Message-ID: <20250814122206.109096-1-gpaoloni@redhat.com>
Date: Thu, 14 Aug 2025 14:22:06 +0200
From: Gabriele Paoloni <gpaoloni@...hat.com>
To: rostedt@...dmis.org,
	mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org
Cc: Gabriele Paoloni <gpaoloni@...hat.com>
Subject: [PATCH v4] tracing: add testable specifications for event_enable_write/read

This patch implements the documentation of event_enable_write and
event_enable_read in the form of testable function's expectations.

Signed-off-by: Gabriele Paoloni <gpaoloni@...hat.com>
---
Changes from v1:
- Added "Context:" information and "Assumptions of use"

Changes from v2 and v3:
- Addressed comments from: https://lore.kernel.org/linux-trace-kernel/CA+wEVJbjhLmA4ZR5w7s6QDCfjET=Pf2J9PsFhC2wdO1nQ5YY+A@mail.gmail.com/T/#ma46663274411b08c14f380a69363dc6225b09a45
---
 kernel/trace/trace_events.c | 84 +++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 9f3e9537417d..af89eaed6a43 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -1772,6 +1772,49 @@ static void p_stop(struct seq_file *m, void *p)
 	mutex_unlock(&event_mutex);
 }
 
+/**
+ * event_enable_read - read from a trace event file to retrieve its status.
+ * @filp: file pointer associated with the target trace event.
+ * @ubuf: user space buffer where the event status is copied to.
+ * @cnt: number of bytes to be copied to the user space buffer.
+ * @ppos: the current position in the buffer.
+ *
+ * This is a way for user space executables to retrieve the status of a
+ * specific event
+ *
+ * Function's expectations:
+ * - The global event_mutex shall be taken before performing any operation
+ *   on the target event;
+ *
+ * - The string copied to user space shall be formatted according to the
+ *   status flags from the target event file:
+ *   - If the enable flag is set AND the soft_disable flag is not set then
+ *     the first character shall be set to "1" ELSE it shall be set to "0";
+ *
+ *   - If either the soft_disable flag or the soft_mode flag is set then the
+ *     second character shall be set to "*" ELSE it is skipped;
+ *
+ *   - The string shall be terminated by a newline ("\n") and any remaining
+ *     character shall be set to "0";
+ *
+ * - This function shall invoke simple_read_from_buffer() to perform the copy
+ *   of the kernel space string to ubuf.
+ *
+ * Assumptions of Use:
+ * - The caller shall pass cnt equal or greater than the length of the string
+ *   to be copied to user space;
+ *
+ * - Any read operation on a file descriptor, unless it is the first operation
+ *   following a trace event file open, shall be preceded by an lseek
+ *   invocation to reposition the file offset to zero.
+ *
+ * Context: process context, locks and unlocks event_mutex.
+ *
+ * Return:
+ * * the number of copied bytes on success
+ * * %-ENODEV - the event file cannot be retrieved from the input filp
+ * * any error returned by simple_read_from_buffer
+ */
 static ssize_t
 event_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
 		  loff_t *ppos)
@@ -1801,6 +1844,47 @@ event_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
 	return simple_read_from_buffer(ubuf, cnt, ppos, buf, strlen(buf));
 }
 
+/**
+ * event_enable_write - write to a trace event file to enable/disable it.
+ * @filp: file pointer associated with the target trace event.
+ * @ubuf: user space buffer where the enable/disable value is copied from.
+ * @cnt: number of bytes to be copied from the user space buffer.
+ * @ppos: the current position in the buffer.
+ *
+ * This is a way for user space executables to enable or disable event
+ * recording.
+ *
+ * Function's expectations:
+ * - This function shall copy cnt bytes from the input ubuf buffer to a kernel
+ *   space buffer and shall convert the string within the kernel space buffer
+ *   into a decimal base format number;
+ *
+ * - The global event_mutex shall be taken before performing any operation
+ *   on the target event;
+ *
+ * - This function shall check the size of the per-cpu ring-buffers used for
+ *   the event trace data and, if smaller than TRACE_BUF_SIZE_DEFAULT, expand
+ *   them to TRACE_BUF_SIZE_DEFAULT bytes (sizes larger than
+ *   TRACE_BUF_SIZE_DEFAULT are not allowed);
+ *
+ * - This function shall invoke ftrace_event_enable_disable to enable or
+ *   disable the target trace event according to the value read from user space
+ *   (0 - disable, 1 - enable);
+ *
+ * - This function shall increase the file position pointed by ppos by the
+ *   number of bytes specified by cnt.
+ *
+ * Context: process context, locks and unlocks event_mutex.
+ *
+ * Return:
+ * * the number of written bytes on success
+ * * any error returned by kstrtoul_from_user
+ * * %-ENODEV - the event file cannot be retrieved from the input filp
+ * * any error returned by tracing_update_buffers
+ * * any error returned by ftrace_event_enable_disable
+ * * %-EINVAL - the value copied from the user space ubuf is different
+ * from 0 or 1
+ */
 static ssize_t
 event_enable_write(struct file *filp, const char __user *ubuf, size_t cnt,
 		   loff_t *ppos)
-- 
2.48.1

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