[PATCH V3 5/9] tracing/trace: Add a generic function to read/write u64 values from tracefs

[Daniel Bristot de Oliveira <bristot@redhat.com>]

Provides a generic read and write implementation to save/read u64 values
from a file on tracefs. The trace_ull_config structure defines where to
read/write the value, the min and the max acceptable values, and a lock
to protect the write.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Clark Willaims <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
---
 kernel/trace/trace.c | 87 ++++++++++++++++++++++++++++++++++++++++++++
 kernel/trace/trace.h | 19 ++++++++++
 2 files changed, 106 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 560e4c8d3825..b4cd89010813 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7516,6 +7516,93 @@ static const struct file_operations snapshot_raw_fops = {
 
 #endif /* CONFIG_TRACER_SNAPSHOT */
 
+/*
+ * trace_ull_config_write - Generic write function to save u64 value
+ * @filp: The active open file structure
+ * @ubuf: The userspace provided buffer to read value into
+ * @cnt: The maximum number of bytes to read
+ * @ppos: The current "file" position
+ *
+ * This function provides a generic write implementation to save u64 values
+ * from a file on tracefs. The filp->private_data must point to a
+ * trace_ull_config structure that defines where to write the value, the
+ * min and the max acceptable values, and a lock to protect the write.
+ */
+static ssize_t
+trace_ull_config_write(struct file *filp, const char __user *ubuf, size_t cnt,
+		       loff_t *ppos)
+{
+	struct trace_ull_config *config = filp->private_data;
+	u64 val;
+	int err;
+
+	if (!config)
+		return -EFAULT;
+
+	err = kstrtoull_from_user(ubuf, cnt, 10, &val);
+	if (err)
+		return err;
+
+	if (config->lock)
+		mutex_lock(config->lock);
+
+	if (config->min && val < *config->min)
+		err = -EINVAL;
+
+	if (config->max && val > *config->max)
+		err = -EINVAL;
+
+	if (!err)
+		*config->val = val;
+
+	if (config->lock)
+		mutex_unlock(config->lock);
+
+	if (err)
+		return err;
+
+	return cnt;
+}
+
+/*
+ * trace_ull_config_read - Generic write function to read u64 value via tracefs
+ * @filp: The active open file structure
+ * @ubuf: The userspace provided buffer to read value into
+ * @cnt: The maximum number of bytes to read
+ * @ppos: The current "file" position
+ *
+ * This function provides a generic read implementation to read a u64 value
+ * from a file on tracefs. The filp->private_data must point to a
+ * trace_ull_config structure with valid data.
+ */
+static ssize_t
+trace_ull_config_read(struct file *filp, char __user *ubuf, size_t cnt,
+		      loff_t *ppos)
+{
+	struct trace_ull_config *config = filp->private_data;
+	char buf[ULL_STR_SIZE];
+	u64 val;
+        int len;
+
+        if (!config)
+                return -EFAULT;
+
+	val = *config->val;
+
+        if (cnt > sizeof(buf))
+                cnt = sizeof(buf);
+
+        len = snprintf(buf, sizeof(buf), "%llu\n", val);
+
+        return simple_read_from_buffer(ubuf, cnt, ppos, buf, len);
+}
+
+const struct file_operations trace_ull_config_fops = {
+	.open		= tracing_open_generic,
+	.read		= trace_ull_config_read,
+	.write		= trace_ull_config_write,
+};
+
 #define TRACING_LOG_ERRS_MAX	8
 #define TRACING_LOG_LOC_MAX	128
 
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index cd80d046c7a5..44fa25c1264a 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -1952,4 +1952,23 @@ static inline bool is_good_name(const char *name)
 	return true;
 }
 
+/*
+ * This is a generic way to read and write a u64 config value from a file
+ * in tracefs.
+ *
+ * The value is stored on the variable pointed by *val. The value needs
+ * to be at least *min and at most *max. The write is protected by an
+ * existing *lock.
+ */
+struct trace_ull_config {
+	struct mutex *lock;
+	u64 *val;
+	u64 *min;
+	u64 *max;
+};
+
+#define ULL_STR_SIZE		22	/* 20 digits max */
+
+extern const struct file_operations trace_ull_config_fops;
+
 #endif /* _LINUX_KERNEL_TRACE_H */
-- 
2.26.3