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ftrace: deprecate disabling via ftrace_enabled sysctl

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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.

Link: https://patch.msgid.link/20260806153000.4184871-2-andrey.grodzovsky@crowdstrike.com
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Andrey Grodzovsky 2026-08-06 11:29:59 -04:00 committed by Steven Rostedt
parent 075b74841b
commit b0c8570491
2 changed files with 11 additions and 37 deletions
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5
Documentation/trace/ftrace.rst
View File

@ -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

43
kernel/trace/ftrace.c
View File

@ -9370,38 +9370,10 @@ static void ftrace_startup_sysctl(void)
}
}
static void ftrace_shutdown_sysctl(void)
{
int command;
if (unlikely(ftrace_disabled))
return;
/* ftrace_start_up is true if ftrace is running */
if (ftrace_start_up) {
command = FTRACE_DISABLE_CALLS;
if (ftrace_graph_active)
command |= FTRACE_STOP_FUNC_RET;
ftrace_run_update_code(command);
}
}
#else
# define ftrace_startup_sysctl() do { } while (0)
# define ftrace_shutdown_sysctl() do { } while (0)
#endif /* CONFIG_DYNAMIC_FTRACE */
static bool is_permanent_ops_registered(void)
{
struct ftrace_ops *op;
do_for_each_ftrace_op(op, ftrace_ops_list) {
if (op->flags & FTRACE_OPS_FL_PERMANENT)
return true;
} while_for_each_ftrace_op(op);
return false;
}
static int
ftrace_enable_sysctl(const struct ctl_table *table, int write,
void *buffer, size_t *lenp, loff_t *ppos)
@ -9428,15 +9400,12 @@ 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;