Edit variable values in ELF file?

Question

I need to change a couple of variables in a compiled ELF file. Trying to explain this clearly I'll use a simple C struct as an example.

The single source file is compiled and linked (@ 0x1000) into MyFile.elf from MyFile.c:

typedef struct {
    uint32_t SerialNumber;      /* Increments for every time it's programmed */
    uint32_t PartNumber;        /* Always the same */
    char     ProdDateTime[32];  /* "YYYY-MM-DD HH:MM:SS" date/time when programmed */
    uint32_t CalcCrc32;         /* Checksum of the above data */
} MyData_T;

const MyData_T MyData = {
    /* SerialNumber      */ 0x11111111,
    /* PartNumber        */ 0x12345678,
    /* ProdDateTime[32]  */ "2013-11-10 12:49:30",
    /* CalcCrc32         */ 0xC0CAC01A
                        };

Now I need a "console-tool" that (without compiling):

1. Writes a new serial number to 0x1000
2. Writes a new string to 0x1008
3. Updates the checksum at 0x1028.

I have not been able to find a tool (objcopy etc?) that even does the first (1) task. Seems this should be a rather common scenario? I've written my own tool for now but would prefer a open source tool or similar.

Any suggestions / ideas / comments / criticisms are highly appreciated :D Thanks you!!

Condescending

Do you have adb (the assembly language debugger, not the android bridge)? Back in the day, we'd do something like echo "0x1000?W 0x11111111" | adb -w MyFile.elf – Living

I haven't tried/heard of adb. I don't think I have access to it for my project but I'll look into it regardless. Thanks! – Condescending

Having to adjust a compiled binary is not as common as you think. How would a "universal" tool know where to find the values to change? In this case, if you need to do this on a regular basis(1), writing a custom utility is exactly what I'd do. (1) For one-offs I'd simply use a hex editor. – Archiepiscopal

Answer 1

"gdb --write /your/application/binary" should be able to change value of initialized data and write it back to the executable.

Add "-batch" and "-x command_file" and you should be able to get it to do what you want.

Carmacarmack

I haven't tried this but several people has suggested gdb. I'm programming in a WIN environment and the toolchain was not delivered with gdb but I suppose I could find another toolchain for a similar CPU? Thanks a lot for your help :) – Condescending

What toolchain do you use and what's the target (x86? ARM? something else)? cygwin and mingw both provide pre-compiled gdb binaries, but that would only work for the host binaries. – Carmacarmack

ARM, Freescale Kinetis K61 to be more precise :) I suppose that if I find a pre-compiled gdb for a similar CPU I could possibly use that. Thank you! – Condescending

I forgot to mention I (am forced to) do my development on a WIN7-x64 machine :( – Condescending

Answer 2

QNX has a built-in tool called "spatch" that allows you to do exactly this. The other suggestions to use gdb or a hex editor are equally valid.

While patching binary code is totally possible, it sounds like you're doing it wrong :-). Perhaps these values would be better suited to be stored in some data file distributed with the binary and read in during a constructor of some sort? Unless you have some compelling reason to require this to be in the binary, I would seriously look at the design and see if you really need to do this.

If the answer you come up with is "yes, I really need to do this," then great. You've got a couple very valid methods suggested for accomplishing this. Good luck.

Apologist

Thanks for your ideas. There's no file system etc. The image I'm editing basically constitutes this data file and is programmed straight to flash (using JTAG). – Condescending

@Condescending Ah. Then you are not "doing it wrong", as I incorrectly indicated. Good luck. :-) – Apologist