The Byte Order Fiasco

Post by userbinator » Sat, May 08, 2021, 9:23 PM UTC

jart wrote: post #243

ajross wrote: post #74

Earlier quoted context omitted.

> There is a huge mismatch between the assumptions of the C spec and actual machine code. Right, which is why the kind of UB pedantry in the linked article is hurting and not helping. Cranky old man perspective here: Folks: the fact that compilers will routinely exploit edge cases in undefined behavior in the language specification to miscompile obvious idiomatic code is a terrible bug in the compilers . Period. And…

I agree but language of the standard very unambiguously lets them do it. Quoth X3.159-1988 * Undefined behavior --- behavior, upon use of a nonportable or erroneous program construct, of erroneous data, or of indeterminately-valued objects, for which the Standard imposes no requirements. Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during transla…

Post by londons_explore » Sat, May 08, 2021, 9:37 PM UTC

thristian wrote: post #142

ghoward wrote: post #105

Earlier quoted context omitted.

Sorry, dumb question: what is bit banging?

In order to exchange data over a serial connection, the ones and zeroes have to be sent with exact timing, so the receiver can reliably tell where one bit ends and the next begins. Because of this, the hardware that's doing the communication can't do anything else at the same time. And since the actual mechanics of the process are simple and straightforward, most computers with a serial connection have special serial…

Post by pornel » Sat, May 08, 2021, 9:40 PM UTC

jart wrote: post #239

patrakov wrote: post #228

Earlier quoted context omitted.

There was a blog post and a FOSDEM presentation by (misguided) Gentoo developers a few years ago, and it was retracted, because sanitizers add their own exploitable vulnerabilities due to the way they work. https://blog.hboeck.de/archives/879-Safer-use-of-C-code-runn... https://www.openwall.com/lists/oss-security/2016/02/17/9

Sanitizers have the ability to bring Rust-like safety assurances to all the C/C++ code that exists. The fact that existing ASAN runtimes weren't designed for setuid binaries shouldn't dissuade us from pursuing those benefits. We just need a production-worthy runtime that does less things. For example, here's the ASAN runtime that's used for the redbean web server: https://github.com/jart/cosmopolitan/blob/master/libc…

Post by tsimionescu » Sat, May 08, 2021, 9:44 PM UTC

cbmuser wrote: post #75

captainmuon wrote: post #41

It is a ridiculous feature of modern C that you have to write the super verbose "mask and shift" code, which then gets compiled to a simple `mov` and maybe a `bswap`. Wheras, the direct equivalent in C, an assignment with a (type changing) cast, is illegal. There is a huge mismatch between the assumptions of the C spec and actual machine code. One of the few reasons I ever even reached to C is the ability to slurp in…

> One of the few reasons I ever even reached to C is the ability to slurp in data and reinterpret it as a struct, or the ability to reason in which registers things will show up and mix in some `asm` with my C. Which results in undefined behavior according to the C ISO standard. Quote: “2 All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined.” From…

  union reinterpret {
    char raw[100];
    struct myStruct interpreted;
  } example;

  read(fd, &example.raw)
  struct myStruct dest = interpreted;

Post by bitwize » Sat, May 08, 2021, 9:59 PM UTC

mrlonglong wrote: post #164

bitwize wrote: post #77

Earlier quoted context omitted.

Little endian. There is no extant big-endian CPU that matters.

I did say in an ideal world.