[llvm-dev] Linker Option support for ELF

Sun Jan 7 21:14:15 PST 2018

I think it makes sense to allow specifying "-lNAME" in an ELF object file,
the same way it makes sense on Apple and MS systems.

But I'm very much against allowing any sort of option that specifies full
paths to be embedded, including "-L". Typically, object files are
location-independent -- if you want to copy them to another system and run
the linker there (with an appropriate sysroot path), that's going to work
perfectly fine. But as soon as you start allowing path options to be
embedded, you're in a whole new world, with a whole new set of possible
pain. And, AFAICT, other systems do not allow that, either.

Other issues:

There will need to be a new linker option, "-ignore-auto-link" or something
like that, for people who want to disable this feature.

Also, what happens with "-r" and "-Ur"? Probably that should propagate the
options to the output .o file, instead of using them, but I'm not certain.
Maybe that needs a new option as well.

An additional complication that I think nobody has mentioned yet: what
happens with static libraries containing such object files? I'd hope that
linking against a .a which includes .o files with linker options, only adds
the options from a .o files _actually included_ in the link. That is true
of the Apple implementation, but I don't know about the MS one. But that
really needs to be specified.

A short survey of some existing implementations:

1. Apple

ld64 allows precisely two linker options to be embedded: "-lNAME" and
"-framework NAME". Note that -framework is almost identical to "-l". It
simply looks for a library called "NAME.framework/NAME" in the
framework-search-path, instead of a library called libNAME.{a,so} in the
library-search-path.

Apple has an option called "-ignore_auto_link", to ignore these embedded
options from object files.

2. MSVC

MS allows only a few options as well:
https://docs.microsoft.com/en-us/cpp/preprocessor/comment-c-cpp
- /DEFAULTLIB (equivalent to "-l")
- /EXPORT (export a symbol)
- /INCLUDE (equivalent to "-u")
- /MANIFESTDEPENDENCY (add data to manifest)
- /MERGE (merge two sections)
- /SECTION (specify attributes for sections)

MS has a linker option "/NODEFAULTLIB", which ignores any /DEFAULTLIB
options (from object files or the command-line). MS also allows you to
_selectively_ ignore certain libraries, with "/NODEFAULTLIB:name.lib". Also
note that "/NODEFAULTLIB" doesn't ignore ALL the options specified in an
object files, only the additional libraries to link against.

