[LLVMdev] Some thought on handling ELF shared libraries in lld (original) (raw)

Davide Italiano davide at freebsd.org
Tue Jul 21 17:35:48 PDT 2015

On Tue, Jul 21, 2015 at 5:46 AM, Rafael Espíndola <rafael.espindola at gmail.com> wrote:

Most ELF shared libraries can be sliced in two ways. One is following the information in the program headers (ephoff). The other is following the information in the section headers (eshoff).

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/gnuhashelfsections). 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.

I agree we should use sections. Also, there's no reason we diverge from what other linker do unless there's a real reason.

-- Davide

"There are no solved problems; there are only problems that are more or less solved" -- Henri Poincare

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