[PATCH] tracing: Fix trace_pipe_raw read panic

[Chunyu Hu <chuhu@redhat.com>]

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@redhat.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);
+}
+
 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);
+
 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;
+	struct ring_buffer *buffer = tb->buffer;
 	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;
 
+	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;
-- 
1.8.3.1