linux-kernel - [PATCH v2] init/gcov: allow CONFIG_CONSTRUCTORS on UML to fix module gcov

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Message-Id: <20210120172041.c246a2cac2fb.I1358f584b76f1898373adfed77f4462c8705b736@changeid>
Date:   Wed, 20 Jan 2021 17:20:41 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     linux-um@...ts.infradead.org
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>, Jessica Yu <jeyu@...nel.org>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Berg <johannes.berg@...el.com>
Subject: [PATCH v2] init/gcov: allow CONFIG_CONSTRUCTORS on UML to fix module gcov

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

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.

Signed-off-by: Johannes Berg <johannes.berg@...el.com>
---
Tested with a kernel configured with CONFIG_GCOV_KERNEL, without
the patch nothing ever appears in /sys/kernel/debug/gcov/ (apart
from the reset file), and with it we get the files and they work.

v2:
 * special-case UML instead of splitting the CONSTRUCTORS config
---
 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 b77c60f8b963..29ad68325028 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -76,7 +76,6 @@ config CC_HAS_ASM_INLINE
 
 config CONSTRUCTORS
 	bool
-	depends on !UML
 
 config IRQ_WORK
 	bool
diff --git a/init/main.c b/init/main.c
index c68d784376ca..a626e78dbf06 100644
--- a/init/main.c
+++ b/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++)
diff --git a/kernel/gcov/Kconfig b/kernel/gcov/Kconfig
index 3110c77230c7..f62de2dea8a3 100644
--- a/kernel/gcov/Kconfig
+++ b/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
-- 
2.26.2