On Tue, Jan 12, 2010 at 08:22:48AM +0000, Duncan Coutts wrote:
> man ld.so says:
>
> The shared libraries needed by the program are searched for in the
> following order:
>
> * (ELF only) Using the directories specified in the DT_RPATH
> dynamic section attribute of the binary if present and
> DT_RUNPATH attribute does not exist. Use of DT_RPATH is
> deprecated.
> * Using the environment variable LD_LIBRARY_PATH. Except if the
> executable is a set-user-ID/set-group-ID binary, in which case
> it is ignored.
> * (ELF only) Using the directories specified in the DT_RUNPATH
> dynamic section attribute of the binary if present.
glibc says (it's sufficient to read the comments):
------------------------------------------------------------------------
/* Search for NAME in several places. */
size_t namelen = strlen (name) + 1;
if (__builtin_expect (GLRO(dl_debug_mask) & DL_DEBUG_LIBS, 0))
_dl_debug_printf ("find library=%s [%lu]; searching\n", name, nsid);
fd = -1;
/* When the object has the RUNPATH information we don't use any
RPATHs. */
if (loader == NULL || loader->l_info[DT_RUNPATH] == NULL)
{
/* This is the executable's map (if there is one). Make sure that
we do not look at it twice. */
struct link_map *main_map = GL(dl_ns)[LM_ID_BASE]._ns_loaded;
bool did_main_map = false;
/* First try the DT_RPATH of the dependent object that caused NAME
to be loaded. Then that object's dependent, and on up. */
for (l = loader; l; l = l->l_loader)
if (cache_rpath (l, &l->l_rpath_dirs, DT_RPATH, "RPATH"))
{
fd = open_path (name, namelen, preloaded, &l->l_rpath_dirs,
&realname, &fb, loader, LA_SER_RUNPATH,
&found_other_class);
if (fd != -1)
break;
did_main_map |= l == main_map;
}
/* If dynamically linked, try the DT_RPATH of the executable
itself. NB: we do this for lookups in any namespace. */
if (fd == -1 && !did_main_map
&& main_map != NULL && main_map->l_type != lt_loaded
&& cache_rpath (main_map, &main_map->l_rpath_dirs, DT_RPATH,
"RPATH"))
fd = open_path (name, namelen, preloaded, &main_map->l_rpath_dirs,
&realname, &fb, loader ?: main_map, LA_SER_RUNPATH,
&found_other_class);
}
/* Try the LD_LIBRARY_PATH environment variable. */
if (fd == -1 && env_path_list.dirs != (void *) -1)
fd = open_path (name, namelen, preloaded, &env_path_list,
&realname, &fb,
loader ?: GL(dl_ns)[LM_ID_BASE]._ns_loaded,
LA_SER_LIBPATH, &found_other_class);
/* Look at the RUNPATH information for this binary. */
if (fd == -1 && loader != NULL
&& cache_rpath (loader, &loader->l_runpath_dirs,
DT_RUNPATH, "RUNPATH"))
fd = open_path (name, namelen, preloaded,
&loader->l_runpath_dirs, &realname, &fb, loader,
LA_SER_RUNPATH, &found_other_class);
if (fd == -1
&& (__builtin_expect (! preloaded, 1)
|| ! INTUSE(__libc_enable_secure)))
{
/* Check the list of libraries in the file /etc/ld.so.cache,
for compatibility with Linux's ldconfig program. */
const char *cached = _dl_load_cache_lookup (name);
<snip>
}
/* Finally, try the default path. */
if (fd == -1
&& ((l = loader ?: GL(dl_ns)[nsid]._ns_loaded) == NULL
|| __builtin_expect (!(l->l_flags_1 & DF_1_NODEFLIB), 1))
&& rtld_search_dirs.dirs != (void *) -1)
fd = open_path (name, namelen, preloaded, &rtld_search_dirs,
&realname, &fb, l, LA_SER_DEFAULT, &found_other_class);
------------------------------------------------------------------------
> Indeed you can test this and find that this is exactly how it behaves.
>
> $ ./main
> 42
>
> $ LD_LIBRARY_PATH=bar ./main
> 43
>
> This is a main linking to libfoo.so via rpath but overridden to link to
> a separate ./bar/libfoo.so that exports a different implementation of
> extern int foo();
asuffi...@cyclone:~/tmp$ echo 'int foo = 42;' > foo42.c
asuffi...@cyclone:~/tmp$ echo 'int foo = 43;' > foo43.c
asuffi...@cyclone:~/tmp$ gcc -shared -o libfoo.so foo42.c
asuffi...@cyclone:~/tmp$ gcc -shared -o bar/libfoo.so foo43.c
asuffi...@cyclone:~/tmp$ printf '#include <stdio.h>\nextern int foo; int
main(void){printf("%%d\\n",foo);}\n' > main.c
asuffi...@cyclone:~/tmp$ gcc -o main main.c libfoo.so -Wl,-rpath .
asuffi...@cyclone:~/tmp$ ./main
42
asuffi...@cyclone:~/tmp$ LD_LIBRARY_PATH=bar ./main
42
asuffi...@cyclone:~/tmp$ LD_DEBUG=libs LD_LIBRARY_PATH=bar ./main
20013: find library=libfoo.so [0]; searching
20013: search path=./tls/x86_64:./tls:./x86_64:. (RPATH
from file ./main)
20013: trying file=./tls/x86_64/libfoo.so
20013: trying file=./tls/libfoo.so
20013: trying file=./x86_64/libfoo.so
20013: trying file=./libfoo.so
20013:
<snip>
Are you perhaps not on a glibc system? The manpage you quoted is not
the one normally found there (which also happens to be wrong, sigh). I
did say that this behaviour is totally non-portable. glibc follows the
ELF specification and, approximately, Solaris, but most other things
do not.
> > It overrides everything, which is why it's so bad. The only thing you
> > can do is use --inhibit-rpath, but that requires you to maintain a
> > full list of the broken objects that have rpath entries, and use a
> > wrapper script to start every binary - in which case you might as well
> > just forget the whole thing and use LD_PRELOAD.
>
> Is your concern about old .so libs that have an DT_RPATH but no
> DT_RUNPATH entry and thus invoke the old behaviour?
ghc uses DT_RPATH with no DT_RUNPATH, because it relies on dlopen()
for addDLL() to work [see previous mail re: DT_RUNPATH does not work
with dlopen(), by design]
I am not sure why you think there are any 'new' libraries that use
DT_RUNPATH. It's a little-known and rarely-used feature; the effort to
replace DT_RPATH is widely regarded to have been a failure.
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