[PATCH AUTOSEL 4.9 4/4] init/gcov: allow CONFIG_CONSTRUCTORS on UML to fix module gcov

[Sasha Levin <sashal@kernel.org>]

From: Johannes Berg <johannes.berg@intel.com>

[ Upstream commit 55b6f763d8bcb5546997933105d66d3e6b080e6a ]

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 <johannes.berg@intel.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 init/Kconfig        | 1 -
 init/main.c         | 8 +++++++-
 kernel/gcov/Kconfig | 2 +-
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 0a615bdc203a4..34fb7b8798aa7 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -18,7 +18,6 @@ config DEFCONFIG_LIST
 
 config CONSTRUCTORS
 	bool
-	depends on !UML
 
 config IRQ_WORK
 	bool
diff --git a/init/main.c b/init/main.c
index 7ad08957dd180..649220593244e 100644
--- a/init/main.c
+++ b/init/main.c
@@ -669,7 +669,13 @@ asmlinkage __visible void __init 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++)
diff --git a/kernel/gcov/Kconfig b/kernel/gcov/Kconfig
index 1276aabaab550..2aef7d9e3e5c2 100644
--- a/kernel/gcov/Kconfig
+++ b/kernel/gcov/Kconfig
@@ -3,7 +3,7 @@ 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
-- 
2.27.0