When looking at how dlopen for various libs is implemented, I found that the
macros hide too much. I find it much easier to see what is going on if 'extern'
and '= NULL' are written explicitly. After all, we don't hide those for other
definitions, e.g. our style guide says that static variables should be
initialized with '= NULL'. With that change, it's much more obvious what is
a variable declaration and what is a variable initialization.
This allows code to declare "weak" dlopen() style deps via an ELF
section following the just added specification.
The idea is that any user of dlopen() will place ELF_NOTE_DLOPEN(…)
somewhere close which will synthesize the note.
Tools such as rpm/dpkg package builders as well as initrd generators
(such as dracut) can then automatically pick up these weak deps of
suggested dependencies for their purposes.
Co-authored-by: Luca Boccassi <bluca@debian.org>
I'm going to dlopen_many_sym_or_warn() in src/basic/compress.c, this
will introduce a circular dependency because libshared already depends
from libbasic.
To avoid this, move dlfcn-util.c from libshared to libbasic.
This was added in 88d775b734,
with the apparent intent of using in shared/ and the rest of our code.
It doesn't matter much for our code, since libdl is part of glibc anyway,
but moving it removes one linkage from libsystemd. (libshared was already
linking to libdl explicitly).
If the cleanup function returns the appropriate type, use that to reset the
variable. For other functions (usually the foreign ones which return void), add
an explicit value to reset to.
This causes a bit of code churn, but I think it might be worth it. In a
following patch static destructors will be called from a fuzzer, and this
change allows them to be called multiple times. But I think such a change might
help with detecting unitialized code reuse too. We hit various bugs like this,
and things are more obvious when a pointer has been set to NULL.
I was worried whether this change increases text size, but it doesn't seem to:
-Dbuildtype=debug:
before "tree-wide: return NULL from freeing functions":
-rwxrwxr-x 1 zbyszek zbyszek 4117672 Feb 16 14:36 build/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 4494520 Feb 16 15:06 build/systemd*
after "tree-wide: return NULL from freeing functions":
-rwxrwxr-x 1 zbyszek zbyszek 4117672 Feb 16 14:36 build/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 4494576 Feb 16 15:10 build/systemd*
now:
-rwxrwxr-x 1 zbyszek zbyszek 4117672 Feb 16 14:36 build/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 4494640 Feb 16 15:15 build/systemd*
-Dbuildtype=release:
before "tree-wide: return NULL from freeing functions":
-rwxrwxr-x 1 zbyszek zbyszek 5252256 Feb 14 14:47 build-rawhide/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 1834184 Feb 16 15:09 build-rawhide/systemd*
after "tree-wide: return NULL from freeing functions":
-rwxrwxr-x 1 zbyszek zbyszek 5252256 Feb 14 14:47 build-rawhide/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 1834184 Feb 16 15:10 build-rawhide/systemd*
now:
-rwxrwxr-x 1 zbyszek zbyszek 5252256 Feb 14 14:47 build-rawhide/libsystemd.so.0.30.0*
-rwxrwxr-x 1 zbyszek zbyszek 1834184 Feb 16 15:16 build-rawhide/systemd*
I would expect that the compiler would be able to elide the setting of a
variable if the variable is never used again. And this seems to be the case:
in optimized builds there is no change in size whatsoever. And the change in
size in unoptimized build is negligible.
Something strange is happening with size of libsystemd: it's bigger in
optimized builds. Something to figure out, but unrelated to this patch.
This just adds a _cleanup_ helper call encapsulating dlclose().
This also means libsystemd-shared is linked against libdl now. I don't
think this is much of an issue, since libdl is part of glibc anyway, and
anything from exotic. It's not an optional part of the OS (think: NSS
requires dynamic linking), hence this pulls in no deps and is almost
certainly loaded into all process' memory anyway.
[zj: use DEFINE_TRIVIAL_CLEANUP_FUNC().]
