[systemd-devel] [RFC 04/12] build: add target to link binary sources

[Greg KH]

On Wed, Nov 27, 2013 at 07:48:39PM +0100, David Herrmann wrote:
> In several situations we want to link a binary file into our executable
> and access it from our C code. The easiest way is to transform it into a
> C-array and compile it as usual. However, for large files (>1MB) such
> compilations can take a considerable amount of time or even fail on
> low-memory systems.
> 
> This adds a new automake-target to link binary sources directly. Instead
> of transforming it into a C-array, we simply use "ld -r" to create an
> object file via:
>   ld -r -o my-source.bin.o --format=binary my-source.bin
> We also use "-z noexecstack" to mark "my-source.bin.o" to not require an
> executable stack.
> 
> As we only want to support read-only data sources here, we do some
> post-processing to mark the object as read-only via:
>   objcopy --rename-section .data=.rodata,alloc,load,readonly,data,contents my-source.bin.o
> 
> As libtool requires "*.lo" files, we cannot link this object-file
> directly. Thus, we also create a fake "*.lo" file for such objects which
> libtool can use. Note that libtool actually *requires* the comment-section
> in "*.lo" files (ugh?!) so we need to fake that, too.
> 
> How to use this helper?
>  - put your binary source file into the tree as:
>      src/somewhere/something.bin
>  - for the library you want to link that to, add this to mylib_LIBADD:
>      src/somewhere/something.bin.lo
> This causes the helper to create src/somewhere/something.bin.[o,lo] and
> it will be linked as a normal object file.
> 
> GNU-ld automatically creates 3 symbols for such objects, but the important
> symbols are:
>   extern const char _binary_src_somewhere_something_bin_start[];
>   extern const char _binary_src_somewhere_something_bin_end[];
> Use these to access start/end of the binary section.
> 
> I tested this with in-tree and out-of-tree builds, with GNU-ld and
> GNU-gold and cross-compilation. All worked fine..

That's crazy, very nice job in doing this :)

I tried to do this a while ago, and gave up and just used a perl script
and a .c file with a big array, like 'xxd -i' can create.  I think I'll
"steal" this idea for other projects if you don't mind, as it makes
things easier just to use the linker.

greg k-h