Linux kernel: Dumping a module’s content for regression check

This post was written by eli on October 29, 2020
Posted Under: Linux,Linux kernel

After making a lot of whitespace reorganization in a kernel module (indentation, line breaks, fixing things reported by sparse and checkpatch), I wanted to make sure I didn’t really change anything. All edits were of the type that the compiler should be indifferent about, but how can I be sure I didn’t change anything accidentally?

It would have been nice if the compiler’s object files were identical before and after the changes, but that doesn’t happen. So instead, let’s hope it’s enough to verify that the executable assembly code didn’t change, and neither did the string literals.

The idea is to make a disassembly of the executable part and dump the part that contains the literal strings, and output everything into a single file. Do that before and after the changes (git helps here, of course), and run a plain diff on the couple of files.

Which boils down to this little script:


objdump -d $1
objdump -s -j .rodata -j .rodata.str1.1 $1

and run it on the compiled module, e.g.

$ ./ themodule.ko > original.txt

The script first makes the disassembly, and then makes a hex dump of two sections in the ELF file. Most interesting is the .rodata.str1.1 section, which contains the string literals. That’s the name of this section on an v5.7 kernel, anyhow.

Does it cover everything? Can I be sure that I did nothing wrong if the outputs before and after the changes are identical? I don’t really know. I know for sure that it detects the smallest change in the code, as well as a change in any error message string I had (and that’s where I made a lot of changes), but maybe there are some accidents that this check doesn’t cover.

As for how I found the names of the sections: Pretty much trying them all. The list of sections in the ELF file can be found with

$ readelf -S themodule.ko

However only those marked with PROGBITS type can be dumped with objdump -s (or more precisely, will be found with the -j flag). I think. It’s not like I really understand what I’m doing here.

Bottom line: This check is definitely better than nothing.

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