On Fri, 31 Oct 2025 12:52:56 +0100
Valentin Schneider [off-list ref] wrote:

On 30/10/25 11:25, Petr Tesarik wrote:

quoted

On Fri, 10 Oct 2025 17:38:16 +0200
Valentin Schneider [off-list ref] wrote:
 

quoted

From: Josh Poimboeuf <jpoimboe@kernel.org>

Deferring a code patching IPI is unsafe if the patched code is in a
noinstr region.  In that case the text poke code must trigger an
immediate IPI to all CPUs, which can rudely interrupt an isolated NO_HZ
CPU running in userspace.

If a noinstr static call only needs to be patched during boot, its key
can be made ro-after-init to ensure it will never be patched at runtime.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 include/linux/static_call.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index 78a77a4ae0ea8..ea6ca57e2a829 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h

@@ -192,6 +192,14 @@ extern long __static_call_return0(void);
     };								\
     ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)

+#define DEFINE_STATIC_CALL_RO(name, _func)				\
+	DECLARE_STATIC_CALL(name, _func);				\
+	struct static_call_key __ro_after_init STATIC_CALL_KEY(name) = {\
+		.func = _func,						\
+		.type = 1,						\
+	};								\
+	ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
+
 #define DEFINE_STATIC_CALL_NULL(name, _func)				\
     DECLARE_STATIC_CALL(name, _func);				\
     struct static_call_key STATIC_CALL_KEY(name) = {		\

@@ -200,6 +208,14 @@ extern long __static_call_return0(void);
     };								\
     ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)

+#define DEFINE_STATIC_CALL_NULL_RO(name, _func)				\
+	DECLARE_STATIC_CALL(name, _func);				\
+	struct static_call_key __ro_after_init STATIC_CALL_KEY(name) = {\
+		.func = NULL,						\
+		.type = 1,						\
+	};								\
+	ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
+  

I think it would be a good idea to add a comment describing when these
macros are supposed to be used, similar to the explanation you wrote for
the _NOINSTR variants. Just to provide a clue for people adding a new
static key in the future, because the commit message may become a bit
hard to find if there are a few cleanup patches on top.
 

I was about to write such a comment but I had another take; The _NOINSTR
static key helpers are special and only relevant to IPI deferral; whereas
the _RO helpers actually change the backing storage for the keys and as a
bonus are used by the IPI deferral instrumentation.

IMO it's the same here for the static calls, it makes sense to mark the
relevant ones as _RO regardless of IPI deferral.

I could however add a comment to ANNOTATE_NOINSTR_ALLOWED() itself,
something like:

/*
 * This is used to tell objtool that a given static key is safe to be used
 * within .noinstr code, and it doesn't need to generate a warning about it.
 *
 * For more information, see tools/objtool/Documentation/objtool.txt,
 * "non-RO static key usage in noinstr code"
 */
#define ANNOTATE_NOINSTR_ALLOWED(key) __ANNOTATE_NOINSTR_ALLOWED(key)

I agree, this makes more sense. Thank you!

Petr T