Commit 7162431d authored by Miroslav Benes's avatar Miroslav Benes Committed by Steven Rostedt
Browse files

ftrace: Introduce PERMANENT ftrace_ops flag 

Livepatch uses ftrace for redirection to new patched functions. It means
that if ftrace is disabled, all live patched functions are disabled as
well. Toggling global 'ftrace_enabled' sysctl thus affect it directly.
It is not a problem per se, because only administrator can set sysctl
values, but it still may be surprising.

Introduce PERMANENT ftrace_ops flag to amend this. If the
FTRACE_OPS_FL_PERMANENT is set on any ftrace ops, the tracing cannot be
disabled by disabling ftrace_enabled. Equally, a callback with the flag
set cannot be registered if ftrace_enabled is disabled.

Link: http://lkml.kernel.org/r/20191016113316.13415-2-mbenes@suse.cz



Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Reviewed-by: default avatarKamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: default avatarMiroslav Benes <mbenes@suse.cz>
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
parent a99d8080

Documentation/trace/ftrace-uses.rst

+8 −0
Original line number Diff line number Diff line
@@ -170,6 +170,14 @@ FTRACE_OPS_FL_RCU 
	a callback may be executed and RCU synchronization will not protect

	it.



FTRACE_OPS_FL_PERMANENT

        If this is set on any ftrace ops, then the tracing cannot disabled by

        writing 0 to the proc sysctl ftrace_enabled. Equally, a callback with

        the flag set cannot be registered if ftrace_enabled is 0.



        Livepatch uses it not to lose the function redirection, so the system

        stays protected.





Filtering which functions to trace

==================================

Documentation/trace/ftrace.rst

+3 −1
Original line number Diff line number Diff line
@@ -2976,7 +2976,9 @@ 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

disabled. This includes not only the function tracers for ftrace, but

also for any other uses (perf, kprobes, stack tracing, profiling, etc).

also for any other uses (perf, kprobes, stack tracing, profiling, etc). It

cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set

registered.



Please disable this with care.

include/linux/ftrace.h

+3 −0
Original line number Diff line number Diff line
@@ -142,6 +142,8 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops); 
 * PID     - Is affected by set_ftrace_pid (allows filtering on those pids)

 * RCU     - Set when the ops can only be called when RCU is watching.

 * TRACE_ARRAY - The ops->private points to a trace_array descriptor.

 * PERMANENT - Set when the ops is permanent and should not be affected by

 *             ftrace_enabled.

 */

enum {

	FTRACE_OPS_FL_ENABLED			= 1 << 0,
@@ -160,6 +162,7 @@ enum { 
	FTRACE_OPS_FL_PID			= 1 << 13,

	FTRACE_OPS_FL_RCU			= 1 << 14,

	FTRACE_OPS_FL_TRACE_ARRAY		= 1 << 15,

	FTRACE_OPS_FL_PERMANENT                 = 1 << 16,

};



#ifdef CONFIG_DYNAMIC_FTRACE

kernel/livepatch/patch.c

+2 −1
Original line number Diff line number Diff line
@@ -196,7 +196,8 @@ static int klp_patch_func(struct klp_func *func) 
		ops->fops.func = klp_ftrace_handler;

		ops->fops.flags = FTRACE_OPS_FL_SAVE_REGS |

				  FTRACE_OPS_FL_DYNAMIC |

				  FTRACE_OPS_FL_IPMODIFY;

				  FTRACE_OPS_FL_IPMODIFY |

				  FTRACE_OPS_FL_PERMANENT;



		list_add(&ops->node, &klp_ops);

kernel/trace/ftrace.c

+21 −2
Original line number Diff line number Diff line
@@ -326,6 +326,8 @@ int __register_ftrace_function(struct ftrace_ops *ops) 
	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED)

		ops->flags |= FTRACE_OPS_FL_SAVE_REGS;

#endif

	if (!ftrace_enabled && (ops->flags & FTRACE_OPS_FL_PERMANENT))

		return -EBUSY;



	if (!core_kernel_data((unsigned long)ops))

		ops->flags |= FTRACE_OPS_FL_DYNAMIC;
@@ -6754,6 +6756,18 @@ int unregister_ftrace_function(struct ftrace_ops *ops) 
}

EXPORT_SYMBOL_GPL(unregister_ftrace_function);



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;

}



int

ftrace_enable_sysctl(struct ctl_table *table, int write,

		     void __user *buffer, size_t *lenp,
@@ -6771,8 +6785,6 @@ ftrace_enable_sysctl(struct ctl_table *table, int write, 
	if (ret || !write || (last_ftrace_enabled == !!ftrace_enabled))

		goto out;



	last_ftrace_enabled = !!ftrace_enabled;



	if (ftrace_enabled) {



		/* we are starting ftrace again */
@@ -6783,12 +6795,19 @@ ftrace_enable_sysctl(struct ctl_table *table, int write, 
		ftrace_startup_sysctl();



	} else {

		if (is_permanent_ops_registered()) {

			ftrace_enabled = true;

			ret = -EBUSY;

			goto out;

		}



		/* stopping ftrace calls (just send to ftrace_stub) */

		ftrace_trace_function = ftrace_stub;



		ftrace_shutdown_sysctl();

	}



	last_ftrace_enabled = !!ftrace_enabled;

 out:

	mutex_unlock(&ftrace_lock);

	return ret;

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