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

[Mark Wielaard <mark@klomp.org>]

Hi Dmitry,

On Fri, Nov 19, 2021 at 01:20:26AM +0300, Dmitry V. Levin wrote:
> On Thu, Nov 18, 2021 at 10:23:41PM +0100, Mark Wielaard wrote:
> > dwfl-proc-attach uses (overrides) dlopen (so it does nothing).  This
> > seems to cause a versioned dlopen symbol to be pulled in when building
> > with LTO. Resulting in a link failure (when dlopen isn't integrated
> > into libc):
> >
> > /usr/bin/ld: dwfl-proc-attach.o (symbol from plugin): undefined
> > reference to symbol 'dlopen@@GLIBC_2.2.5'
> > /usr/bin/ld: /usr/lib64/libdl.so.2: error adding symbols: DSO missing
> > from command line collect2: error: ld returned 1 exit status
> >
> > So simply explicitly add -ldl to the LDFLAGS.
> [...]
> 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.

Agreed. It isn't totally clear why lto causes dlopen to be "pulled in"
(which is probably the wrong technical term). But it really
does. Instead of adding -ldl to LDFLAGS you can also remove the dlopen
from the testcase:

diff --git a/tests/dwfl-proc-attach.c b/tests/dwfl-proc-attach.c
index d02e9fc0..0c5e8622 100644
--- a/tests/dwfl-proc-attach.c
+++ b/tests/dwfl-proc-attach.c
@@ -114,9 +114,5 @@ main (int argc __attribute__ ((unused)),
    So simply override dlopen and always return NULL so libdebuginfod
    (and libcurl) are never loaded.  This test doesn't rely on
    libdebuginfod anyway.  */
-void *dlopen (void)
-{
-  return NULL;
-}
 
 #endif /* __linux__ */

Which also makes the linker happy, but removes the valgrind workaround.

Cheers,

Mark