[lld] r295632 - Add more comments about copy relocations.

[Hal Finkel via llvm-commits]

On 02/20/2017 08:16 PM, Rui Ueyama wrote:
> Are you suggesting I bring it up to the LLVM blog? This comment is my 
> opinion, and I guess not everybody agrees we should avoid exporting 
> globals from DSOs.

Either the LLVM blog or some other. If you feel that your opinion is 
controversial, you could stick to the facts about what becomes part of 
the ABI.

  -Hal

>
> On Mon, Feb 20, 2017 at 7:49 AM, Hal Finkel <hfinkel at anl.gov 
> <mailto:hfinkel at anl.gov>> wrote:
>
>     Hi Rui,
>
>     I certainly found this informative, thanks for writing this. I
>     think this would also be a good topic for a blog post; those will
>     get a larger audience.
>
>      -Hal
>
>
>
>     On 02/19/2017 08:22 PM, Rui Ueyama via llvm-commits wrote:
>
>         Author: ruiu
>         Date: Sun Feb 19 20:22:56 2017
>         New Revision: 295632
>
>         URL: http://llvm.org/viewvc/llvm-project?rev=295632&view=rev
>         <http://llvm.org/viewvc/llvm-project?rev=295632&view=rev>
>         Log:
>         Add more comments about copy relocations.
>
>         Modified:
>              lld/trunk/ELF/Relocations.cpp
>
>         Modified: lld/trunk/ELF/Relocations.cpp
>         URL:
>         http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Relocations.cpp?rev=295632&r1=295631&r2=295632&view=diff
>         <http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Relocations.cpp?rev=295632&r1=295631&r2=295632&view=diff>
>         ==============================================================================
>         --- lld/trunk/ELF/Relocations.cpp (original)
>         +++ lld/trunk/ELF/Relocations.cpp Sun Feb 19 20:22:56 2017
>         @@ -461,6 +461,21 @@ static std::vector<SharedSymbol<ELFT> *>
>           // dynamic linker, and the relocation contains not only
>         symbol name but
>           // various other informtion about the symbol. So, such
>         attributes become a
>           // part of the ABI.
>         +//
>         +// Note for application developers: I can give you a piece of
>         advice if
>         +// you are writing a shared library. You probably should
>         export only
>         +// functions from your library. You shouldn't export variables.
>         +//
>         +// As an example what can happen when you export variables
>         without knowing
>         +// the semantics of copy relocations, assume that you have an
>         exported
>         +// variable of type T. It is an ABI-breaking change to add
>         new members at
>         +// end of T even though doing that doesn't change the layout
>         of the
>         +// existing members. That's because the space for the new
>         members are not
>         +// reserved in .bss unless you recompile the main program.
>         That means they
>         +// are likely to overlap with other data that happens to be
>         laid out next
>         +// to the variable in .bss. This kind of issue is sometimes
>         very hard to
>         +// debug. What's a solution? Instead of exporting a varaible
>         V from a DSO,
>         +// define an accessor getV().
>           template <class ELFT> static void
>         addCopyRelSymbol(SharedSymbol<ELFT> *SS) {
>             typedef typename ELFT::uint uintX_t;
>           @@ -482,10 +497,10 @@ template <class ELFT> static void addCop
>             // Look through the DSO's dynamic symbol table for aliases
>         and create a
>             // dynamic symbol for each one. This causes the copy
>         relocation to correctly
>             // interpose any aliases.
>         -  for (SharedSymbol<ELFT> *Alias : getSymbolsAt(SS)) {
>         -    Alias->NeedsCopy = true;
>         -    Alias->Section = ISec;
>         -    Alias->symbol()->IsUsedInRegularObj = true;
>         +  for (SharedSymbol<ELFT> *Sym : getSymbolsAt(SS)) {
>         +    Sym->NeedsCopy = true;
>         +    Sym->Section = ISec;
>         +    Sym->symbol()->IsUsedInRegularObj = true;
>             }
>               In<ELFT>::RelaDyn->addReloc({Target->CopyRel, ISec, 0,
>         false, SS, 0});
>
>
>         _______________________________________________
>         llvm-commits mailing list
>         llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org>
>         http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>         <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits>
>
>
>     -- 
>     Hal Finkel
>     Lead, Compiler Technology and Programming Languages
>     Leadership Computing Facility
>     Argonne National Laboratory
>
>

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170221/844f7a9f/attachment.html>