[llvm-dev] llvm-objcopy proposal

[Jake Ehrlich via llvm-dev]

Fantastic! Thanks for all of the input! I'll be considering all of it going
forward. The plan right now is just to worry about ELF executables and
nothing else. I'm very sympathetic to the "llvm-objtool" change. If
everyone is cool with it I'll change the name in the next CL to
"llvm-objtool".

To start out I implemented a very basic ELF64LE specific bit of code. I'm
currently looking for reviewers on it. The phabricator link is here:
https://reviews.llvm.org/D33964. I'd like to find people willing to review
this as I work on this going forward as well. I haven't bothered worrying
about it but I imagine that this will template fairly easily to support
ELF32LE, ELF32BE, and ELF64BE.

Would anyone be willing to let me set them as a reviewer going forward for
future CLs?

On Sun, Jun 4, 2017 at 6:07 PM Sean Silva via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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