[llvm-dev] Linker Option support for ELF

Saleem Abdulrasool via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 26 15:08:43 PST 2018

> On Jan 18, 2018, at 9:43 PM, Saleem Abdulrasool <compnerd at compnerd.org> wrote:
> For those interested, I've updated the patch on D40849.  It is significantly simpler, emitting the options as pairs of C-style strings in a section named `.linker-options` as the agreement here was.

I’ve updated the patch, its pretty much ready to go.  I’ve added a new section type, documentation, and some additional tests.  Please give it a final look, I’d like to get this in soon.


> On Fri, Jan 12, 2018 at 8:41 PM, Rui Ueyama via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> On Fri, Jan 12, 2018 at 5:56 PM, Cary Coutant via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> > Given how ELF works I would expect an unknown section to simply end up
> > in the output, but we can use SHF_EXCLUDE to avoid that.
> Yes, gold currently treats unknown section types pretty much the same
> as PROGBITS sections. The SHF_ALLOCATE and SHF_EXCLUDE flags would
> control where and whether the section goes into the output file.
> Another thing that we need to work out is the link order. The link
> order is basically a topologically-ordered list of objects, ordered so
> that if A depends on B, A precedes B in the link order. Today, in the
> absence of any dependency information at all, we rely on the user and
> compiler to come up with a reasonably correct link order and pass a
> linear list of files and libraries to the linker. In an ideal world
> (e.g., one where you can just type "ld main.o"), we'd have explicit
> dependencies for every object, and we could construct a topological
> order automatically. But with this feature, we will have a partial
> list of explicit dependencies, and without a complete list, we have no
> good way of adding new objects into the link order.
> One way to approximate a proper link order would be to place each
> added object immediately after the last object that requests it. For
> example, if you run "ld a.o b.o c.o -lc", and both a.o and b.o request
> libm, you would insert libm (i.e., any and all objects extracted from
> libm if it's an archive library) after b.o and before c.o. But this
> approach wouldn't work -- we'd have to read and process the directive
> section from every object before establishing the final link order,
> which means we can't start building our symbol table until we've read
> all the objects, which means we can't search archive libraries.
> I'm little confused. Even if the final command line becomes something like this:
>   ld -static a.o -lm b.o -lm c.o -lc
> it links without any problem, no? I mean no files will be pulled out from libm.a more than once.
> FWIW the notion of the link order is very different in lld because our linking order is similar to Windows linkers rather than the traditional Unix linker.
> I think what would work is to insert each requested object or shared
> library into the link order immediately after the object that requests
> it, but only if the object hasn't already been inserted and isn't
> already listed on the command line (i.e., we won't try to load the
> same file twice); and to search each requested archive library
> immediately after each object that requests it (of course, because of
> how library searching works, we would load a given archive member once
> at most). With this method, libm would be searched after both a.o and
> b.o, so we'd load any members needed by a.o before b.o, and any
> remaining members needed by b.o before c.o.
> The difference between this and a proper topological ordering would be
> small, but would have a subtle effect on symbol interposition. I'm
> willing to require anyone who depends on symbol interposition to
> control their link order explicitly via the command line.
> In my ideal world, archive libraries would carry dependency
> information rather than the individual objects within them. I suspect
> that's too much to ask.
> I see no need for shared libraries to carry any dependency information
> beyond the DT_NEEDED entries they already have.
> (It would be so much easier to build a self-driving car if we could
> immediately jump to the point where all cars are self-driving, right?)
> -cary
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> -- 
> Saleem Abdulrasool
> compnerd (at) compnerd (dot) org

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