[PATCH RFC 11/11] samples/kfifo: let procfs copy out reads

From: Christian Brauner <brauner@kernel.org>
Date: 2026-11-02 15:34:58
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Reply to: Christian Brauner, 2026-11-02

Implement ->proc_read_buf() so procfs copies what is read to userspace
after it unpinned the entry, and take the data out of the fifo with
kfifo_out() instead of kfifo_to_user(). A reader that passes a bad
buffer loses what was taken out for it now, kfifo_to_user() left that in
the fifo. The samples can be spliced now too, and a sendfile() from them
into a target that fails or takes less than was read drops what was
taken out in between, like for the rtas error log.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 samples/kfifo/bytestream-example.c |  9 +++------
 samples/kfifo/inttype-example.c    | 10 ++++------
 samples/kfifo/record-example.c     |  9 +++------
 3 files changed, 10 insertions(+), 18 deletions(-)

diff --git a/samples/kfifo/bytestream-example.c b/samples/kfifo/bytestream-example.c
index 8c58fb46a2bf..214e46a64cc1 100644
--- a/samples/kfifo/bytestream-example.c
+++ b/samples/kfifo/bytestream-example.c

@@ -125,26 +125,23 @@ static ssize_t fifo_write(struct file *file, char *buf,
 	return copied;
 }
 
-static ssize_t fifo_read(struct file *file, char __user *buf,
+static ssize_t fifo_read(struct file *file, char *buf,
 						size_t count, loff_t *ppos)
 {
-	int ret;
 	unsigned int copied;
 
 	if (mutex_lock_interruptible(&read_access))
 		return -ERESTARTSYS;
 
-	ret = kfifo_to_user(&test, buf, count, &copied);
+	copied = kfifo_out(&test, buf, count);
 
 	mutex_unlock(&read_access);
-	if (ret)
-		return ret;
 
 	return copied;
 }
 
 static const struct proc_ops fifo_proc_ops = {
-	.proc_read	= fifo_read,
+	.proc_read_buf	= fifo_read,
 	.proc_write_buf	= fifo_write,
 	.proc_lseek	= noop_llseek,
 };

diff --git a/samples/kfifo/inttype-example.c b/samples/kfifo/inttype-example.c
index c5d16b815e73..8384d7f8de6c 100644
--- a/samples/kfifo/inttype-example.c
+++ b/samples/kfifo/inttype-example.c

@@ -119,26 +119,24 @@ static ssize_t fifo_write(struct file *file, char *buf,
 	return copied;
 }
 
-static ssize_t fifo_read(struct file *file, char __user *buf,
+static ssize_t fifo_read(struct file *file, char *buf,
 						size_t count, loff_t *ppos)
 {
-	int ret;
 	unsigned int copied;
 
 	if (mutex_lock_interruptible(&read_access))
 		return -ERESTARTSYS;
 
-	ret = kfifo_to_user(&test, buf, count, &copied);
+	copied = kfifo_out(&test, (int *)buf, count / sizeof(int)) *
+		 sizeof(int);
 
 	mutex_unlock(&read_access);
-	if (ret)
-		return ret;
 
 	return copied;
 }
 
 static const struct proc_ops fifo_proc_ops = {
-	.proc_read	= fifo_read,
+	.proc_read_buf	= fifo_read,
 	.proc_write_buf	= fifo_write,
 	.proc_lseek	= noop_llseek,
 };

diff --git a/samples/kfifo/record-example.c b/samples/kfifo/record-example.c
index dcc749247c64..29d08e90a63d 100644
--- a/samples/kfifo/record-example.c
+++ b/samples/kfifo/record-example.c

@@ -132,26 +132,23 @@ static ssize_t fifo_write(struct file *file, char *buf,
 	return copied;
 }
 
-static ssize_t fifo_read(struct file *file, char __user *buf,
+static ssize_t fifo_read(struct file *file, char *buf,
 						size_t count, loff_t *ppos)
 {
-	int ret;
 	unsigned int copied;
 
 	if (mutex_lock_interruptible(&read_access))
 		return -ERESTARTSYS;
 
-	ret = kfifo_to_user(&test, buf, count, &copied);
+	copied = kfifo_out(&test, buf, count);
 
 	mutex_unlock(&read_access);
-	if (ret)
-		return ret;
 
 	return copied;
 }
 
 static const struct proc_ops fifo_proc_ops = {
-	.proc_read	= fifo_read,
+	.proc_read_buf	= fifo_read,
 	.proc_write_buf	= fifo_write,
 	.proc_lseek	= noop_llseek,
 	/* kfifo_from_user() cut a record to what its length byte holds */

-- 
2.53.0

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