ARM Assembly: ∞ Ways to Return

October 19, 2017 Assembly No comments

ARM is unusual among the processors by having the program counter available as a “general purpose” register. Most other processors have the program counter hidden, and its value will only be disclosed as the return address when calling a function. If you want to modify it, a jumping instruction is used.

For example, on the x86, the program counter is called the instruction pointer, and is stored in eip, which is not an accessible register. After a function call, eip is pushed onto the stack, at which point it could be examined. Return is done through the ret instruction which pops the return address off the stack, and jumps there.

Another example: on the MIPS, the program counter is stored into register 31 after executing a JALR instruction, which is used for function calling. The value in there can be examined, and a return is a register jump JR to that register.

ARM’s unusual design allows many, many ways of returning from functions. But first, we must understand how function calls work on the ARM.


private and final fields: Can you actually hide data in Java?

October 12, 2017 Java No comments

Sometimes, after many attempts, you realized that to complete your mission, you must access private fields, or perhaps change final fields.

There are many reasons imaginable: the accessors copy the entire object before returning, and that takes a very long time, the authors forgot to provide an accessor, the library function is highly inefficient and you need to do better, …

Are you out of luck? Fortunately, no.


Online Judging Sandbox: From Linux to FreeBSD

October 7, 2017 Code, Security No comments

As most probably know, DMOJ uses a sandbox to protect itself from potentially malicious user submissions. An overview of the Linux sandbox has been published by my friend Tudor. However, it doesn’t go deep into the implementation details, many of which differ between Linux and FreeBSD.

At its core, the sandbox, cptbox, uses the ptrace(2) API to intercept system calls before and after they are executed, denying access and manipulating results. The core is written in C, hence the name cptbox.

Perhaps the most obvious difference between Linux and FreeBSD is that on Linux, ptrace(2) subfunctions are invoked as ptrace(PTRACE_*), while on FreeBSD, it is ptrace(PT_*). But this difference is rather superficial compared to the significant internal differences.


Effective Assembly: Bitwise Shifts

Most people, when first starting assembly, still carry over a lot of high level constructs in their assembly programs. A common pattern is to multiply and divide when a bit shift would suffice.

For example, a lot of people would write a program to write out the binary representation of an integer using the divide and modulo operations. This is rather inefficient compared to using shifts. For example, the divide by 2 can be replaced with a right shift by 1, and modulo 2 can be replaced by a bitwise AND with 1.

Aside: interestingly, taking any number modulo a power of two m is equivalent to doing a bitwise AND with m-1. The proof of this is left as an exercise for the reader.

This post will address the basics you need to know about shifts to get up to speed on writing good assembly.


A new “Hello, World!” for C

August 27, 2017 Code No comments

Most of us have a good idea how to write a simple “Hello, World!” program in C, but sometimes it feels a little too easy. Luckily, we can always make it more of a challenge!

Consider a hypothetical situation where many symbols are banned, such as ", ', \, #, {, and }, and we aren’t allowed the string Hello, World! as a subsequence in the code. How would we write a “Hello, World!” program then?

Is it impossible, because we can no longer use {} to write a block of code for a function? Is it impossible, because we can’t actually embed the string?


Using the Visual C++ compiler on Linux

August 11, 2017 Code No comments

It is a fairly common practice to compile Windows application on Linux build servers. However, this is usually done through an approach called cross-compiling. The essence of this approach is using a compiler for Windows applications, but the compiler itself is a Linux application. Usually, the compiler used for this is MinGW (or MinGW-w64 these days), a GCC implementation for Windows.

This works great when porting traditional Unix applications to Windows, since it meshes nicely with the traditional build system on Unix-like systems. But it is rather poor for standalone single .exe applications, which are more common in the Windows world. MinGW has a few DLLs that are needed to run the applications it compiles, and that ruins the single executable experience.

The traditional way to build these simple applications in the Windows world is with the Microsoft compilers, usually in the form of Visual C++. These compilers are fairly nice, but they have one problem: they do not exist as cross compilers. (Well, they can cross compile between different processors, but the compilers themselves will only run on Windows.) What do we do then? Do we resign ourselves into not having single executable applications, or do give up and buy a Windows build machine?


A polyglot header for Python and cmd.exe

August 4, 2017 Code No comments

After seeing Raymond’s post on polyglot launchers for Perl and JScript with batch files, I decided to present one for Python:

@python -x "%~f0" %* & goto :eof
# Your Python code here.

This one simply use the special python flag -x to ignore the first line, which is somewhat analogous to the -x Perl flag, but much simpler.

I also have an alternative Perl polyglot header that does not require the special flag -x.

@rem = '--*-Perl-*--
@perl "%~f0" %*
@goto :eof
undef @rem;
# Your Perl code here.