# \[PATCH RFC 04/23\] PCI/proc: let procfs copy out config space reads

- From: Christian Brauner \<brauner@kernel.org\>
- Date: 2026-11-02 15:11:31 
- List: linux-fbdev
- Page: https://ratatoskr.run/linux-fbdev/2026/11/17845419
- Thread: https://ratatoskr.run/linux-fsdevel/2026/11/17844895/t/2.md

```text
Implement ->proc_read_buf() so procfs copies what is read to userspace
after it unpinned the entry, and read the config space into procfs'
buffer with put_unaligned_le16() and put_unaligned_le32() instead of
put_user(). A read into a buffer that can't be written used to report
success. It fails with -EFAULT now. A page holds all of the config
space, so a single read of it still works, libpci treats a short read as
an error.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 drivers/pci/proc.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/drivers/pci/proc.c b/drivers/pci/proc.c
index d420353ee2b3..a8d7469f222b 100644
--- a/drivers/pci/proc.c
+++ b/drivers/pci/proc.c
@@ -29,8 +29,8 @@ static loff_t proc_bus_pci_lseek(struct file *file, loff_t off, int whence)
 	return fixed_size_llseek(file, off, whence, dev->cfg_size);
 }
 
-static ssize_t proc_bus_pci_read(struct file *file, char __user *buf,
-				 size_t nbytes, loff_t *ppos)
+static ssize_t proc_bus_pci_read(struct file *file, char *buf, size_t nbytes,
+				 loff_t *ppos)
 {
 	struct pci_dev *dev = pde_data(file_inode(file));
 	unsigned int pos, cnt, size;
@@ -60,15 +60,12 @@ static ssize_t proc_bus_pci_read(struct file *file, char __user *buf,
 		nbytes = size - pos;
 	cnt = nbytes;
 
-	if (!access_ok(buf, cnt))
-		return -EINVAL;
-
 	pci_config_pm_runtime_get(dev);
 
 	if ((pos & 1) && cnt) {
 		unsigned char val;
 		pci_user_read_config_byte(dev, pos, &val);
-		put_user(val, buf);
+		*buf = val;
 		buf++;
 		pos++;
 		cnt--;
@@ -77,7 +74,7 @@ static ssize_t proc_bus_pci_read(struct file *file, char __user *buf,
 	if ((pos & 3) && cnt > 2) {
 		unsigned short val;
 		pci_user_read_config_word(dev, pos, &val);
-		put_user(cpu_to_le16(val), (__le16 __user *) buf);
+		put_unaligned_le16(val, buf);
 		buf += 2;
 		pos += 2;
 		cnt -= 2;
@@ -86,7 +83,7 @@ static ssize_t proc_bus_pci_read(struct file *file, char __user *buf,
 	while (cnt >= 4) {
 		unsigned int val;
 		pci_user_read_config_dword(dev, pos, &val);
-		put_user(cpu_to_le32(val), (__le32 __user *) buf);
+		put_unaligned_le32(val, buf);
 		buf += 4;
 		pos += 4;
 		cnt -= 4;
@@ -96,7 +93,7 @@ static ssize_t proc_bus_pci_read(struct file *file, char __user *buf,
 	if (cnt >= 2) {
 		unsigned short val;
 		pci_user_read_config_word(dev, pos, &val);
-		put_user(cpu_to_le16(val), (__le16 __user *) buf);
+		put_unaligned_le16(val, buf);
 		buf += 2;
 		pos += 2;
 		cnt -= 2;
@@ -105,7 +102,7 @@ static ssize_t proc_bus_pci_read(struct file *file, char __user *buf,
 	if (cnt) {
 		unsigned char val;
 		pci_user_read_config_byte(dev, pos, &val);
-		put_user(val, buf);
+		*buf = val;
 		pos++;
 	}
 
@@ -327,7 +324,7 @@ static int proc_bus_pci_release(struct inode *inode, struct file *file)
 static const struct proc_ops proc_bus_pci_ops = {
 	.proc_flags	= PROC_ENTRY_SHORT_WRITES,
 	.proc_lseek	= proc_bus_pci_lseek,
-	.proc_read	= proc_bus_pci_read,
+	.proc_read_buf	= proc_bus_pci_read,
 	.proc_write_buf	= proc_bus_pci_write,
 	.proc_write_size = PCI_CFG_SPACE_EXP_SIZE,
 	.proc_ioctl	= proc_bus_pci_ioctl,

-- 
2.53.0

```