On Sun, Jan 7, 2018 at 2:52 PM, Saleem Abdulrasool via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Jan 6, 2018, at 6:55 PM, Rui Ueyama <ruiu at google.com> wrote:
>
> On Sat, Jan 6, 2018 at 8:56 AM, Saleem Abdulrasool <compnerd at compnerd.org>
>  wrote:
>
>>
>> On Fri, Jan 5, 2018 at 2:30 AM Rui Ueyama <ruiu at google.com> wrote:
>>
>>> Thank you for starting the discussion thread.
>>>
>>> In general I'm in favor of the proposal. Defining a generic way to
>>> convey some information from the compiler to the linker is useful, and it
>>> looks like it is just a historical reason that the ELF lacks the feature at
>>> the moment.
>>>
>>> This is a scenario in which the feature is useful: when you include
>>> math.h, a compiler (which is driven by some pragma) could added `-lm` to
>>> the note section so that a linker automatically links libm.
>>>
>>> I think I'm also in favor of the format, which is essentially runs of
>>> null-terminated strings (*1) that are basically opaque to compilers.
>>>
>>
>> Yes.  However, I think I want to clarify that we want this to be
>> completely opaque to the backend.  The front end could possibly have some
>> enhancements to make this better.  But, that will be a separate change, and
>> that discussion should take place then.  We shouldn’t paint ourselves into
>> a corner.  Basically, I think that there is some legitimate concerns here,
>> but they would not be handled at this layer, but above.
>>
>> However, you should define as a spec what options are allowed and what
>>> their semantics are. We should not accept arbitrary linker options because
>>> semantics of some linker options cannot be clearly defined when they appear
>>> as embedded options. Just saying "this feature allows you to embed linker
>>> options to object files" is too weak as a specification. You need to
>>> clearly define a list of options that will be supported by linkers with
>>> their clear semantics.
>>>
>>
>> Personally, I would like to see the ability to add support for additional
>> options without having to modify the compiler.  That said, I think that
>> there are options which can be scary (e.g. -nopie).  I think that the
>> linker should make the decision of what it supports and error out on
>> others.  This allows for us to enhance the support over time without a huge
>> overhead.  As a starting point, I think that -l and -L are two that would
>> be interesting.  I can see -u being useful as well, but the point is that
>> we can slowly grow the support after consideration by delaying the
>> validation of the options.
>>
>
> I think no one including me opposed the idea of designing the feature so
> that new options can be added without having to modify the compiler. That's
> fine to me.
>
>
> Excellent.  I think that the TLV approach requires that to a certain
> extent (the enumeration would be hardcoded).
>
> What I said is that you still have to specify a list of options that are
> allowed in the "linker options" section in your *specification*. Your
> specification is incomplete and underspecified if it doesn't explicitly
> list linker options with their semantics. If you don't do that, people
> would start using arbitrary linker options whose semantics are vague/that
> are dangerous/that are simply wrong (e.g. unclosed `-whole-archive` option)
>  after you would implement the feature. We really shouldn't let that mess
> happen.
>
>
> I think we all agree that blindly allowing the linker to honor the options
> would be scary.  I agree that we should whitelist the options, and am of
> the opinion that we should force validation on the linker side (use of any
> option which the linker doesn't support in this form can be fatal).
> Starting small is the best way, with `-l` and `-L` as a starting point.  I
> want to retain the ability to add additional options which may not be
> available in all linkers.  However, whitelisting obviously requires working
> with the linker as would adding such options, so that could be handled at
> that time.
>
> Furthermore, note that the LLVM side of the implementation does not
> provide a way to add anything from the user - that is a change to clang,
> and is something that I intend to be a follow up change.  As I mentioned in
> a previous email, I have my own preferences for how to handle those, but, I
> think that when we start adding support to the frontend for it is the right
> time to discuss the syntax of how to support that.  If we go with the
> approach of tokenizing the options and doing a C-string per option, I think
> it leaves sufficient flexibility to permit the frontend proper control to
> emit the necessary argument and not have the linker parse the options
> having to worry about quoting and space handling which I believe to be a
> good compromise on all sides.
>
> Concretely, which options do you want to use? I could imagine everyone
> wants to use `-l`. `-L` is arguable because I do not see an obvious reason
> to add a search path from the compiler. You mentioned `-u`. Is there any
> other flag that you have in your mind?
>
>
> I’m thinking about future enhancements.  MachO does actually provide
> something like `-L` -`l` in a single go via `-framework`.  But, no such
> option exists for ELF since it doesn’t have the concept of framework
> bundles (but the layout itself is interesting), and I just want to try to
> keep the door open for such features.
>
>
> (*1) One of the big annoyances that I noticed when I was implementing the
>>> same feature for COFF is that the COFF's .drctve section that contains
>>> linker options have to be tokenized in the same way as the Windows command
>>> line does. So it needs to interpret double quotes and backslashes correctly
>>> especially when handling space-containing pathnames. This is a design
>>> failure that a COFF file contains just a single string instead of runs of
>>> strings that have already been tokenized.
>>>
>>
>> I think that there is room for refinement on this.  The best part is that
>> the refinement for that is delayed!  It would be best done in the front end
>> IMO, and we can actually  further discuss and design the feature there.  I
>> don’t think that we need to be completely blind to the issues we have seen,
>> but we shouldn’t over constrain either.
>>
>> On Thu, Jan 4, 2018 at 9:02 AM, Saleem Abdulrasool via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hello all,
>>>>
>>>> There was some interest from a number of a few people about adding
>>>> support for embedded linker options to ELF.  This would be an extension
>>>> that requires linker support to actually work, but has significant prior
>>>> art with PE/COFF as well as MachO both having support for this.
>>>>
>>>> The desire here is to actually add support to LLVM to pass along the
>>>> necessary information into the object file.  In order to keep this focused
>>>> on that, this thread is specifically for the *backend*, we are not
>>>> discussing how to get the information to the backend here at all, but
>>>> assuming that the information is present in the same LLVM IR encoding
>>>> (llvm.linker-options module metadata string).
>>>>
>>>> In order to have compatibility with existing linkers, I am suggesting
>>>> the use of an ELF note.  These are implicitly dropped by the linker so we
>>>> can be certain that the options will not end up in the final binary even if
>>>> the extension is not supported.  The payload would be a 4-byte version
>>>> identifier (to allow future enhancements) and a null-terminated string of
>>>> options.
>>>>
>>>> This allows for the backend to be entirely oblivious to the data as the
>>>> other backends and allows for extensions in the future without having to
>>>> teach the backend anything about the new functionality (again, something
>>>> which both of the other file formats support).
>>>>
>>>> As an example of how this can be useful, it would help with Swift
>>>> support on Linux where currently the linker options are pushed into a
>>>> custom section, and a secondary program is used to extract the options from
>>>> this section prior to the linker being invoked.  This adds a lot of
>>>> complexity to the driver as well as additional tools being invoked in the
>>>> build chain slowing down the build.
>>>>
>>>> --
>>>> Saleem Abdulrasool
>>>> compnerd (at) compnerd (dot) org
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>> --
>> Saleem Abdulrasool
>> compnerd (at) compnerd (dot) org
>>
>
>
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