linux-kernel - Re: [PATCH] tracing: Fix trace_pipe

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Message-ID: <CABATaM63t_6V=FZV697USLjra0vOShxNf1412MQkUyw9kd-kPw@mail.gmail.com>
Date:   Wed, 2 Aug 2017 19:03:50 +0800
From:   Chunyu Hu <chuhu.ncepu@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Chunyu Hu <chuhu@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Fix trace_pipe_raw read panic

Thanks for the comments and review!

On 2 August 2017 at 00:22, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> Hmm, I applied this without taking too much of a look at it. I'm going
> to modified it a little (sorry, I'm about to take another trip and need
> to get this tested before I leave).
>
> Comments below...
>
>
> On Mon, 24 Jul 2017 15:21:06 +0800
> Chunyu Hu <chuhu@...hat.com> wrote:
>
>> per_cpu trace directories and files are created for all possible cpus,
>> but only the cpus which have ever been on-lined have their own per cpu
>> ring buffer (allocated by cpuhp threads). While trace_buffers_open, the
>> open handler for trace file 'trace_pipe_raw' is always trying to access
>> field of ring_buffer_per_cpu, and would panic with the NULL pointer.
>>
>> Align the behavior of trace_pipe_raw with trace_pipe, that returns -NODEV
>> when openning it if that cpu does not have trace ring buffer.
>>
>> Reproduce:
>> cat /sys/kernel/debug/tracing/per_cpu/cpu31/trace_pipe_raw
>> (cpu31 is never on-lined, this is a 16 cores x86_64 box)
>>
>> Tested with:
>> 1) boot with maxcpus=14, read trace_pipe_raw of cpu15.
>>    Got -NODEV.
>> 2) oneline cpu15, read trace_pipe_raw of cpu15.
>>    Get the raw trace data.
>>
>> Call trace:
>> [ 5760.950995] RIP: 0010:ring_buffer_alloc_read_page+0x32/0xe0
>> [ 5760.961678]  tracing_buffers_read+0x1f6/0x230
>> [ 5760.962695]  __vfs_read+0x37/0x160
>> [ 5760.963498]  ? __vfs_read+0x5/0x160
>> [ 5760.964339]  ? security_file_permission+0x9d/0xc0
>> [ 5760.965451]  ? __vfs_read+0x5/0x160
>> [ 5760.966280]  vfs_read+0x8c/0x130
>> [ 5760.967070]  SyS_read+0x55/0xc0
>> [ 5760.967779]  do_syscall_64+0x67/0x150
>> [ 5760.968687]  entry_SYSCALL64_slow_path+0x25/0x25
>>
>> Signed-off-by: Chunyu Hu <chuhu@...hat.com>
>> ---
>>  kernel/trace/ring_buffer.c |  5 +++++
>>  kernel/trace/trace.c       | 14 +++++++++++++-
>>  2 files changed, 18 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
>> index 4ae268e..66c109e 100644
>> --- a/kernel/trace/ring_buffer.c
>> +++ b/kernel/trace/ring_buffer.c
>> @@ -3002,6 +3002,11 @@ int ring_buffer_write(struct ring_buffer *buffer,
>>  }
>>  EXPORT_SYMBOL_GPL(ring_buffer_write);
>>
>> +bool rb_per_cpu_allocated(struct ring_buffer *buffer, int cpu)
>> +{
>> +     return !!cpumask_test_cpu(cpu, buffer->cpumask);
>
> The "!!" is not needed. The compiler will convert it for you.

OK. After researching, agreed.

>
>> +}
>> +
>>  static bool rb_per_cpu_empty(struct ring_buffer_per_cpu *cpu_buffer)
>>  {
>>       struct buffer_page *reader = cpu_buffer->reader_page;
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index 42b9355..508a1ca 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -6542,11 +6542,16 @@ static int snapshot_raw_open(struct inode *inode, struct file *filp)
>>
>>  #endif /* CONFIG_TRACER_SNAPSHOT */
>>
>> +extern bool rb_per_cpu_allocated(struct ring_buffer *buffer, int cpu);
>> +
>
> This must be in a header file.

OK. I hope that won't change symver.

>
>
>>  static int tracing_buffers_open(struct inode *inode, struct file *filp)
>>  {
>>       struct trace_array *tr = inode->i_private;
>> +     struct trace_buffer *tb = &tr->trace_buffer;
>
> Other places use the variable "trace_buf" for this. Need to stay
> consistent.

Agreed.


>
>> +     struct ring_buffer *buffer = tb->buffer;
>
> Actually, why even have that variable? It's not used anywhere else.

Agreed.

>
>>       struct ftrace_buffer_info *info;
>>       int ret;
>> +     int cpu_file;
>>
>>       if (tracing_disabled)
>>               return -ENODEV;
>> @@ -6554,6 +6559,13 @@ static int tracing_buffers_open(struct inode *inode, struct file *filp)
>>       if (trace_array_get(tr) < 0)
>>               return -ENODEV;
>
> Not to mention, the tr can not be dereferenced until after the above
> get.

Ah, my bad. I missed this.

>
> I'll make the changes, and I'll keep you as author.

Thanks.

>
> Thanks!
>
> -- Steve
>
>>
>> +     cpu_file = tracing_get_cpu(inode);
>> +
>> +     /* Make sure, ring buffer for this cpu is allocated. */
>> +     if (cpu_file != RING_BUFFER_ALL_CPUS &&
>> +                     !rb_per_cpu_allocated(buffer, cpu_file))
>> +             return -ENODEV;
>> +
>>       info = kzalloc(sizeof(*info), GFP_KERNEL);
>>       if (!info) {
>>               trace_array_put(tr);
>> @@ -6563,7 +6575,7 @@ static int tracing_buffers_open(struct inode *inode, struct file *filp)
>>       mutex_lock(&trace_types_lock);
>>
>>       info->iter.tr           = tr;
>> -     info->iter.cpu_file     = tracing_get_cpu(inode);
>> +     info->iter.cpu_file     = cpu_file;
>>       info->iter.trace        = tr->current_trace;
>>       info->iter.trace_buffer = &tr->trace_buffer;
>>       info->spare             = NULL;
>