libxt_coova: Use constructor instead of _init()

[hauke]

The _init() function collides with a _init() function in crti.o.
This results in the following error in OpenWrt:
arc-openwrt-linux-gnu/bin/ld: libxt_coova.o: in function `_init':
/lib/gcc/arc-openwrt-linux-gnu/11.3.0/crti.o:(.init+0x4): first defined here
collect2: error: ld returned 1 exit status
make[7]: *** [Makefile:22: libxt_coova.so] Error 1

Fix this by suing a constructor attribute instead. This function will
still be called after the shared library was loaded.

Fixes: #553
Signed-off-by: Hauke Mehrtens hauke@hauke-m.de

[sevan]

I was wondering if the correct fix for this is actually to change the function signature from void _init(void) to static void _init(void). The Writing Netfilter modules book has the following to say:

"Each library must register to the running ip6tables (or iptables) program by calling xtables_register_match. The _init function is called when the module is loaded by iptables. For more information about it, see dlopen(3). As a tiny implementation detail, note that _init is actually defined as a macro for iptables, and the keyword will be replaced by appropriate logic to wire it up with iptables, as we cannot strictly use _init, because the Glibc CRT (common runtime) stubs that will be linked into shared libraries, already do.
void _init(void) { xtables_register_match(&ipaddr_mt_reg); }
When iptables is built, this will expand to:
void __attribute__((constructor)) libxt_ipaddr_init(void)
so you may not use the name libxt_ipaddr_init for other functions, or you will get an unfortunate compile error.
In case you use the Xtables-addons framework, just directly write
static void _init(void)
i. e. with the static keyword and without the extra prototype above it, because modules are always compiled as shared library objects (.so) in Xtables-addons, so no symbols need to be globally visible."

Unfortunately, I don't have OpenWRT setup to test, can you try making the following change and report back?

--- a/src/linux/libxt_coova.c
+++ b/src/linux/libxt_coova.c
@@ -126,7 +126,7 @@ static struct xtables_match coova_mt6_reg = {
        .extra_opts    = coova_opts,
 };
 
-void _init(void)
+static void _init(void)
 {
        xtables_register_match(&coova_mt_reg);
        xtables_register_match(&coova_mt6_reg);

[sevan]

@hauke @teslamint @neheb ^^

[neheb]

I see

json/json_object.c:static void json_object_init(void) __attribute__ ((constructor));

No idea if both are desired to be honest.

Having it static would probably result in an unused function warning.

edit: ah disregard that.

edit2: I tried the static version. No unused function warning.

[neheb]

I was wrong

  3 libxt_coova.c:129:13: warning: '_init' defined but not used [-Wunused-function]
  2   129 | static void _init(void)
  1       |             ^~~~~

edit: xtables.h has all the logic hidden behind a define. Which is probably why it doesn't work:

#ifdef XTABLES_INTERNAL

#if defined(ALL_INCLUSIVE) || defined(NO_SHARED_LIBS)
#       ifdef _INIT
#               undef _init
#               define _init _INIT
#       endif
        extern void init_extensions(void);
        extern void init_extensions4(void);
        extern void init_extensions6(void);
        extern void init_extensionsa(void);
        extern void init_extensionsb(void);
#else
#       define _init __attribute__((constructor)) _INIT
#       define EMPTY_FUNC_DEF(x) static inline void x(void) {}
        EMPTY_FUNC_DEF(init_extensions)
        EMPTY_FUNC_DEF(init_extensions4)
        EMPTY_FUNC_DEF(init_extensions6)
        EMPTY_FUNC_DEF(init_extensionsa)
        EMPTY_FUNC_DEF(init_extensionsb)
#       undef EMPTY_FUNC_DEF
#endif

extern void _init(void);

#endif /* XTABLES_INTERNAL */

[Neustradamus]

Dear @coova team, @sevan, @jobezic,

Have you progressed on this PR?