lkml.org 
[lkml]   [2022]   [Nov]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v3] tracing: Fix infinite loop in tracing_read_pipe on overflowed print_trace_line
From
Date


On 2022/11/29 0:46, Steven Rostedt wrote:
> On Thu, 24 Nov 2022 20:58:50 +0800
> Yang Jihong <yangjihong1@huawei.com> wrote:
>
>> print_trace_line may overflow seq_file buffer. If the event is not
>> consumed, the while loop keeps peeking this event, causing a infinite loop.
>>
>> Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
>> ---
>> kernel/trace/trace.c | 22 +++++++++++++++++++++-
>> 1 file changed, 21 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index a7fe0e115272..55733224fa88 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -6787,7 +6787,27 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>>
>> ret = print_trace_line(iter);
>> if (ret == TRACE_TYPE_PARTIAL_LINE) {
>> - /* don't print partial lines */
>> + /*
>> + * If one trace_line of the tracer overflows seq_file
>> + * buffer, trace_seq_to_user returns -EBUSY.
>> + * In this case, we need to consume it, otherwise,
>> + * while loop will peek this event next time,
>> + * resulting in an infinite loop.
>> + */
>> + if (trace_seq_has_overflowed(&iter->seq)) {
>
> The only way to get here is if the above is true, and that is not going to
> cause the infinite loop. What does is if save_len == 0. In fact, that's
> all you need to check for:
>
> if (save_len == 0) {
>
> Should do the trick.
Yes, Has been tested to be feasible, will change in next version.

By the way, the problem mentioned in the v1 patch:
"Anyway, what is triggering this?"

The problem is caused by the print_line callback function
(blk_tracer_print_line) of blktrace.
The blktrace does not filter out the events whose type is !=TRACK_BLK.
It parses and prints all trace events in the ring buffer as blktrace
events. As a result, the seq_file buffer may overflow in the
blk_log_dump_pdu function:

static void blk_log_dump_pdu()
{
...
for (i = 0; i < pdu_len; i++) { // pdu_len may exceed the
seq_file buffer size.

trace_seq_printf(s, "%s%02x",
i == 0 ? "" : " ", pdu_buf[i]);

/*
* stop when the rest is just zeros and indicate so
* with a ".." appended
*/
if (i == end && end != pdu_len - 1) {
trace_seq_puts(s, " ..) ");
return;
}
}
...
}

The fixed patch has been sent:
https://lore.kernel.org/lkml/20221122040410.85113-1-yangjihong1@huawei.com/T/


Thanks,
Yang

\
 
 \ /
  Last update: 2022-11-29 12:33    [W:0.074 / U:0.220 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site