tools/nolibc: add support for constructors and destructors

With the startup code moved to C, implementing support for
constructors and deconstructors is fairly easy to implement.

Examples for code size impact:

   text	   data	    bss	    dec	    hex	filename
  21837	    104	     88	  22029	   560d	nolibc-test.before
  22135	    120	     88	  22343	   5747	nolibc-test.after
  21970	    104	     88	  22162	   5692 nolibc-test.after-only-crt.h-changes

The sections are defined by [0].

[0] https://refspecs.linuxfoundation.org/elf/gabi4+/ch5.dynamic.htmlSigned-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/lkml/20231007-nolibc-constructors-v2-1-ef84693efbc1@weissschuh.net/

tools/include/nolibc/crt.h

@@ -13,12 +13,23 @@ const unsigned long *_auxv __attribute__((weak));
 static void __stack_chk_init(void);
 static void exit(int);
 
+extern void (*const __preinit_array_start[])(void) __attribute__((weak));
+extern void (*const __preinit_array_end[])(void) __attribute__((weak));
+
+extern void (*const __init_array_start[])(void) __attribute__((weak));
+extern void (*const __init_array_end[])(void) __attribute__((weak));
+
+extern void (*const __fini_array_start[])(void) __attribute__((weak));
+extern void (*const __fini_array_end[])(void) __attribute__((weak));
+
 __attribute__((weak))
 void _start_c(long *sp)
 {
 	long argc;
 	char **argv;
 	char **envp;
+	int exitcode;
+	void (* const *func)(void);
 	const unsigned long *auxv;
 	/* silence potential warning: conflicting types for 'main' */
 	int _nolibc_main(int, char **, char **) __asm__ ("main");
@@ -55,8 +66,18 @@ void _start_c(long *sp)
 		;
 	_auxv = auxv;
 
+	for (func = __preinit_array_start; func < __preinit_array_end; func++)
+		(*func)();
+	for (func = __init_array_start; func < __init_array_end; func++)
+		(*func)();
+
 	/* go to application */
-	exit(_nolibc_main(argc, argv, envp));
+	exitcode = _nolibc_main(argc, argv, envp);
+
+	for (func = __fini_array_end; func > __fini_array_start;)
+		(*--func)();
+
+	exit(exitcode);
 }
 
 #endif /* _NOLIBC_CRT_H */

tools/testing/selftests/nolibc/nolibc-test.c

@@ -57,6 +57,9 @@ static int test_argc;
 /* will be used by some test cases as readable file, please don't write it */
 static const char *argv0;
 
+/* will be used by constructor tests */
+static int constructor_test_value;
+
 /* definition of a series of tests */
 struct test {
 	const char *name;              /* test name */
@@ -594,6 +597,19 @@ int expect_strne(const char *expr, int llen, const char *cmp)
 #define CASE_TEST(name) \
 	case __LINE__: llen += printf("%d %s", test, #name);
 
+/* constructors validate that they are executed in definition order */
+__attribute__((constructor))
+static void constructor1(void)
+{
+	constructor_test_value = 1;
+}
+
+__attribute__((constructor))
+static void constructor2(void)
+{
+	constructor_test_value *= 2;
+}
+
 int run_startup(int min, int max)
 {
 	int test;
@@ -630,6 +646,7 @@ int run_startup(int min, int max)
 		CASE_TEST(environ_HOME);     EXPECT_PTRNZ(1, getenv("HOME")); break;
 		CASE_TEST(auxv_addr);        EXPECT_PTRGT(test_auxv != (void *)-1, test_auxv, brk); break;
 		CASE_TEST(auxv_AT_UID);      EXPECT_EQ(1, getauxval(AT_UID), getuid()); break;
+		CASE_TEST(constructor);      EXPECT_EQ(1, constructor_test_value, 2); break;
 		case __LINE__:
 			return ret; /* must be last */
 		/* note: do not set any defaults so as to permit holes above */