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

[Rafael Espíndola]

On 21 July 2015 at 09:08, Hal Finkel <hfinkel at anl.gov> wrote:
> ----- 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?

To find all the defined ones. See the above example.

Cheers,
Rafael