init/gcov: allow CONFIG_CONSTRUCTORS on UML to fix module gcov

On ARCH=um, loading a module doesn't result in its constructors getting
called, which breaks module gcov since the debugfs files are never
registered.  On the other hand, in-kernel constructors have already been
called by the dynamic linker, so we can't call them again.

Get out of this conundrum by allowing CONFIG_CONSTRUCTORS to be
selected, but avoiding the in-kernel constructor calls.

Also remove the "if !UML" from GCOV selecting CONSTRUCTORS now, since we
really do want CONSTRUCTORS, just not kernel binary ones.

Link: https://lkml.kernel.org/r/20210120172041.c246a2cac2fb.I1358f584b76f1898373adfed77f4462c8705b736@changeid
Signed-off-by: Johannes Berg <[email protected]>
Reviewed-by: Peter Oberparleiter <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Jessica Yu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

init/Kconfig

@@ -76,7 +76,6 @@ config CC_HAS_ASM_INLINE
 
 config CONSTRUCTORS
 	bool
-	depends on !UML
 
 config IRQ_WORK
 	bool

init/main.c

@@ -1066,7 +1066,13 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
 /* Call all constructor functions linked into the kernel. */
 static void __init do_ctors(void)
 {
-#ifdef CONFIG_CONSTRUCTORS
+/*
+ * For UML, the constructors have already been called by the
+ * normal setup code as it's just a normal ELF binary, so we
+ * cannot do it again - but we do need CONFIG_CONSTRUCTORS
+ * even on UML for modules.
+ */
+#if defined(CONFIG_CONSTRUCTORS) && !defined(CONFIG_UML)
 	ctor_fn_t *fn = (ctor_fn_t *) __ctors_start;
 
 	for (; fn < (ctor_fn_t *) __ctors_end; fn++)

kernel/gcov/Kconfig

@@ -4,7 +4,7 @@ menu "GCOV-based kernel profiling"
 config GCOV_KERNEL
 	bool "Enable gcov-based kernel profiling"
 	depends on DEBUG_FS
-	select CONSTRUCTORS if !UML
+	select CONSTRUCTORS
 	default n
 	help
 	This option enables gcov-based code profiling (e.g. for code coverage