spcharc · HackerTrans

Who needs hackers if you have IT experts like this

And the chinese tea cup in the picture apparently has a base plate. It also isolates heat from the cup so whatever underneath it won't get too hot

spcharc

2 anni fadiscuss

I do have a couple of asserts in my code, but that does not explain why every stack trace begins and ends with "assert()"

Actually I call syscalls from assembly, and my _start() is also an assembly function. That is why this happens I guess. strace somehow doesn't play well with code written in .s file?

spcharc

2 anni fadiscuss

Yeah, you are right. Android is a variant of Linux.

Just like MacOS and iPadOS, both run Darwin kernel, but they are very different. Even though iPads have very capable hardware, there are a lot you cannot do on iPadOS.

The same applies to Linux. People want a real desktop operating system.

spcharc

2 anni fadiscuss

> It's easy enough to learn "base + index*scale +- displacement".

Well, you already know this requirement, so it is not a problem for you. However for learners, who don't have even the remotest idea of machine instructions, all they get is some invalid expression error messages. Same message for all (1) (2) and (3) cases I listed above.

Maybe Intel syntax works better for experienced programmers. But at least for me, when I was a newbie to assembly, AT&T was better than Intel syntax. It made learning process easier.

> AT&T syntax forces you to learn this before writing or even reading a single line of code that references memory

I do believe that rules forced by language is a good thing. At least it helped me understand why (1) (2) and (3) didn't work

Many people believe Rust can help people write safer code. Why? Because of its rules. Incorrect ownership will be discovered by the borrow checker so a compilation error will force the programmer to correct it.

Maybe an experienced C++ programmer will be able to handle memory management correctly in C++, and loves the extra freedom that C++ brings. But Rust can be helpful for learners. It provides clear error message helping them understand why their code is wrong. While in C++? Segmentation fault.

(Needless to say, Rust also helps experienced programmers. Even experienced programmers write buggy C++ code.)

spcharc

2 anni fadiscuss

It can be useful I think.

Trying this on my toy project. This is a single thread program so it cannot do syscall from multiple cores.

It is not very helpful without debug info:

  nanosleep({tv_sec=1, tv_nsec=299127292}, NULL) = 0
   > /home/spcharc/proj/out() [0x80f1]
  write(1, "g1() ===== end =====\n", 21g1() ===== end =====
  )  = 21
   > /home/spcharc/proj/out() [0x80f1]
  munmap(0x7a267b491000, 66580)           = 0
   > /home/spcharc/proj/out() [0x80ec]
  exit(0)                                 = ?
  +++ exited with 0 +++
   > /home/spcharc/proj/out(+0x0) [0x80e7]

With debug info added, the output looks much better:

  nanosleep({tv_sec=1, tv_nsec=297422159}, NULL) = 0
   > /home/spcharc/proj/out(assert(bool, char const*)+0x14a) [0x87a8]
   > /home/spcharc/proj/out(fd_manager::wait_event(timespec const*, timespec&, epoll_event*, unsigned int)+0xbe) [0x5598]
   > /home/spcharc/proj/out(event_loop::execute_tasks()+0x56) [0x6a10]
   > /home/spcharc/proj/out(event_loop::main_loop()+0x5b) [0x6b19]
   > /home/spcharc/proj/out(main+0x184) [0x77c6]
   > /home/spcharc/proj/out(assert(bool, char const*)+0xc0) [0x871e]
  write(1, "g1() ===== end =====\n", 21g1() ===== end =====
  )  = 21
   > /home/spcharc/proj/out(assert(bool, char const*)+0x14f) [0x87ad]
   > /home/spcharc/proj/out(static_file_buffered_printer<1u, 1012u>::flush()+0x75) [0x5071]
   > /home/spcharc/proj/out(output_stream<static_file_buffered_printer<1u, 1012u> >::operator<<(Manip)+0x74) [0x3624]
   > /home/spcharc/proj/out(g1(event_loop*, uptr_t)+0x14c) [0x74bc]
   > /home/spcharc/proj/out(assert(bool, char const*)+0x12d) [0x878b]
  munmap(0x7b80db5fd000, 66580)           = 0
   > /home/spcharc/proj/out(assert(bool, char const*)+0x14a) [0x87a8]
   > /home/spcharc/proj/out(main+0x1bd) [0x77ff]
   > /home/spcharc/proj/out(assert(bool, char const*)+0xc0) [0x871e]
  exit(0)                                 = ?
  +++ exited with 0 +++
   > /home/spcharc/proj/out(assert(bool, char const*)+0x145) [0x87a3]
   > /home/spcharc/proj/out(assert(bool, char const*)+0xc8) [0x8726]

Interesting that every syscall is from assert(). My assert() is basically "if (!cond) {print(msg); exit(1);}".

I guess it traced to some unrecognized area and stopped there.

spcharc

2 anni fadiscuss

> 1,283 with known malicious code (229 million installs)

Not expecting that many.

I sometimes randomly try interesting extensions out. Will not do this again. Should only install extensions recommended by Microsoft I guess.

Maybe the same applies for browser extensions?

spcharc

2 anni fadiscuss

> Because it's translated into a single machine instruction!!!

Actually we never learned the x86 binary representation of instructions in that assembly language class. The textbook also did not cover that.

All I wanted to say is: Intel syntax hides the fact that there are only 4 things in address calculation: displacement, base, index, scale. The compilation error is also hard to understand (at least for the compiler I used). It says something like "the expression is invalid" but you never know what went wrong.

AT&T syntax exposed the underlying requirement, and the compilation error is easy to understand.

Now I am okay with both AT&T and Intel, but when I was learning, I appreciated AT&T syntax more. Assembly is mandatory for CS major in that college, and AT&T syntax made my semester easier.

spcharc

2 anni fadiscuss

Only if the user base is large enough to gather enough interest. I expect someone reverse-engineering popular devices like nintendo switch but not bionic eyes used by a few hundred people.

spcharc

2 anni fadiscuss

I love AT&T syntax.

I originally started with Intel syntax when learning assembly since it was just in my textbook. I sometimes ran into weird syntax issues I didn't yet understand.

For example, why "mov eax, [ebx + 2 * ecx + 4]" is allowed, but not "mov eax, [ebx + ecx + edx]" (1)? Or maybe "mov eax, [ebx + 3 * ecx]" (2)? Or maybe "mov eax, [2 * ebx + 4 * ecx]" (3)?

As long as it is a math expression it should work right? Why did the compiler keeps telling me the expression is invalid?

Later when I learned about AT&T syntax, everything started to make sense. The syntax ensures you cannot construct (1) and (3), and when trying to use (2) it explicitly tells you it expects 1, 2, 4, or 8 but got 3.

I started to use AT&T syntax since then.

In my opinion, there is one most confusing part about AT&T syntax, which is the condition based instructions. I guess the original author of the article did not do much programming with AT&T syntax so they did not notice.

  cmpl %eax, %ebx
  jae label

Now tell me if eax is bigger, should you jump to label?

Instead, with Intel syntax it is pretty straight forward. The "jae ..." following "cmp ebx, eax" translates to "if (ebx >= eax) goto ..."

(BTW, the cat or whatever following your mouse pointer is super annoying)

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