Programming

I gave it a whirrl. While it gave me a nice tree-view of the elf binary, it was not so simple to add or change section data. It appears you have to do it "by hand" -- ie, create a table, calculate the correct pointer, and change the entry in the table for that pointer. Not particularly time-saving.

You can use the GNU linker (ld) to create a new executable that's identical as the old one, but additionally contains the section from a new file. Well, in theory. So far I haven't gotten it to work as hoped. Attached are the files you can use for playing around.

First, you take a C source file that contains only static strings:

Code:

static char* __version = "sample 1.0";
static char* __author = "otheus@gmail.com";

Second, you compile it as usual with a C compiler to an object file.

Code:

gcc -o version_tmp.o -c version.c

Third, create a ld script file that will be used by ldto create an object file containing the static strings in your source, but in a ".comment" section.

Code:

SECTIONS
{
  .comment       0 : { *(.rodata) }
  /DISCARD/        : { *(.text .bss .data .comment .note.GNU-stack) }
}

Fourth, use the linker with this special ld script file ("commentsonly.lds") on your object file (created above with the gcc command). Now, instead of the strings being in the ".rodata" section, they are stored in the ".comment" section. Use the -r option so an object file is created.

Code:

ld -r -T commentsonly.lds -o version.o version_tmp.o

Finally, relink the original program with the new object file:

Code:

ld -o new_executable original_executable version.o

Now you can confirm the data is stored in this file using readelf or objdump:

Code:

objdump -s -j .comment new_executable

Here you should see the data you stored in your .c program.

So what happens. When I tried it, I got a segmentation fault. I suspect you have to do some address juggling to get this to work right. Let me know if you get anywhere?

If you want to add a new section to an object file from an outside object file can you not do

Code:

objdump --target=<your triplet> --add-section SECTIONNAME==version.o \ 
   old_exe new_exe

objdump will identify the triplet of BFDNAME for the original for you ahead of time. This approoach will only work if .comment is a valid sectionname for your format. It should be a readonly section.

I cannot try this -- I'm on HPUX for the near future.