* RTLD_LAZY deferred symbol binding
@ 2019-12-11 10:09 Scherbatiy Alexander
2019-12-11 10:35 ` Szabolcs Nagy
0 siblings, 1 reply; 5+ messages in thread
From: Scherbatiy Alexander @ 2019-12-11 10:09 UTC (permalink / raw)
To: musl
Hello,
musl libc release 1.1.17 has new feature [1]:
- RTLD_LAZY deferred symbol binding, functionally equivalent to lazy binding
The lazy bindings section [2] gives more details on it:
Newer versions of musl implement “deferred binding” in place of lazy binding, whereby binding is deferred until a subsequent dlopen call that introduces new symbols, rather than at the point of the function call.
It is still is not clear for me what is a difference between of lazy and deferred binding.
I wrote a simple example there a shared library with an unresolved symbols is loaded by dlopen with RTLD_LAZY option (source code is at the end of the email).
It works on my Ubuntu desktop but fails on Alpine linux 3.10.3 with musl libc 1.1.22 (x86_64) with message:
"dlopen failed: Error relocating bin/shared/libshared_lib.so: unresolved_function: symbol not found"
What is a good example that can show how the new "deferred symbol binding" feature works so it fails before muls libc 1.1.17 and starts working after it?
[1] https://git.musl-libc.org/cgit/musl/tree/WHATSNEW
[2] https://wiki.musl-libc.org/functional-differences-from-glibc.html
Thanks,
Alexander.
Loading a shared library with unresolved symbols example:
--- include/resolved_lib.h ---
void resolved_function();
--- include/unresolved_lib.h ---
void unresolved_function();
--- include/shared_lib.h ---
void call_resolved_function();
void call_unresolved_function();
--- src/resolved_impl.c ---
#include <stdio.h>
#include "resolved_lib.h"
void resolved_function() {
printf("call resolved function.\n");
}
--- src/shared_lib.c ---
#include "shared_lib.h"
#include "resolved_lib.h"
#include "unresolved_lib.h"
void call_resolved_function() {
resolved_function();
}
void call_unresolved_function() {
unresolved_function();
}
--- src/main.c ---
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
void call_resolved_function_dynamic() {
const char *lib_path = "bin/shared/libshared_lib.so";
void (*call_resolved_function)(void);
void *handle = dlopen(lib_path, RTLD_LAZY);
if (!handle) {
fprintf(stderr, "dlopen failed: %s\n", dlerror());
exit(EXIT_FAILURE);
}
*(void **) (&call_resolved_function) = dlsym(handle, "call_resolved_function");
char *error = dlerror();
if (error != NULL) {
fprintf(stderr, "%s\n", error);
exit(EXIT_FAILURE);
}
(*call_resolved_function)();
dlclose(handle);
}
int main(int argc, char* argv[]) {
printf("call main.\n");
call_resolved_function_dynamic();
}
--- ---
# build sources
gcc -c -fPIC src/resolved_impl.c -Iinclude -o bin/shared/resolved_impl.o
gcc -c -fPIC src/shared_lib.c -Iinclude -o bin/shared/shared_lib.o
gcc -shared bin/shared/shared_lib.o bin/shared/resolved_impl.o -Iinclude -o bin/shared/libshared_lib.so
gcc -c src/main.c -Iinclude -o bin/main.o
gcc bin/main.o -ldl -o bin/main
# run
export LD_LIBRARY_PATH=./bin/shared
./bin/main
--- ---
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: RTLD_LAZY deferred symbol binding
2019-12-11 10:09 RTLD_LAZY deferred symbol binding Scherbatiy Alexander
@ 2019-12-11 10:35 ` Szabolcs Nagy
2019-12-11 11:55 ` Scherbatiy Alexander
[not found] ` <4372011576065223@myt5-7210d748eb79.qloud-c.yandex.net>
0 siblings, 2 replies; 5+ messages in thread
From: Szabolcs Nagy @ 2019-12-11 10:35 UTC (permalink / raw)
To: musl; +Cc: Scherbatiy Alexander
* Scherbatiy Alexander <alexander.scherbatiy@bell-sw.com> [2019-12-11 13:09:36 +0300]:
> # build sources
> gcc -c -fPIC src/resolved_impl.c -Iinclude -o bin/shared/resolved_impl.o
> gcc -c -fPIC src/shared_lib.c -Iinclude -o bin/shared/shared_lib.o
> gcc -shared bin/shared/shared_lib.o bin/shared/resolved_impl.o -Iinclude -o bin/shared/libshared_lib.so
^^^^^^^^^^^^^^^
you need to pass -Wl,-z,lazy (and verify it with readelf -d )
because alpine (and various other distros) defaults to -z now
(and then libc obviously cant do lazy binding no matter what
you specified in dlopen).
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: RTLD_LAZY deferred symbol binding
2019-12-11 10:35 ` Szabolcs Nagy
@ 2019-12-11 11:55 ` Scherbatiy Alexander
2019-12-11 13:19 ` Rich Felker
[not found] ` <4372011576065223@myt5-7210d748eb79.qloud-c.yandex.net>
1 sibling, 1 reply; 5+ messages in thread
From: Scherbatiy Alexander @ 2019-12-11 11:55 UTC (permalink / raw)
To: Szabolcs Nagy, musl
Thank you. It works.
