remove undef weak refs to init/fini array symbols in libc.so

commit ad1cd43a86645ba2d4f7c8747240452a349d6bc1 eliminated
preprocessor-level omission of references to the init/fini array
symbols from object files going into libc.so. the references are weak,
and the intent was that the linker would resolve them to zero in
libc.so, but instead it leaves undefined references that could be
satisfied at runtime. normally these references would be harmless,
since the code using them does not even get executed, but some older
binutils versions produce a linking error: when linking a program
against libc.so, ld first tries to use the hidden init/fini array
symbols produced by the linker script to satisfy the references in
libc.so, then produces an error because the definitions are hidden.

ideally ld would have already provided definitions of these symbols
when linking libc.so, but the linker script for -shared omits them.

to avoid this situation, the dynamic linker now provides its own dummy
definitions of the init/fini array symbols for libc.so. since they are
hidden, everything binds at ld time and no references remain in the
dynamic symbol table. with modern binutils and --gc-sections, both
the dummy empty array objects and the code referencing them get
dropped at link time, anyway.

the _init and _fini symbols are also switched back to using weak
definitions rather than weak references since the latter behave
somewhat problematically in general, and the weak definition approach
was known to work well.

src/env/__libc_start_main.c

@@ -8,9 +8,11 @@
 
 void __init_tls(size_t *);
 
-extern void _init() __attribute__((weak));
-extern void (*const __init_array_start)() __attribute__((weak));
-extern void (*const __init_array_end)() __attribute__((weak));
+static void dummy(void) {}
+weak_alias(dummy, _init);
+
+__attribute__((__weak__, __visibility__("hidden")))
+extern void (*const __init_array_start)(void), (*const __init_array_end)(void);
 
 static void dummy1(void *p) {}
 weak_alias(dummy1, __init_ssp);
@@ -53,7 +55,7 @@ void __init_libc(char **envp, char *pn)
 
 static void libc_start_init(void)
 {
-	if (_init) _init();
+	_init();
 	uintptr_t a = (uintptr_t)&__init_array_start;
 	for (; a<(uintptr_t)&__init_array_end; a+=sizeof(void(*)()))
 		(*(void (**)())a)();

src/exit/exit.c

@@ -10,17 +10,17 @@ static void dummy()
  * as a consequence of linking either __toread.c or __towrite.c. */
 weak_alias(dummy, __funcs_on_exit);
 weak_alias(dummy, __stdio_exit);
+weak_alias(dummy, _fini);
 
-extern void _fini() __attribute__((weak));
-extern void (*const __fini_array_start)() __attribute__((weak));
-extern void (*const __fini_array_end)() __attribute__((weak));
+__attribute__((__weak__, __visibility__("hidden")))
+extern void (*const __fini_array_start)(void), (*const __fini_array_end)(void);
 
 static void libc_exit_fini(void)
 {
 	uintptr_t a = (uintptr_t)&__fini_array_end;
 	for (; a>(uintptr_t)&__fini_array_start; a-=sizeof(void(*)()))
 		(*(void (**)())(a-sizeof(void(*)())))();
-	if (_fini) _fini();
+	_fini();
 }
 
 weak_alias(libc_exit_fini, __libc_exit_fini);

src/internal/vis.h

@@ -8,8 +8,7 @@
 
 /* Conceptually, all symbols should be protected, but some toolchains
  * fail to support copy relocations for protected data, so exclude all
- * exported data symbols. Also omit visibility for possibly-undefined
- * weak references. */
+ * exported data symbols. */
 
 __attribute__((__visibility__("default")))
 extern struct _IO_FILE *const stdin, *const stdout, *const stderr;
@@ -23,10 +22,6 @@ extern long timezone, __timezone;
 __attribute__((__visibility__("default")))
 extern char *optarg, **environ, **__environ, *tzname[2], *__tzname[2], *__progname, *__progname_full;
 
-__attribute__((__visibility__("default")))
-extern void (*const __init_array_start)(), (*const __init_array_end)(),
-	(*const __fini_array_start)(), (*const __fini_array_end)();
-
 #pragma GCC visibility push(protected)
 
 #endif

src/ldso/dynlink.c

@@ -134,6 +134,15 @@ static struct fdpic_dummy_loadmap app_dummy_loadmap;
 
 struct debug *_dl_debug_addr = &debug;
 
+__attribute__((__visibility__("hidden")))
+void (*const __init_array_start)(void)=0, (*const __fini_array_start)(void)=0;
+
+__attribute__((__visibility__("hidden")))
+extern void (*const __init_array_end)(void), (*const __fini_array_end)(void);
+
+weak_alias(__init_array_start, __init_array_end);
+weak_alias(__fini_array_start, __fini_array_end);
+
 static int dl_strcmp(const char *l, const char *r)
 {
 	for (; *l==*r && *l; l++, r++);