GDB Patching results in "Cannot access memory at address 0x
gdb, patch
Solution
`Cannot access memory at address 0x7ffff7acb377`
You are trying to write to an address where some shared library resides. You can find out which library that is with `info sym 0x7ffff7acb377`.
At the time when you are trying to perform the patch, the said shared library has not been loaded yet, which explains the message you get.
Run the program to `main`. Then you should be able to write to the address. However, you'll need to have write permission on the library to make your write "stick".
Problem
I have a program that I need to patch using GDB. The issue is there is a line of code that makes a "less than or equal test" and fails causing the program to end with a Segmentation fault. The program is already compiled and I do not have the source so I cannot change the source code obviously. However, using GDB, I was able to locate where the <= test is done and then I was able to locate the memory address which you can see below. ``` (gdb) x/100i $pc ... removed extra lines ... 0x7ffff7acb377: jle 0x7ffff7acb3b1 .... ``` All I need to do is change the test to a 'greater than or equal to' test and then the program should run fine. The opcode for jle is 0x7e and I need to change it to 0x7d. My assignment gives instructions on how to do this as follows: ``` $ gdb -write -q programtomodify (gdb) set {unsigned char} 0x8040856f = 0x7d (gdb) quit ``` So I try it and get... ``` $ gdb -write -q player (gdb) set {unsigned char} 0x7ffff7acb377 = 0x7d Cannot access memory at address 0x7ffff7acb377 ``` I have tried various other memory addresses and no matter what I try I get the same response. That is my only problem, I don't care if it's the wrong address or wrong opcode instruction at this point, I just want to be able to modify the memory. I am running Linux Mint 14 via VMware Player Thank