[PATCH v8 1/5] scsi: core: Protect INQUIRY sysfs attributes with mutex

[Brian Bunker <brian@purestorage.com>]

All INQUIRY-derived sysfs attributes (type, scsi_level, vendor, model,
rev, cdl_supported, modalias, and the binary inquiry attribute) read
data that can be updated during device rescan. These reads must be
protected against concurrent updates.

Use the existing inquiry_mutex to protect access to these sysfs
attributes. This ensures that userspace always sees consistent INQUIRY
data, even if a rescan is updating the buffer concurrently.

Update the sdev_rd_attr macro to take inquiry_mutex around the field
access and switch to sysfs_emit. Since vendor, model, and rev are
NUL-terminated fixed-size arrays, %s format handles all field types
correctly.

scsi_bus_uevent() gets a const device and only needs the single-byte
type for MODALIAS, so read it with READ_ONCE() instead of taking the
mutex.

This is preparatory work for adding INQUIRY data update support during
device rescan operations.

Co-developed-by: Krishna Kant <krishna.kant@purestorage.com>
Signed-off-by: Krishna Kant <krishna.kant@purestorage.com>
Signed-off-by: Brian Bunker <brian@purestorage.com>
---
 drivers/scsi/scsi_sysfs.c | 37 +++++++++++++++++++++++++++----------
 1 file changed, 27 insertions(+), 10 deletions(-)

diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
index fe54792ed69e..5f3e3fcdbad3 100644
--- a/drivers/scsi/scsi_sysfs.c
+++ b/drivers/scsi/scsi_sysfs.c
@@ -551,7 +551,13 @@ static int scsi_bus_uevent(const struct device *dev, struct kobj_uevent_env *env
 
 	sdev = to_scsi_device(dev);
 
-	add_uevent_var(env, "MODALIAS=" SCSI_DEVICE_MODALIAS_FMT, sdev->type);
+	/*
+	 * sdev->type is a single byte that a rescan may update under
+	 * inquiry_mutex, so a lockless read sees either the old or the new
+	 * type.
+	 */
+	add_uevent_var(env, "MODALIAS=" SCSI_DEVICE_MODALIAS_FMT,
+		       READ_ONCE(sdev->type));
 	return 0;
 }
 
@@ -638,21 +644,30 @@ sdev_show_##field (struct device *dev, struct device_attribute *attr,	\
 
 /*
  * sdev_rd_attr: macro to create a function and attribute variable for a
- * read only field.
+ * read-only field. inquiry_mutex protects INQUIRY-derived fields against
+ * concurrent updates during device rescan.
  */
 #define sdev_rd_attr(field, format_string)				\
-	sdev_show_function(field, format_string)			\
-static DEVICE_ATTR(field, S_IRUGO, sdev_show_##field, NULL);
+static ssize_t								\
+sdev_show_##field(struct device *dev, struct device_attribute *attr,	\
+		  char *buf)						\
+{									\
+	struct scsi_device *sdev = to_scsi_device(dev);			\
+									\
+	guard(mutex)(&sdev->inquiry_mutex);				\
+	return sysfs_emit(buf, format_string, sdev->field);		\
+}									\
+static DEVICE_ATTR(field, S_IRUGO, sdev_show_##field, NULL)
 
 /*
  * Create the actual show/store functions and data structures.
  */
-sdev_rd_attr (type, "%d\n");
-sdev_rd_attr (scsi_level, "%d\n");
-sdev_rd_attr (vendor, "%.8s\n");
-sdev_rd_attr (model, "%.16s\n");
-sdev_rd_attr (rev, "%.4s\n");
-sdev_rd_attr (cdl_supported, "%d\n");
+sdev_rd_attr(type, "%d\n");
+sdev_rd_attr(scsi_level, "%d\n");
+sdev_rd_attr(cdl_supported, "%d\n");
+sdev_rd_attr(vendor, "%s\n");
+sdev_rd_attr(model, "%s\n");
+sdev_rd_attr(rev, "%s\n");
 
 static ssize_t
 sdev_show_device_busy(struct device *dev, struct device_attribute *attr,
@@ -917,6 +932,7 @@ static ssize_t show_inquiry(struct file *filep, struct kobject *kobj,
 	struct device *dev = kobj_to_dev(kobj);
 	struct scsi_device *sdev = to_scsi_device(dev);
 
+	guard(mutex)(&sdev->inquiry_mutex);
 	if (!sdev->inquiry)
 		return -EINVAL;
 
@@ -963,6 +979,7 @@ sdev_show_modalias(struct device *dev, struct device_attribute *attr, char *buf)
 {
 	struct scsi_device *sdev;
 	sdev = to_scsi_device(dev);
+	guard(mutex)(&sdev->inquiry_mutex);
 	return snprintf (buf, 20, SCSI_DEVICE_MODALIAS_FMT "\n", sdev->type);
 }
 static DEVICE_ATTR(modalias, S_IRUGO, sdev_show_modalias, NULL);
-- 
2.56.0