Setting a constant in rodata

Question

I am trying to understand how to set the value of a string in the rodata segment as loading it using an environment variable gives me issues. I want to externally set a constant string in the rodata section. This function should be independent of the code executed. So, when I do

"objdump -c foo"

the rodata section must enlist this string without the file foo.c having to do it. How do I set a constant in the .rodata section ?

Edit: Linux OS and using GCC

I cannot use an environment var as that would mean that the c code is modified, I want the c code untouched and add the constant, say "Goo" to the rodata segment.

Could you please clarify what you want to accomplish? As given it is very unclear. Normally, data is automatically placed into the `rodata` _section_ ("segments" are something different) by the compiler. These are `const` variables and such like _string-literals_. If you want your program write into a variable in this section: this is undefined behaviour and on architectures with protected memory normally not allowed/possible. In C, writing to string literals is explicitly undefined behaviour. — too honest for this site, Aug 29 '15 at 18:38
Sorry for being vague.. I want to externally set a constant string in the rodata section. This function should be independent of the code executed. So, when I do "objdump -c foo", the rodata section must enlist this string without the file foo.c having to do it. — LockStock, Aug 29 '15 at 18:41
Please edit your question, not in a comment. However, I think I got the idea (it is still not completely clear). You want to expand the `rodata` section by another field, i.e. manipulate the object file. This is likely dependent on the file-format and the toolchain. Also: is that to be done static or when loading? If you just want to pass something, the enviroment variable might be the easier&better approach. If that is an option, perhaps you better try find out why it did not work. — too honest for this site, Aug 29 '15 at 18:43
Can it be done statically ? The location of the environment variable seems to change when I load the file . — LockStock, Aug 29 '15 at 18:49
What do you mean by "location" Please state OS, tollchain (GNU or CLAng, I presume?), how you read that variable, how you set it. Do you use [`getenv`](http://port70.net/~nsz/c/c11/n1570.html#7.22.4.6)? — too honest for this site, Aug 29 '15 at 18:51
But if the C code is not aware: how does it access this variable? And yhy not edit the code? Sorry if I'm wrong, but that starts sounding a bit suspisious now. Even more, as binaries are normally `r-x`. — too honest for this site, Aug 29 '15 at 18:59
I am just experimenting with the code so it's not of any use specifically. In any case, I do not want the c code to access the value, I just want to modif the segment. — LockStock, Aug 29 '15 at 19:01
What is the bigger context? What problem are you trying to solve (and how did you arrive at the solution of directly modifying executables)? — melpomene, Aug 29 '15 at 19:01

score 0 · Answer 1 · answered Aug 29 '15 at 21:33

0

Then you need to write a program that lets you modify the binary file.

Read the ELF file specifications.

Then write a program that modifies the ELF program and section headers and adds the data to the .rodata section.

answered Aug 29 '15 at 21:33

John Hammond

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score 0 · Answer 2 · edited Jun 20 '20 at 09:12

I've managed to write a small bash script that does more or less what I think you want.

First let's consider this sample program:

test.c

#include <stdio.h>
const char message[1024] = "world";
int main()
{
    printf("hello %s\n", message);
}

The target variable will be message. Note that I will not change the size of the variable, that would be a mess, you be careful to reserve as much memory as you will ever need.

Now the script:

patchsym

#!/bin/bash
# usage: patchsym PROGRAM SYMBOL < NEWCONTENT
EXE="$1"
SYMBOL="$2"
OFFS=$((0x$(objdump -t "$EXE" | grep " $SYMBOL$" | cut -d ' ' -f 1)))
OFFS=2176
dd of="$EXE" bs=1 seek=$OFFS conv=notrunc

The new message content will be:

newmsg

universe^@

(where ^@ is actually a NUL character).

Now just do:

$ gcc test.c -o test
$ ./test
hello world
$ ./patchsym test message < newmsg
$ ./test
hello universe

Setting a constant in rodata

2 Answers2

test.c

patchsym

newmsg