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SubjectRe: [PATCH 4/5] tracing: Adding new functions for kernel access to Ftrace instances
On Wed, 13 Nov 2019 13:16:01 -0800
Divya Indi <divya.indi@oracle.com> wrote:

> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -297,12 +297,24 @@ static void __trace_array_put(struct trace_array *this_tr)
> this_tr->ref--;
> }
>
> +/**
> + * trace_array_put - Decrement the reference counter for this trace array.
> + *
> + * NOTE: Use this when we no longer need the trace array returned by
> + * trace_array_get_by_name(). This ensures the trace array can be later
> + * destroyed.
> + *
> + */
> void trace_array_put(struct trace_array *this_tr)
> {
> + if (!this_tr)
> + return;
> +
> mutex_lock(&trace_types_lock);
> __trace_array_put(this_tr);
> mutex_unlock(&trace_types_lock);
> }
> +EXPORT_SYMBOL_GPL(trace_array_put);
>
> int call_filter_check_discard(struct trace_event_call *call, void *rec,
> struct ring_buffer *buffer,
> @@ -8302,24 +8314,17 @@ static void update_tracer_options(struct trace_array *tr)
> mutex_unlock(&trace_types_lock);
> }
>
> -struct trace_array *trace_array_create(const char *name)
> +static struct trace_array *trace_array_create(const char *name)
> {
> struct trace_array *tr;
> int ret;
>
> - mutex_lock(&event_mutex);
> - mutex_lock(&trace_types_lock);
> -
> - ret = -EEXIST;
> - list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> - if (tr->name && strcmp(tr->name, name) == 0)
> - goto out_unlock;
> - }
> -
> ret = -ENOMEM;
> tr = kzalloc(sizeof(*tr), GFP_KERNEL);
> if (!tr)
> - goto out_unlock;
> + return ERR_PTR(ret);
> +
> + mutex_lock(&event_mutex);
>
> tr->name = kstrdup(name, GFP_KERNEL);
> if (!tr->name)
> @@ -8364,7 +8369,8 @@ struct trace_array *trace_array_create(const char *name)
>
> list_add(&tr->list, &ftrace_trace_arrays);
>
> - mutex_unlock(&trace_types_lock);
> + tr->ref++;
> +
> mutex_unlock(&event_mutex);
>
> return tr;
> @@ -8375,24 +8381,74 @@ struct trace_array *trace_array_create(const char *name)
> kfree(tr->name);
> kfree(tr);
>
> - out_unlock:
> - mutex_unlock(&trace_types_lock);
> mutex_unlock(&event_mutex);
>
> return ERR_PTR(ret);
> }
> -EXPORT_SYMBOL_GPL(trace_array_create);
>
> static int instance_mkdir(const char *name)
> {
> - return PTR_ERR_OR_ZERO(trace_array_create(name));
> + struct trace_array *tr;
> + int ret;
> +
> + mutex_lock(&trace_types_lock);
> +
> + ret = -EEXIST;
> + list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> + if (tr->name && strcmp(tr->name, name) == 0)
> + goto out_unlock;
> + }
> +
> + tr = trace_array_create(name);


You just changed the locking order here, which can cause a deadlock.
You can't take event_mutex after taking trace_types_lock. I applied
this, booted with lockdep enabled, loaded your sample module and
triggered this:


e1000e: em1 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: Rx/Tx
IPv6: ADDRCONF(NETDEV_CHANGE): em1: link becomes ready
L1TF CPU bug present and SMT on, data leak possible. See CVE-2018-3646 and https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/l1tf.html for details.

======================================================
WARNING: possible circular locking dependency detected
5.4.0-rc6-test+ #23 Not tainted
------------------------------------------------------
modprobe/1569 is trying to acquire lock:
ffffffff90677a80 (event_mutex){+.+.}, at: trace_array_create+0x47/0x230

but task is already holding lock:
ffffffff90674660 (trace_types_lock){+.+.}, at: trace_array_get_by_name+0x13/0x80

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (trace_types_lock){+.+.}:
__mutex_lock+0x95/0x920
trace_add_event_call+0x23/0xd0
trace_probe_register_event_call+0x22/0x50
trace_kprobe_create+0x681/0xa90
create_or_delete_trace_kprobe+0xd/0x30
trace_run_command+0x72/0x90
kprobe_trace_self_tests_init+0x53/0x41c
do_one_initcall+0x5d/0x314
kernel_init_freeable+0x218/0x2dd
kernel_init+0xa/0x100
ret_from_fork+0x3a/0x50

-> #0 (event_mutex){+.+.}:
__lock_acquire+0xd17/0x14c0
lock_acquire+0x9e/0x190
__mutex_lock+0x95/0x920
trace_array_create+0x47/0x230
trace_array_get_by_name+0x4c/0x80
sample_trace_array_init+0x12/0xfa8 [sample_trace_array]
do_one_initcall+0x5d/0x314
do_init_module+0x5a/0x220
load_module+0x2172/0x2480
__do_sys_finit_module+0xa8/0x110
do_syscall_64+0x60/0x210
entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(trace_types_lock);
lock(event_mutex);
lock(trace_types_lock);
lock(event_mutex);

