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!!

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.

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.