Re: [PATCH] kbuild: use objcopy to generate asm-offsets

[Vegard Nossum]

On 28/08/2024 10:36, Vegard Nossum wrote:
> In order to give assembly code access to C structs without having to
> hardcore member offsets, the kernel compiles a C source file listing all
> the structs and offsets that are needed in assembly code. Using some
> C preprocessor trickery and a sed script, the compiled assembly code is
> turned back into C preprocessor code that in turn can be used by the
> asssembly code.
>
> This sed script is very hard to read and understand.
>
> Remove the sed script and compile the C source listing structs and
> offsets to an object file (instead of assembly code) that embeds C source
> directly. Then extract the C source using objcopy.
>
> The resulting code is more readable, less fragile, and sligthly shorter.
>
> Note to reviewers: The 'objcopy ... /dev/stdout | cat' bit is needed to
> force the correct ordering of the objcopy lines vs. the surrounding echo
> commands; without it, objcopy will open /dev/stdout (which refers to a
> temporary file created by kbuild) and reset the file offset to 0. In
> other words, the pipe ensures that objcopy doesn't overwrite the lines
> that already exist in /dev/stdout.

Turns out LLVM's objcopy doesn't support writing to /dev/stdout, so we
might have to use a temporary file. I'll look into it and send a v2.


Vegard