[PATCH v7 21/31] stackleack: Mark stack_erasing_bypass key as allowed in .noinstr

[Valentin Schneider]

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 at kernel.org>
Signed-off-by: Valentin Schneider <vschneid at redhat.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