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>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 29a2e4d..8e64aaa 100644 (file)
--- a/kernel/trace/trace.c
+++ b/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:

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 95956f7..73eaec1 100644 (file)
--- a/kernel/trace/trace.h
+++ b/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