[RFC PATCH bpf-next v2 1/2] ftrace: deprecate disabling via ftrace_enabled sysctl

[Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>]

Writing 0 to kernel.ftrace_enabled has not reliably disabled ftrace
for years (FTRACE_OPS_FL_PERMANENT users already block it, and more
callers rely on ftrace always being on). Refuse the write instead of
leaving it in an inconsistent "disables some, not all" state: return
-EOPNOTSUPP and log a message. Reads and enabling (writing 1) are
unaffected.

Update the docs to note the deprecation up front.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
---
 Documentation/trace/ftrace.rst |  5 +++++
 kernel/trace/ftrace.c          | 19 ++++++++-----------
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 84f06bf0da9b..7261f25f8b4b 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -3313,6 +3313,11 @@ this special filter via::
 ftrace_enabled
 --------------
 
+.. note::
+   Disabling ftrace via this switch is deprecated. Writing 0 is refused
+   with -EOPNOTSUPP and logs a warning; writing 1 and reading the value
+   are unaffected.
+
 Note, the proc sysctl ftrace_enable is a big on/off switch for the
 function tracer. By default it is enabled (when function tracing is
 enabled in the kernel). If it is disabled, all function tracing is
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index f93e34dd2328..82bb7356ccce 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -9357,7 +9357,7 @@ static void ftrace_startup_sysctl(void)
 	}
 }
 
-static void ftrace_shutdown_sysctl(void)
+static void __maybe_unused ftrace_shutdown_sysctl(void)
 {
 	int command;
 
@@ -9377,7 +9377,7 @@ static void ftrace_shutdown_sysctl(void)
 # define ftrace_shutdown_sysctl()      do { } while (0)
 #endif /* CONFIG_DYNAMIC_FTRACE */
 
-static bool is_permanent_ops_registered(void)
+static bool __maybe_unused is_permanent_ops_registered(void)
 {
 	struct ftrace_ops *op;
 
@@ -9415,15 +9415,12 @@ ftrace_enable_sysctl(const struct ctl_table *table, int write,
 		ftrace_startup_sysctl();
 
 	} else {
-		if (is_permanent_ops_registered()) {
-			ftrace_enabled = true;
-			return -EBUSY;
-		}
-
-		/* stopping ftrace calls (just send to ftrace_stub) */
-		ftrace_trace_function = ftrace_stub;
-
-		ftrace_shutdown_sysctl();
+		/*
+		 * Disabling ftrace at runtime via this knob is deprecated.
+		 */
+		ftrace_enabled = true;
+		pr_warn_once("The ftrace_enabled file is deprecated and no longer disables ftrace\n");
+		return -EOPNOTSUPP;
 	}
 
 	last_ftrace_enabled = !!ftrace_enabled;
-- 
2.34.1