tracing: Introduce pipe_cpumask to avoid race on trace_pipes

There is race issue when concurrently splice_read main trace_pipe and
per_cpu trace_pipes which will result in data read out being different
from what actually writen.

As suggested by Steven:
  > I believe we should add a ref count to trace_pipe and the per_cpu
  > trace_pipes, where if they are opened, nothing else can read it.
  >
  > Opening trace_pipe locks all per_cpu ref counts, if any of them are
  > open, then the trace_pipe open will fail (and releases any ref counts
  > it had taken).
  >
  > Opening a per_cpu trace_pipe will up the ref count for just that
  > CPU buffer. This will allow multiple tasks to read different per_cpu
  > trace_pipe files, but will prevent the main trace_pipe file from
  > being opened.

But because we only need to know whether per_cpu trace_pipe is open or
not, using a cpumask instead of using ref count may be easier.

After this patch, users will find that:
 - Main trace_pipe can be opened by only one user, and if it is
   opened, all per_cpu trace_pipes cannot be opened;
 - Per_cpu trace_pipes can be opened by multiple users, but each per_cpu
   trace_pipe can only be opened by one user. And if one of them is
   opened, main trace_pipe cannot be opened.

Link: https://lore.kernel.org/linux-trace-kernel/20230818022645.1948314-1-zhengyejian1@huawei.com

Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -6718,10 +6718,36 @@ tracing_max_lat_write(struct file *filp, const char __user *ubuf,
 
 #endif
 
+static int open_pipe_on_cpu(struct trace_array *tr, int cpu)
+{
+	if (cpu == RING_BUFFER_ALL_CPUS) {
+		if (cpumask_empty(tr->pipe_cpumask)) {
+			cpumask_setall(tr->pipe_cpumask);
+			return 0;
+		}
+	} else if (!cpumask_test_cpu(cpu, tr->pipe_cpumask)) {
+		cpumask_set_cpu(cpu, tr->pipe_cpumask);
+		return 0;
+	}
+	return -EBUSY;
+}
+
+static void close_pipe_on_cpu(struct trace_array *tr, int cpu)
+{
+	if (cpu == RING_BUFFER_ALL_CPUS) {
+		WARN_ON(!cpumask_full(tr->pipe_cpumask));
+		cpumask_clear(tr->pipe_cpumask);
+	} else {
+		WARN_ON(!cpumask_test_cpu(cpu, tr->pipe_cpumask));
+		cpumask_clear_cpu(cpu, tr->pipe_cpumask);
+	}
+}
+
 static int tracing_open_pipe(struct inode *inode, struct file *filp)
 {
 	struct trace_array *tr = inode->i_private;
 	struct trace_iterator *iter;
+	int cpu;
 	int ret;
 
 	ret = tracing_check_open_get_tr(tr);
@@ -6729,13 +6755,16 @@ static int tracing_open_pipe(struct inode *inode, struct file *filp)
 		return ret;
 
 	mutex_lock(&trace_types_lock);
+	cpu = tracing_get_cpu(inode);
+	ret = open_pipe_on_cpu(tr, cpu);
+	if (ret)
+		goto fail_pipe_on_cpu;
 
 	/* create a buffer to store the information to pass to userspace */
 	iter = kzalloc(sizeof(*iter), GFP_KERNEL);
 	if (!iter) {
 		ret = -ENOMEM;
-		__trace_array_put(tr);
-		goto out;
+		goto fail_alloc_iter;
 	}
 
 	trace_seq_init(&iter->seq);
@@ -6758,7 +6787,7 @@ static int tracing_open_pipe(struct inode *inode, struct file *filp)
 
 	iter->tr = tr;
 	iter->array_buffer = &tr->array_buffer;
-	iter->cpu_file = tracing_get_cpu(inode);
+	iter->cpu_file = cpu;
 	mutex_init(&iter->mutex);
 	filp->private_data = iter;
 
@@ -6768,12 +6797,15 @@ static int tracing_open_pipe(struct inode *inode, struct file *filp)
 	nonseekable_open(inode, filp);
 
 	tr->trace_ref++;
-out:
+
 	mutex_unlock(&trace_types_lock);
 	return ret;
 
 fail:
 	kfree(iter);
+fail_alloc_iter:
+	close_pipe_on_cpu(tr, cpu);
+fail_pipe_on_cpu:
 	__trace_array_put(tr);
 	mutex_unlock(&trace_types_lock);
 	return ret;
@@ -6790,7 +6822,7 @@ static int tracing_release_pipe(struct inode *inode, struct file *file)
 
 	if (iter->trace->pipe_close)
 		iter->trace->pipe_close(iter);
-
+	close_pipe_on_cpu(tr, iter->cpu_file);
 	mutex_unlock(&trace_types_lock);
 
 	free_cpumask_var(iter->started);
@@ -9454,6 +9486,9 @@ static struct trace_array *trace_array_create(const char *name)
 	if (!alloc_cpumask_var(&tr->tracing_cpumask, GFP_KERNEL))
 		goto out_free_tr;
 
+	if (!alloc_cpumask_var(&tr->pipe_cpumask, GFP_KERNEL))
+		goto out_free_tr;
+
 	tr->trace_flags = global_trace.trace_flags & ~ZEROED_TRACE_FLAGS;
 
 	cpumask_copy(tr->tracing_cpumask, cpu_all_mask);
@@ -9495,6 +9530,7 @@ static struct trace_array *trace_array_create(const char *name)
  out_free_tr:
 	ftrace_free_ftrace_ops(tr);
 	free_trace_buffers(tr);
+	free_cpumask_var(tr->pipe_cpumask);
 	free_cpumask_var(tr->tracing_cpumask);
 	kfree(tr->name);
 	kfree(tr);
@@ -9597,6 +9633,7 @@ static int __remove_instance(struct trace_array *tr)
 	}
 	kfree(tr->topts);
 
+	free_cpumask_var(tr->pipe_cpumask);
 	free_cpumask_var(tr->tracing_cpumask);
 	kfree(tr->name);
 	kfree(tr);
@@ -10394,12 +10431,14 @@ __init static int tracer_alloc_buffers(void)
 	if (trace_create_savedcmd() < 0)
 		goto out_free_temp_buffer;
 
+	if (!alloc_cpumask_var(&global_trace.pipe_cpumask, GFP_KERNEL))
+		goto out_free_savedcmd;
+
 	/* TODO: make the number of buffers hot pluggable with CPUS */
 	if (allocate_trace_buffers(&global_trace, ring_buf_size) < 0) {
 		MEM_FAIL(1, "tracer: failed to allocate ring buffer!\n");
-		goto out_free_savedcmd;
+		goto out_free_pipe_cpumask;
 	}
-
 	if (global_trace.buffer_disabled)
 		tracing_off();
 
@@ -10452,6 +10491,8 @@ __init static int tracer_alloc_buffers(void)
 
 	return 0;
 
+out_free_pipe_cpumask:
+	free_cpumask_var(global_trace.pipe_cpumask);
 out_free_savedcmd:
 	free_saved_cmdlines_buffer(savedcmd);
 out_free_temp_buffer:

kernel/trace/trace.h

@@ -377,6 +377,8 @@ struct trace_array {
 	struct list_head	events;
 	struct trace_event_file *trace_marker_file;
 	cpumask_var_t		tracing_cpumask; /* only trace on set CPUs */
+	/* one per_cpu trace_pipe can be opened by only one user */
+	cpumask_var_t		pipe_cpumask;
 	int			ref;
 	int			trace_ref;
 #ifdef CONFIG_FUNCTION_TRACER