[PATCH 5/5] PCI/sysfs: Use user config accessors for serial_number

[Matthew Wood <thepacketgeek@gmail.com>]

serial_number_show() reads the device serial number with pci_get_dsn(),
which uses the kernel config accessors which don't check
block_cfg_access, so a read that races with a reset goes to the device
mid-reset and can fail with -EIO or report an all-ones serial number.

Do the capability walk and DSN reads with the user config accessors
instead, as the "config" attribute does, so each access waits for any
pci_cfg_access_lock() holder (e.g. pci_reset_function()) to finish.
Also wrap the reads in pci_config_pm_runtime_{get,put}() like the other
attributes that read config space.

A reset can still start between two accesses, but the next access just
waits for it to finish, and the DSN capability is unchanged by a reset.

Unlike pci_find_ext_capability(), the walk doesn't check dev->cfg_size
first. serial_number is only visible if pci_get_dsn() found the DSN
capability when the device was added, and cfg_size is final by then.

Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
Tested-by: Nikita Vetoshkin <nekto0n@meta.com>
---
 drivers/pci/pci-sysfs.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index a272f8583a8f..cbdd56b92d1a 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -696,10 +696,23 @@ static ssize_t serial_number_show(struct device *dev,
 				  struct device_attribute *attr, char *buf)
 {
 	struct pci_dev *pci_dev = to_pci_dev(dev);
-	u64 dsn;
+	u32 dword;
+	u64 dsn = 0;
 	u8 bytes[8];
+	int pos;
+
+	pci_config_pm_runtime_get(pci_dev);
+	pos = PCI_FIND_NEXT_EXT_CAP(pci_user_read_config, 0,
+				    PCI_EXT_CAP_ID_DSN, NULL, pci_dev);
+	if (pos) {
+		pos += 4;
+		pci_user_read_config_dword(pci_dev, pos, &dword);
+		dsn = (u64)dword;
+		pci_user_read_config_dword(pci_dev, pos + 4, &dword);
+		dsn |= ((u64)dword) << 32;
+	}
+	pci_config_pm_runtime_put(pci_dev);
 
-	dsn = pci_get_dsn(pci_dev);
 	if (!dsn)
 		return -EIO;
 
-- 
2.53.0-Meta