A Gentle Primer on Reverse Engineering • Hacker News Board

Re: A Gentle Primer on Reverse Engineering

#11

Post by sdevlin » Thu, Jan 29, 2015, 12:16 AM UTC

First: great article.

One nit, though. There's a subtle error in the main function:

    char* input;
    printf("Please input a word: ");
    scanf("%s", input);

Local variables are not automatically initialized in C, and we never assign input to point to any particular block of memory. This means it's probably pointing off to some random location - basically whatever address happened to be sitting on the stack when main was called. So when scanf writes the user password to input, it's going to go to some unpredictable location with unpredictable results. This could lead to code execution, or at least a straightforward denial of service.

Re: A Gentle Primer on Reverse Engineering

#12

Post by jasode » Thu, Jan 29, 2015, 12:20 AM UTC

i5rider wrote: post #8

The title seems a bit misleading, e.g. one could reverse engineer source code into UML. Perhaps a more appropriate title would be: A Gentle Primer on Code disassembling.

For what she's describing, "reverse engineering" is actually the more common phrase rather than "code disassembling".

If you search amazon.com, "reverse engineering" is in the titles of the first 2 books:

http://www.amazon.com/s/ref=nb_sb_noss_1?&field-keywords=rev...

The way most people use the terminology now, I'd say "reverse engineering" encompasses all the strategies of analyzing and unraveling the logic of assembler code. On the other hand "code disassembling" is more specific about using a tool such as IDA to disassemble the binary executable into assembly before the intellectual task reverse engineering begins.

Re: A Gentle Primer on Reverse Engineering

#13

Post by zrail » Thu, Jan 29, 2015, 12:21 AM UTC

sdevlin wrote: post #11

First: great article. One nit, though. There's a subtle error in the main function: char* input; printf("Please input a word: "); scanf("%s", input); Local variables are not automatically initialized in C, and we never assign input to point to any particular block of memory. This means it's probably pointing off to some random location - basically whatever address happened to be sitting on the stack when main was cal…

The author addresses this in footnote #2. They're simplifying the C code to get to the point of the article faster.

Re: A Gentle Primer on Reverse Engineering

#14

Post by farmdve » Thu, Jan 29, 2015, 12:35 AM UTC

zrail wrote: post #13

sdevlin wrote: post #11

First: great article. One nit, though. There's a subtle error in the main function: char* input; printf("Please input a word: "); scanf("%s", input); Local variables are not automatically initialized in C, and we never assign input to point to any particular block of memory. This means it's probably pointing off to some random location - basically whatever address happened to be sitting on the stack when main was cal…

The author addresses this in footnote #2. They're simplifying the C code to get to the point of the article faster.

Hmm, that footnote was very hard to notice, so I was quick to point out these errors to the person who maintains the blog, only to get blocked by her on twitter. Not sure why go that far.

Re: A Gentle Primer on Reverse Engineering

#15

Post by zrail » Thu, Jan 29, 2015, 12:38 AM UTC

farmdve wrote: post #14

zrail wrote: post #13

Earlier quoted context omitted.

The author addresses this in footnote #2. They're simplifying the C code to get to the point of the article faster.

Hmm, that footnote was very hard to notice, so I was quick to point out these errors to the person who maintains the blog, only to get blocked by her on twitter. Not sure why go that far.

Because, if you're going to criticize the article, maybe go read everything there is to read? Because every one of your points was addressed somewhere in the article.

Re: A Gentle Primer on Reverse Engineering

#16

Post by farmdve » Thu, Jan 29, 2015, 12:42 AM UTC

zrail wrote: post #15

farmdve wrote: post #14

Earlier quoted context omitted.

Hmm, that footnote was very hard to notice, so I was quick to point out these errors to the person who maintains the blog, only to get blocked by her on twitter. Not sure why go that far.

Because, if you're going to criticize the article, maybe go read everything there is to read? Because every one of your points was addressed somewhere in the article.

The footnote was very difficult to spot.

Furthermore, I am not sure why not write out the complete code, even if the author knew the code was not correct, even when pointed out, it would have been very trivial to initialize the variable.

Re: A Gentle Primer on Reverse Engineering

#17

Post by AlexeyBrin » Thu, Jan 29, 2015, 12:44 AM UTC

C lacks a boolean type

This is false, as of C99 we have booleans in C, just include stdbool.h in your code, e.g.:

    #include 
    
    ...

    bool test = true;

    ...

Re: A Gentle Primer on Reverse Engineering

#18

Post by zrail » Thu, Jan 29, 2015, 12:47 AM UTC

AlexeyBrin wrote: post #17

C lacks a boolean type This is false, as of C99 we have booleans in C, just include stdbool.h in your code, e.g.: #include ... bool test = true; ...

Let's look at the source of stdbool.h:

    #define true 1
    #define false 0

http://clang.llvm.org/doxygen/stdbool_8h_source.html

Re: A Gentle Primer on Reverse Engineering

#19

Post by zrail » Thu, Jan 29, 2015, 12:53 AM UTC

farmdve wrote: post #16

zrail wrote: post #15

Earlier quoted context omitted.

Because, if you're going to criticize the article, maybe go read everything there is to read? Because every one of your points was addressed somewhere in the article.

The footnote was very difficult to spot. Furthermore, I am not sure why not write out the complete code, even if the author knew the code was not correct, even when pointed out, it would have been very trivial to initialize the variable.

Because the author made a stylistic choice to publish slightly incomplete code to get to the point faster (and pointed that out in a footnote, which was just as easy to spot as the "bug"). The point of the essay is not a perfectly executed C program, it's to demonstrate code disassembly and reverse engineering. More attention on perfect C code means less attention (reader attention and author attention alike) on the essential facts the author is trying to communicate.

In addition, I'm not an expert at this stuff, but it seems like initializing that variable would make the disassembly more complex and thus obfuscate the point just that much further.

Re: A Gentle Primer on Reverse Engineering

#20

Post by AlexeyBrin » Thu, Jan 29, 2015, 12:54 AM UTC

zrail wrote: post #18

AlexeyBrin wrote: post #17

C lacks a boolean type This is false, as of C99 we have booleans in C, just include stdbool.h in your code, e.g.: #include ... bool test = true; ...

Let's look at the source of stdbool.h: #define true 1 #define false 0 http://clang.llvm.org/doxygen/stdbool_8h_source.html

I know that sdtbool actually contains macros for defining true and false, but _Bool is a new type in C99 ... But it is in the standard, implemented by all major compilers, so it should be used for the sake of clarity.

IMHO this:

    bool is_valid(char* password);

is more clear than this:

    int is_valid(char* password);