# \[PATCH RFC 21/27\] dyndbg: let procfs copy in writes to /proc/dynamic\_debug/control

- From: Christian Brauner \<brauner@kernel.org\>
- Date: 2026-11-02 15:27:15 
- List: linux-modules
- Page: https://ratatoskr.run/linux-modules/2026/11/17845569
- Thread: https://ratatoskr.run/linux-fsdevel/2026/11/17844895/t/2.md

```text
Move the parsing into ddebug_proc_write_buf() and implement
->proc_write_buf() with it so procfs copies in what is written before
it pins the entry. The debugfs file keeps copying in itself. Writes
larger than USER_BUF_PAGE - 1 bytes to the procfs file now fail with
-EINVAL from procfs instead of -E2BIG.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 lib/dynamic_debug.c | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 64a211290872..1d6012c9eb41 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -1002,11 +1002,28 @@ __setup("dyndbg=", dyndbg_setup);
  * command text from userspace, parses and executes it.
  */
 #define USER_BUF_PAGE 4096
+
+static ssize_t ddebug_proc_write_buf(struct file *file, char *buf, size_t len,
+				     loff_t *offp)
+{
+	int ret;
+
+	if (len == 0)
+		return 0;
+	v2pr_info("read %zu bytes from userspace\n", len);
+
+	ret = ddebug_exec_queries(buf, NULL);
+	if (ret < 0)
+		return ret;
+
+	*offp += len;
+	return len;
+}
+
 static ssize_t ddebug_proc_write(struct file *file, const char __user *ubuf,
 				  size_t len, loff_t *offp)
 {
-	char *tmpbuf;
-	int ret;
+	char *tmpbuf __free(kfree) = NULL;
 
 	if (len == 0)
 		return 0;
@@ -1017,15 +1034,7 @@ static ssize_t ddebug_proc_write(struct file *file, const char __user *ubuf,
 	tmpbuf = memdup_user_nul(ubuf, len);
 	if (IS_ERR(tmpbuf))
 		return PTR_ERR(tmpbuf);
-	v2pr_info("read %zu bytes from userspace\n", len);
-
-	ret = ddebug_exec_queries(tmpbuf, NULL);
-	kfree(tmpbuf);
-	if (ret < 0)
-		return ret;
-
-	*offp += len;
-	return len;
+	return ddebug_proc_write_buf(file, tmpbuf, len, offp);
 }
 
 /*
@@ -1196,7 +1205,8 @@ static const struct file_operations ddebug_proc_fops = {
 static const struct proc_ops proc_fops = {
 	.proc_seq_ops = &ddebug_proc_seqops,
 	.proc_state_size = sizeof(struct ddebug_iter),
-	.proc_write = ddebug_proc_write
+	.proc_write_buf = ddebug_proc_write_buf,
+	.proc_write_size = USER_BUF_PAGE - 1,
 };
 
 static void ddebug_attach_module_classes(struct ddebug_table *dt,

-- 
2.53.0

```