[PATCH v4 3/5] PCI/sysfs: Fix trailing newline handling of resource_alignment_param

["Krzysztof Wilczyński" <kw@linux.com>]

The value of the "resource_alignment" can be specified using a kernel
command-line argument (using the "pci=resource_alignment=") or through
the corresponding sysfs object under the /sys/bus/pci path.

Currently, when the value is set via the kernel command-line argument,
and then subsequently accessed through sysfs object, the value read back
will not be correct, as per:

  # grep -oE 'pci=resource_alignment.+' /proc/cmdline
  pci=resource_alignment=20@00:1f.2
  # cat /sys/bus/pci/resource_alignment
  20@00:1f.

This is also true when the value is set through the sysfs object, but
the trailing newline has not been included, as per:

  # echo -n 20@00:1f.2 > /sys/bus/pci/resource_alignment
  # cat /sys/bus/pci/resource_alignment
  20@00:1f.

When the value set through the sysfs object includes the trailing
newline, then reading it back will work as intended, as per:

  # echo 20@00:1f.2 > /sys/bus/pci/resource_alignment
  # cat /sys/bus/pci/resource_alignment
  20@00:1f.2

To fix this inconsistency, append a trailing newline in the show()
function and strip the trailing line in the store() function if one is
present.

Also, allow for the value previously set using either a command-line
argument or through the sysfs object to be cleared at run-time.

Fixes: e499081da1a2 ("PCI: Force trailing new line to resource_alignment_param in sysfs")
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
---
Changes in v2:
  None.
Changes in v3:
  Added Logan Gunthorpe's "Reviewed-by".
Changes in v4:
  None.

 drivers/pci/pci.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 5ed316ea5831..7cde86bdcc8e 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6439,34 +6439,37 @@ static ssize_t resource_alignment_show(struct bus_type *bus, char *buf)
 
 	spin_lock(&resource_alignment_lock);
 	if (resource_alignment_param)
-		count = sysfs_emit(buf, "%s", resource_alignment_param);
+		count = sysfs_emit(buf, "%s\n", resource_alignment_param);
 	spin_unlock(&resource_alignment_lock);
 
-	/*
-	 * When set by the command line, resource_alignment_param will not
-	 * have a trailing line feed, which is ugly. So conditionally add
-	 * it here.
-	 */
-	if (count >= 2 && buf[count - 2] != '\n' && count < PAGE_SIZE - 1) {
-		buf[count - 1] = '\n';
-		buf[count++] = 0;
-	}
-
 	return count;
 }
 
 static ssize_t resource_alignment_store(struct bus_type *bus,
 					const char *buf, size_t count)
 {
-	char *param = kstrndup(buf, count, GFP_KERNEL);
+	char *param, *old, *end;
 
+	param = kstrndup(buf, count, GFP_KERNEL);
 	if (!param)
 		return -ENOMEM;
 
+	end = strchr(param, '\n');
+	if (end)
+		*end = '\0';
+
 	spin_lock(&resource_alignment_lock);
-	kfree(resource_alignment_param);
-	resource_alignment_param = param;
+	old = resource_alignment_param;
+	if (strlen(param)) {
+		resource_alignment_param = param;
+	} else {
+		kfree(resource_alignment_param);
+		resource_alignment_param = NULL;
+	}
 	spin_unlock(&resource_alignment_lock);
+
+	kfree(old);
+
 	return count;
 }
 
-- 
2.31.1