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ftrace: Introduce PERMANENT ftrace_ops flag

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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: Petr Mladek <pmladek@suse.com>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Miroslav Benes 2019-10-16 13:33:13 +02:00 committed by Steven Rostedt
parent a99d8080aa
commit 7162431dcf
5 changed files with 37 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
Documentation/trace/ftrace-uses.rst
View File

@ -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
==================================

4
Documentation/trace/ftrace.rst
View File

@ -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.

3
include/linux/ftrace.h
View File

@ -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

3
kernel/livepatch/patch.c
View File

@ -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);

23
kernel/trace/ftrace.c
View File

@ -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;