I will fite people over this
-
There is no consensus of 0 being part of the natural numbers. There is an ISO standard, but some definitions of natural numbers specify that natural numbers contain all positive whole numbers, and since 0 is neither positive, nor negative, it’s not part of the natural numbers.
-
Peano arithmetic, which I think can be considered fairly standard, is defined in terms of a Zero and a Successor function.
The existence of a zero among the naturals means they can be considered a group, as zero is the identity for addition.
Integers are generally defined as two-tuples of natural numbers where (m,n) represents the difference m-n. The existence of a zero among the naturals means that every integer has a canonical representation as (m,0), (0,n), or (0,0); the first form is simply m, the second is -n, and the third is 0 (which satisfies both, as a bonus showing clearly that 0=-0). This is clearly more elegant than the alternative, where the integral representation of natural m is (m+1,1).
Defining S_0 as the base case of a sequence means that S_n results from n applications of the inductive rule. The simplest example of this is actually peano arithmetic itself, where the number n self-evidently results from n applications of the Successor function to the Zero.
These are just a few examples. I’m unaware of any arguments in favour of excluding zero from the naturals.
-
The existence of a zero among the naturals means they can be considered a group, as zero is the identity for addition.
Not a group but a monoid. Without zero they’d be a semigroup, i.e. lacking identity (or maybe you had some group in mind but with respect to addition the naturals are a monoid, still if people think multiplication is somehow more natural then starting at 1 also gives you a monoid so this argument can go both ways).
I agree with the rest of your comment.
Edit: I guess I’ll put some thoughts here.
The difference between counting the position of something (what is the first number?) and the quantity of something (what is the smallest quantity?) is the difference that matters w.r.t. this indexing question. The reason 0-indexing is more natural is because when you have a positional number system (such as
base whateverthenwhateveris the number of symbols in the system, the first symbol will always have the meaning0because when you run out of symbols likesuc(9)in decimal, then you start again in the next position (apositionalnumber system) and get10).Just to drive the point home: A list in a programming language is a base for a positional number system, the elements in the list are the digits you are using, if you ever find yourself doing index arithmetic of the form
x / (length list)combined withx % (length list)then you are working with abase (length list)number system.Another way to define the natural numbers is in terms of homotopies; if you imagine \mathbb(C) \ {0} then a path that doesn’t circle around the origin is
contractibleand defined as0(a path that has a homotopy to the point it originates from and ends at is contractible) which allows you to prove that 0 = -0, while other numbers can be obtained by looking at how many times you circle around the origin and in which direction the spiral is (you define one direction as positive and the other as negative but I said we’re defining the naturals so you’d ignore directionality, however you’d have have to start at 0 because otherwise extending to the whole numbers would make your earlier system incompatible). This may seem kinda ridiculous but the point is that this kind of inductive number system with a base case is often bidirectional and so it makes sense for the base case to be 0. -
Peano arithmetic, which I think can be considered fairly standard, is defined in terms of a Zero and a Successor function.
While this is commonly the case today, Peano himself originally defined it for positive integers only.
-
-
-
In addition (pun intended) to the rest of the thread, I’ll point out that the Von Neumann ordinals give a correspondence between natural numbers and set cardinalities. The number 1 corresponds to the sets with exactly one element. Similarly, the number 0 corresponds to the empty sets.
Think of the natural numbers as the numbers for counting discrete things. (This is a decategorification!) Counting sheep in a field ala Baez, we count zero sheep, one sheep, two sheep, three sheep, etc. The fact that a field can have zero sheep is still countable with natural numbers.
-
Think of the natural numbers as the numbers for counting discrete things. (This is a decategorification!) Counting sheep in a field ala Baez, we count zero sheep, one sheep, two sheep, three sheep, etc. The fact that a field can have zero sheep is still countable with natural numbers.
Yep.
