[llvm-dev] llvm-objcopy proposal

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Sun Jun 4 18:07:21 PDT 2017

On Fri, Jun 2, 2017 at 3:52 PM, James Y Knight via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Fri, Jun 2, 2017 at 2:34 PM, Ed Maste via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> One additional use case for you: converting from a binary to an ELF
>> object file
>> ```
>> objcopy -I binary -O elf64-x86-64 foo.bin foo.o
>> ```
>> This is sometimes used for embedding binary files for use by drivers and
>> such.
> Yea, unfortunately the command-line you actually end up needing is more
> like:
>   objcopy -I binary -Bi386:x86-64 -Oelf64-x86-64 --rename-section
> .data=.rodata,alloc,load,readonly,data,contents --add-section
> .note.GNU-stack=/dev/null
> Having to manually invoke objcopy and know what to specify for the -B and
> -O options, and to know you need the .note.GNU-stack section, and how to
> move it into rodata...it's really all quite terrible. Nobody should have to
> do that. :(
> There's also the "-b binary" flag to GNU ld (both bfd and gold). But, you
> typically need to do a dedicated "link" for that. You do:
>   ld -r -b binary picture.jpg -o foo.o
> How does ld know what output format to use here? It's gotta just choose
> the default, which is kinda poor...or the user needs to know how to spell
> an "emulation" and output format...

One way to hack around this might be to pass in one of the other object
files in your project, and have the output .o file replace it. Still pretty
hacky and brittle (and hard to integrate into a build system I would think).

> You could imagine trying to use -Wl to put it with the compile command,
> but what do you use to switch back to the normal object format?
>   gcc main.c -Wl,-b -Wl,binary -Wl,picture.jpg -Wl,-b -Wl,<<something to
> undo binary mode?>>
> So, anyways, while this is _possible_ with objcopy, it'd sure be nice if
> you never needed to use it for that...

The other approaches I've seen or can imagine are:

- Assembler `.incbin` directive (could use it from an inline asm).
- Use a "bin2h" type program which takes a binary and spits out a C file
with a giant uint8_t[] literal in it, then include that in one of your
normal .c files. In theory a C++11 raw string literal could bypass most of
the parsing overhead of a big array literal, but the people that care about
including a binary in their program probably don't care about that.

-- Sean Silva

> (BTW, Apple ld actually has an option "-sectcreate SEGNAME SECTNAME
> INPUT_FILE", and the clang driver will pass it through to the linker.)
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