[LLVMdev] Some thought on handling ELF shared libraries in lld

[Hal Finkel]

----- Original Message -----
> From: "Rafael Espíndola" <rafael.espindola at gmail.com>
> To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Tuesday, July 21, 2015 7:46:58 AM
> Subject: [LLVMdev] Some thought on handling ELF shared libraries in lld
> 
> Most ELF shared libraries can be sliced in two ways. One is following
> the information in the program headers (e_phoff). The other is
> following the information in the section headers (e_shoff).
> 
> Regular relocatable objects only have the section header.
> 
> At runtime, the dynamic linker only uses the program headers. In
> fact,
> the section headers is optional.
> 
> When given a shared library, how should the static linker handle it?
> 
> Note that, unlike the dynamic linker, the static one has to find all
> the defined symbol is a shared library. It is not enough to just look
> up the currently undefined symbols. To see that, consider
> 
> $ cat test.c
> void f(void) { }
> $ cat test2.c
> void f(void);
> void g(void) {
>   f();
> }
> 
> $ clang -c test.c test2.c -fPIC
> $ clang -shared test.o -o test.so
> $ rm -f test.a
> $ ar rc test.a test.o
> $ clang test.so test2.o test.a -o t.so -Wl,-t -shared
> $ clang         test2.o test.a -o t.so -Wl,-t -shared
> 
> The second link will include the archive member, the first one will
> not.
> 
> It is tempting to use the program headers in the static linker. Doing
> so would let us support linking with shared libraries with no section
> headers, but there are a few issues:
> 
> * The intention of the spec seems to be for everything static to use
> the section headers and everything dynamic to use the program
> headers.
> * Finding the number of symbols with the program header in a
> traditional ELF file is a hack. One has to read the nchain field of
> the hash table.
> * It doesn't seem even possible to find that information in files
> using the newer gnu hash format
> (https://blogs.oracle.com/ali/entry/gnu_hash_elf_sections).

Why do you need the total number of symbols?

 -Hal

> 
> Given that, it looks like we should use the sections. For what it is
> worth, it looks like that is what every other ELF linker does.
> 
> Cheers,
> Rafael
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory