Hi,
I am trying to change the page protection using mprotect, it throws memory violation
cant seem to understand the problem.

below is the C code

#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>

int main()
{
char *str="my name";
int pagesize = sysconf(_SC_PAGE_SIZE);

int i=mprotect(pagesize*((int)str/pagesize),pagesize,PROT_WRITE); //calculate the page boundary

printf("%x %x \n",pagesize*((int)str/pagesize),str);
if(i==-1)
printf("%d\n",errno);

str[1]='b';

printf(str);
return 0;
}

Exactly what did you want to do with this?
At the very least, be a bit more clear:

Note: of course you cannot protect automatic variables via mprotect...

Is the violation on read or write? Because you didn't pass PROT_READ although you try to read str later...


?

Hi,
I need to change hook certain functions by altering its opcode, (something similar to MS detour or mhook)

anyways from a learning point of view, why does the above attached code crash at mprotect

using Zorin OS 9.1 (Ubuntu)

> > Note: of course you cannot protect automatic variables via mprotect...

> ?

Well, yes, I was wrong, actually he wants to 'unprotect' the string literal.

(What I thought he wanted was switching the stack (or a part of it) to read-only.
Of course that would end the program very quickly.

uhhh...any help
why is mprotect not working.......
can you get mprotect to work,
please post your code

a quick way to get the aligned adress

(int)pointer & ~ 0xFFF
edit: 4k is x86 and amd64, other cpu architectures may use a diff alignment
(so use the getpagesize from above, or better yet if there is a header that declares it)

another, maybe faster, way would be to use two shifts



idk why it fails, not one for thinking tonight

Use gdb to find out where your program stops:

Hi,
I know where/when it stops...I am debugging using QT.

can you post a working code (fixing the code mentioned above), I don't want ideas on how to solve it since I would have tried them already, please copy paste the code to your editor, compile, run and figure out the reason for violation......

i understand the language barrier

people will be willing to help you if you try to do it yourself and post on what you got stuck

also remember to use the CODE ("#" button above) tags when pasting code on the forum

Clearly you didn't try the idea that actually works, so there is at least one you haven't tried already...

1 members found this post helpful.

I am stuck at mprotect , it throws an access violation

please try the code and let me know your inputs on solving it
That is: try the solution on your own machine , then post it on this thread

---------- Post added 03-04-15 at 04:47 PM ----------

which is why I am on this forum.....think you can solve it ;-)

You are trying to change the protection on a page containing a constant string, so it would be in your program's text segment. The value that you gave turns off read and execute access on the whole page, which is likely to instantly kill your program. Allocate a new page and mess with that, not parts of your program text.

1 members found this post helpful.

Thanks....
after a bit of probing the problem is as you described.