linux-kernel - [PATCH v7 21/31] stackleack: Mark stack_erasing

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Message-ID: <20251114151428.1064524-1-vschneid@redhat.com>
Date: Fri, 14 Nov 2025 16:14:18 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	rcu@...r.kernel.org,
	x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
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Cc: Josh Poimboeuf <jpoimboe@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
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	Andrew Morton <akpm@...ux-foundation.org>,
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	Shrikanth Hegde <sshegde@...ux.ibm.com>
Subject: [PATCH v7 21/31] stackleack: Mark stack_erasing_bypass key as allowed in .noinstr

Later commits will cause objtool to warn about static keys being used in
.noinstr sections in order to safely defer instruction patching IPIs
targeted at NOHZ_FULL CPUs.

stack_erasing_bypass is used in .noinstr code, and can be modified at runtime
(proc/sys/kernel/stack_erasing write). However it is not expected that it
will be  flipped during latency-sensitive operations, and thus shouldn't be
a source of interference wrt the text patching IPI.

Mark it to let objtool know not to warn about it.

Reported-by: Josh Poimboeuf <jpoimboe@...nel.org>
Signed-off-by: Valentin Schneider <vschneid@...hat.com>
---
 kernel/kstack_erase.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/kstack_erase.c b/kernel/kstack_erase.c
index e49bb88b4f0a3..99ba44cf939bf 100644
--- a/kernel/kstack_erase.c
+++ b/kernel/kstack_erase.c
@@ -19,7 +19,11 @@
 #include <linux/sysctl.h>
 #include <linux/init.h>
 
-static DEFINE_STATIC_KEY_FALSE(stack_erasing_bypass);
+/*
+ * NOINSTR: This static key can only be modified via its sysctl interface. It is
+ * expected it will remain stable during latency-senstive operations.
+ */
+static DEFINE_STATIC_KEY_FALSE_NOINSTR(stack_erasing_bypass);
 
 #ifdef CONFIG_SYSCTL
 static int stack_erasing_sysctl(const struct ctl_table *table, int write,
-- 
2.51.0