How ObjDump retrieve labels from elf file

[Newbie]

Hello everybody,

Looking around GCC, ELF and OBJDump, I ask myself a question.
When I insert a label in ASM code in a function (the main for example).

In C :

Code: Select all

	int main(int argc, char* argv[])
	{
		.....
		.....
		.....
		.....
		.....
		.....
		.....
		
		__asm__ __volatile__("START:");

		.....
		.....
		.....
		.....
		.....
		.....
		.....
		
		__asm__ __volatile__("END:");

		return 0;	
	}

After compiling it to obtain an ELF then I use OBJDump to examine the code.

In ELF :

8c010242 <START>:
8c010242: .....
8c010244: .....
8c010246: .....
8c010248: .....
8c01024a: .....
8c01024c: .....
8c01024e: .....
8c010250: .....
8c010252: .....
8c010254: .....
8c010256: .....
8c010258: .....
8c01025a <END>:

I found out the written labels in the C code.

But when I try to "Hexedit" the ELF file, searching for the labels ("START:", "START" ...), i did not found anything.

So how could OBJDump write at the correct place the desired label in the code as those labels seem to not exist in ELF file.

Thanks for your explanations.

[BlueCrab]

The labels are stored as debugging symbols in the elf file.

[nymus]

Yes, the .elf file has a specific structure that the gcc tools know about so they can find the debug symbols easily. The labels are still there in text form, but since a "hexdump" program doesn't know the structure, it just reads the data and formats it in its own way e.g as bytes / shorts / longs etc.

A hexedit program "should" be able to find text, though. Which program are you using? Most hex editors have a data frame with raw data and a side frame that displays text. You can even see the symbol names if you execute the command: "grep --binary-files=text file.elf"

Attached is a screenshot of elvis (a vi clone) displaying a section of a .elf file. You can see the text on the far right corresponding to the raw data in the main window.

Note those labels are not structured as they appear in the assembly. They are all placed together in a special section and there are pointers to that text somewhere so that the tools know which text corresponds to which address in the code.

[Newbie]

Hi,

Thanks for answers, I use HXD (https://mh-nexus.de/en/hxd/).

Well, I expected a simpler way of reading labels to figure out position code in the elf file. Perhaps even writing some code to obtain it.

If it uses something like referencing an id used to retrieve the label string from a far located section in the elf file, I did not like that

[nymus]

Try downloading dcload sources and see how t use libelf to decode elf files. It's maintained beside the KOS git repository.