Re: [PATCH] tests: Add -ldl to dwfl_proc_attach_LDFLAGS

[Florian Weimer <fweimer@redhat.com>]

* Dmitry V. Levin:

> Let's make clear what's going on here.  First of all, dwfl-proc-attach.c
> does not use dlopen so it doesn't pull it in and doesn't need -ldl.
> In regular builds, dwfl-proc-attach.o is linked with ../libdw/libdw.so
> which in turn uses dlopen and is already linked with -ldl.
> When elfutils is configured with --enable-gprof or --enable-gcov,
> BUILD_STATIC is enabled and dwfl-proc-attach.o is linked with
> ../libdw/libdw.a -lz $(zip_LIBS) $(libelf) $(libebl) -ldl -lpthread
> which already contains -ldl.
> In any case, I fail to understand why dwfl-proc-attach might need
> an extra -ldl, especially in LDFLAGS which goes before LDADD
> in the linking command.

It may have to do with --as-needed that some builds use.  If there are
no pending undefined references, some linkers drop earlier shared object
references with --as-needed (similar to what happens with static
archives).

The GCC LTO plugin results in ld looking at more objects in greater
detail for some reason.  Without LTO and --as-needed, you probably don't
get a dlopen export (if you do not link with -E) because indirect
dependencies are not consulted, breaking the valgrind workaround because
there is no interposition.

Thanks,
Florian