*** DEADLOCK ***

1 lock held by modprobe/1569:
#0: ffffffff90674660 (trace_types_lock){+.+.}, at: trace_array_get_by_name+0x13/0x80

stack backtrace:
CPU: 2 PID: 1569 Comm: modprobe Not tainted 5.4.0-rc6-test+ #23
Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
Call Trace:
dump_stack+0x85/0xc0
check_noncircular+0x172/0x190
? find_held_lock+0x2d/0x90
__lock_acquire+0xd17/0x14c0
lock_acquire+0x9e/0x190
? trace_array_create+0x47/0x230
__mutex_lock+0x95/0x920
? trace_array_create+0x47/0x230
? fs_reclaim_release.part.99+0x5/0x20
? trace_array_create+0x47/0x230
? trace_array_create+0x2d/0x230
? rcu_read_lock_sched_held+0x52/0x80
? trace_array_create+0x47/0x230
trace_array_create+0x47/0x230
trace_array_get_by_name+0x4c/0x80
? trace_event_define_fields_sample_event+0x58/0x58 [sample_trace_array]
sample_trace_array_init+0x12/0xfa8 [sample_trace_array]
do_one_initcall+0x5d/0x314
? rcu_read_lock_sched_held+0x52/0x80
? kmem_cache_alloc_trace+0x278/0x2b0
do_init_module+0x5a/0x220
load_module+0x2172/0x2480
? vfs_read+0x11d/0x140
? __do_sys_finit_module+0xa8/0x110
__do_sys_finit_module+0xa8/0x110
do_syscall_64+0x60/0x210
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x7f6ba89b0efd
Code: 00 c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 5b 7f 0c 00 f7 d8 64 89 01 48

-- Steve

> +
> + ret = PTR_ERR_OR_ZERO(tr);
> +
> +out_unlock:
> + mutex_unlock(&trace_types_lock);
> + return ret;
> +}
> +
> +/**
> + * trace_array_get_by_name - Create/Lookup a trace array, given its name.
> + * @name: The name of the trace array to be looked up/created.
> + *
> + * Returns pointer to trace array with given name.
> + * NULL, if it cannot be created.
> + *
> + * NOTE: This function increments the reference counter associated with the
> + * trace array returned. This makes sure it cannot be freed while in use.
> + * Use trace_array_put() once the trace array is no longer needed.
> + *
> + */
> +struct trace_array *trace_array_get_by_name(const char *name)
> +{
> + struct trace_array *tr;
> +
> + mutex_lock(&trace_types_lock);
> +
> + list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> + if (tr->name && strcmp(tr->name, name) == 0)
> + goto out_unlock;
> + }
> +
> + tr = trace_array_create(name);
> +
> + if (IS_ERR(tr))
> + tr = NULL;
> +out_unlock:
> + if (tr)
> + tr->ref++;
> + mutex_unlock(&trace_types_lock);
> + return tr;
> }
> +EXPORT_SYMBOL_GPL(trace_array_get_by_name);
>
> static int __remove_instance(struct trace_array *tr)
> {
> int i;
>
> - if (tr->ref || (tr->current_trace && tr->current_trace->ref))
> + /* Reference counter for a newly created trace array = 1. */
> + if (tr->ref > 1 || (tr->current_trace && tr->current_trace->ref))
> return -EBUSY;
>
> list_del(&tr->list);
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 66ff63e..643faaa 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -338,7 +338,6 @@ enum {
> extern struct mutex trace_types_lock;
>
> extern int trace_array_get(struct trace_array *tr);
> -extern void trace_array_put(struct trace_array *tr);
>
> extern int tracing_set_time_stamp_abs(struct trace_array *tr, bool abs);
> extern int tracing_set_clock(struct trace_array *tr, const char *clockstr);
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 2621995..c58ef22 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -834,7 +834,6 @@ static int ftrace_set_clr_event(struct trace_array *tr, char *buf, int set)
>
> return ret;
> }
> -EXPORT_SYMBOL_GPL(ftrace_set_clr_event);
>
> /**
> * trace_set_clr_event - enable or disable an event
> @@ -859,6 +858,32 @@ int trace_set_clr_event(const char *system, const char *event, int set)
> }
> EXPORT_SYMBOL_GPL(trace_set_clr_event);
>
> +/**
> + * trace_array_set_clr_event - enable or disable an event for a trace array.
> + * @tr: concerned trace array.
> + * @system: system name to match (NULL for any system)
> + * @event: event name to match (NULL for all events, within system)
> + * @enable: true to enable, false to disable
> + *
> + * This is a way for other parts of the kernel to enable or disable
> + * event recording.
> + *
> + * Returns 0 on success, -EINVAL if the parameters do not match any
> + * registered events.
> + */
> +int trace_array_set_clr_event(struct trace_array *tr, const char *system,
> + const char *event, bool enable)
> +{
> + int set;
> +
> + if (!tr)
> + return -ENOENT;
> +
> + set = (enable == true) ? 1 : 0;
> + return __ftrace_set_clr_event(tr, NULL, system, event, set);
> +}
> +EXPORT_SYMBOL_GPL(trace_array_set_clr_event);
> +
> /* 128 should be much more than enough */
> #define EVENT_BUF_SIZE 127
>

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