This is what I meant (elsewhere down-thread) when I said that zero is a generalisation of magnitude. Without zero, the question ‘how many [sheep in a field, for example] are there’ has two different kinds of answers: ‘there are n’ or ‘there aren’t any’. That is,
Either None PositiveInt. If zero can be a magnitude, then the answer always takes the form ‘there are n’, with the former ‘none’ case replaced by n=0.(There are additionally generalisations to be had: integers let you reason about deficits, and rationals let you reason about fractional quantities (‘how much’ instead of ‘how many’. But those generalisations come at the context of added complexity, where zero is effectively free.)
-
-
-
One good reason for having a low DNS TTL not mentioned in the article are DDNS setups.
Residential internet connections are flaky and sometimes a router cycles through multiple dynamic IPs in a matter of minutes, without anything the customer can do.
Yes, that’s true. We really need to get over this shitty dynamic IP for home users. IPv6 to the rescue.
In practice stability of IPs has nothing to do with IPv4 vs IPv6. Some providers will give you the same IPv4 address for years, others will rotate your IPv6 prefix all the time.
Yep, anecdote here: AT&T Fiber has given me the same IPv4 address for years, even across multiple of their “gateways” and a plan change.
Anecdata: this is true in theory but I’m not sure in practice? Specifically, I used to get the same IPv4 address for weeks at a time - basically until my modem was rebooted. Then in 2014 ARIN entered phase 4 of their IPv4 exhaustion plan (triggered by them getting down to their last /8 block) and all of a sudden my modem’s IPv4 address refreshed far, far more often, IIRC every couple days.
I guess maybe this was not technically required though, and was potentially just my ISP overreacting? 🤷
CenturyLink in the Seattle area, FWIW.
At least here in Germany will have a different IPv4 address every 24 hours or on reconnect for far most of residential Internet access. The point with IPv6 is that you not only have one public. But again in Germany it‘s even hard to find a not regularly changing IPv6 prefix for residential Interner access. They think it‘s more Staat protection friendly… like cash. Crazy thoughts.
Ideally, a v6 provider would give you two subnets, one for inbound connections that remained stable, one for outbound connections that was changed frequently. Combined with the privacy extensions randomising the low 64 bits, this should make IP-based tracking difficult.
The origin story of the 24h disconnect is that it used to be the differentiator between a leased line and a dial-up line, which belonged in different regulatory regimes (the most obvious aspect to customers has been cost but with a few backend differences, too). The approach has stuck since.
It’s also a (rather barebone) privacy measure against commercial entities, not the government: the latter can relatively easily obtain a mapping from IP to the address by giving a more-or-less refined reason.
Commercial entities have “solved” the tracking issue by using cookies etc.
I.e. customers prefer changing IP addresses for privacy reasons?
Which is really weird, because implementing an “I’d like my IP to change / not change” checkbox would be trivial. I don’t get why that’s not more common.
The checkbox isn’t the complicated part here.
That’s not going to be an easy problem to solve. We’ve embraced the current pseudo standards within home internet connectivity, maybe static, maybe dynamic IP, asymmetric speeds, CGNAT, no IPv6, etc. for so long that many people think that these are real industry standards with cost structures and cost savings behind them and we must suffer with them if we want cost effective internet connectivity at all. A lot of home ISP customers suffer from Stockholm syndrome.
I think that DynDNS.org is using a 60s timeout on A records so for a DynDNS/residential setup I think that the article author would approve of something like this:
foo.mydomain.com 14400 IN CNAME bar.my-dyndns-server.com…
bar.my-dyndns-server.com 60 IN A 192.168.1.100Specifically, I don’t think that the original author is complaining about the 60s TTL on records at
my-dyndns-server.comsince that company has to deal with the lack of caching in their DNS zones. He finds sub 1hr TTLs in the CNAME records to be a problem. And he finds CNAME TTLs shorter than the A record TTLs to be a bigger problem. Honestly even Amazon does this 60s TTL on dynamic resources trick.