[PATCH v2] tools/nolibc: add support for constructors and destructors

[Thomas Weißschuh]

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.html

Signed-off-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
---
Changes in v2:
- Drop RFC status.
- Add comment to constructor test that execution in definition-order is
  expected. (Willy)
- Rework destructor loop to avoid underflow if __fini_array_end is not
  provided by the linker.
- Link to v1: https://lore.kernel.org/r/20231005-nolibc-constructors-v1-1-776d56bbe917@xxxxxxxxxxxxxx
---
 tools/include/nolibc/crt.h                   | 23 ++++++++++++++++++++++-
 tools/testing/selftests/nolibc/nolibc-test.c | 17 +++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/tools/include/nolibc/crt.h b/tools/include/nolibc/crt.h
index a5f33fef1672..b84f5fbb7b58 100644
--- a/tools/include/nolibc/crt.h
+++ b/tools/include/nolibc/crt.h
@@ -13,11 +13,22 @@ 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));
+
 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");
@@ -54,8 +65,18 @@ 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 */
diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index a3ee4496bf0a..88ea8e06e596 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/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;
@@ -631,6 +647,7 @@ int run_startup(int min, int max)
 		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(auxv_AT_PAGESZ);   EXPECT_GE(1, getauxval(AT_PAGESZ), 4096); 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 */

---
base-commit: ab663cc32912914258bc8a2fbd0e753f552ee9d8
change-id: 20231005-nolibc-constructors-b2aebffe9b65

Best regards,
-- 
Thomas Weißschuh <linux@xxxxxxxxxxxxxx>