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Comment by kentuckyduck | Hacker News Reader

I got segmentation fault trying out his hello world example (after changing VMA address). Then again, isn't that the supposed behavior? Not every memory page can be written to, if I remember correctly.

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Yes, segfault for me too after i changed ADDR preprocessor directive to the VMA address from objdump, as the instruction says. I'm on Ubuntu Linux 13.10 x64.

Ubuntu adds a security feature that provides a read-only relocation table area in the final ELF. To be able to run the examples in ubuntu, add this in the command line when compiling

-Wl,-z,norelro

e.g:

gcc -Wl,-z,norelro test.c

ok, now it compiles, thanks for that. However, I'm getting an incomplete response "<h1>hello world</h1" without the trailing closing angled bracket. And when I try to run final.c after setting the FUNCTION_ADDR and ADDR as per your tutorial, I get some stray HTTP/1.0 200 and Content-type text/html being displayed on stdout as I start the program final.c (compiled to a.out by default)

Ah, you have found a bug in my code (I made an error in computing the string length, and didn't notice it because it displayed fine on Chrome). I have fixed my code in git and the blog post.

As for the stray output displayed on the stdout: it is to be expected. The %n format outputs the numbers of character that is written by printf, so it must have written something to the stdout.

Ah! that explains so much (and also why i've wasted a whole hour figuring out why what I observed was happening) I'm a CLI curl guy, rather than relying on these browsers which randomly would add a 0x0d 0x0a to my form submissions, for example. I also noticed that compiling (assembling-linking) the .S to execute it would not print anything (just hang there like a normal webserver), but I was getting stuff written to stdout with the final.c/webserver.c version using its shellcode.

Did you also post this reply using curl?

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