[RFC PATCH bpf-next v2 1/2] ftrace: deprecate disabling via ftrace_enabled sysctl
From: Andrey Grodzovsky <hidden>
Date: 2026-07-31 17:54:28
Also in:
bpf, linux-doc, linux-kselftest, live-patching, stable
Subsystem:
documentation, function hooks (ftrace), the rest, tracing · Maintainers:
Jonathan Corbet, Steven Rostedt, Masami Hiramatsu, Linus Torvalds
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 <redacted> --- 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