I looked at the ld help on linux Alpine and it shows
> ld --help
> -z lazy Mark object lazy runtime binding (default)
Should the lazy option be used by default or the documentation needs to be updated?
Thanks,
Alexander.
11.12.2019, 13:35, "Szabolcs Nagy" <nsz@port70.net>:
> * Scherbatiy Alexander <alexander.scherbatiy@bell-sw.com> [2019-12-11 13:09:36 +0300]:
>> # build sources
>> gcc -c -fPIC src/resolved_impl.c -Iinclude -o bin/shared/resolved_impl.o
>> gcc -c -fPIC src/shared_lib.c -Iinclude -o bin/shared/shared_lib.o
>> gcc -shared bin/shared/shared_lib.o bin/shared/resolved_impl.o -Iinclude -o bin/shared/libshared_lib.so
>
> ^^^^^^^^^^^^^^^
>
> you need to pass -Wl,-z,lazy (and verify it with readelf -d )
> because alpine (and various other distros) defaults to -z now
> (and then libc obviously cant do lazy binding no matter what
> you specified in dlopen).
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: RTLD_LAZY deferred symbol binding
2019-12-11 11:55 ` Scherbatiy Alexander
@ 2019-12-11 13:19 ` Rich Felker
0 siblings, 0 replies; 5+ messages in thread
From: Rich Felker @ 2019-12-11 13:19 UTC (permalink / raw)
To: musl
On Wed, Dec 11, 2019 at 02:55:48PM +0300, Scherbatiy Alexander wrote:
> Thank you. It works.
>
> I looked at the ld help on linux Alpine and it shows
> > ld --help
> > -z lazy Mark object lazy runtime binding (default)
>
> Should the lazy option be used by default
No, it's really a bug to be relying on this behavior, as it precludes
much-wanted hardening and is not well-defined per the supported
standards. Deferred binding was added for the sake of a very small but
important and stubborn set of software (i.e. Xorg) that was depending
on lazy binding and where musl-based dists were needing messy hacks to
make it work without (explicit load order of modules in xorg.conf).
The software that needs this behavior can be built with -z lazy as
needed. This is not unique to musl; it applies to hardened glibc
toolchains/distros where bindnow is default, too.
> or the documentation needs to be updated?
That's the documentation for ld, which applies if you invoke ld as a
command. That's not how you link hosted software. It's linked by
invoking $(CC) (e.g. gcc) which may be the part passing -z now; I
forget. If the default is actually changed in ld, then yes ld should
be patched to print the default it was configured with accurately (and
I would think such a patch would be acceptable for upstream).
Rich
^ permalink raw reply [flat|nested] 5+ messages in thread
[parent not found: <4372011576065223@myt5-7210d748eb79.qloud-c.yandex.net>]
* Re: RTLD_LAZY deferred symbol binding
[not found] ` <4372011576065223@myt5-7210d748eb79.qloud-c.yandex.net>
@ 2019-12-11 13:11 ` Szabolcs Nagy
0 siblings, 0 replies; 5+ messages in thread
From: Szabolcs Nagy @ 2019-12-11 13:11 UTC (permalink / raw)
To: Scherbatiy Alexander; +Cc: musl
* Scherbatiy Alexander <alexander.scherbatiy@bell-sw.com> [2019-12-11 14:53:43 +0300]:
> Thank you. It works.
>
> I looked at the ld help on linux Alpine and it shows
> > ld --help
> > -z lazy Mark object lazy runtime binding (default)
>
> Should the lazy option be used by default or the documentation needs to be updated?
that's standard binutils documentation and it's correct:
i think alpine patches gcc to pass -z now, not ld (and
i think gentoo hardened does the same).
i guess alpine could maintain a toolchain documentation
where it describes the changes compared to upstream
defaults, but you can check their patches
https://git.alpinelinux.org/aports/tree/main/gcc
>
> Thanks,
> Alexander.
>
>
> 11.12.2019, 13:35, "Szabolcs Nagy" <nsz@port70.net>:
> > * Scherbatiy Alexander <alexander.scherbatiy@bell-sw.com> [2019-12-11 13:09:36 +0300]:
> >> # build sources
> >> gcc -c -fPIC src/resolved_impl.c -Iinclude -o bin/shared/resolved_impl.o
> >> gcc -c -fPIC src/shared_lib.c -Iinclude -o bin/shared/shared_lib.o
> >> gcc -shared bin/shared/shared_lib.o bin/shared/resolved_impl.o -Iinclude -o bin/shared/libshared_lib.so
> >
> > ^^^^^^^^^^^^^^^
> >
> > you need to pass -Wl,-z,lazy (and verify it with readelf -d )
> > because alpine (and various other distros) defaults to -z now
> > (and then libc obviously cant do lazy binding no matter what
> > you specified in dlopen).
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2019-12-11 13:19 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-12-11 10:09 RTLD_LAZY deferred symbol binding Scherbatiy Alexander
2019-12-11 10:35 ` Szabolcs Nagy
2019-12-11 11:55 ` Scherbatiy Alexander
2019-12-11 13:19 ` Rich Felker
[not found] ` <4372011576065223@myt5-7210d748eb79.qloud-c.yandex.net>
2019-12-11 13:11 ` Szabolcs Nagy
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