[PATCH] PCI/sysfs: Use kstrtobool() to parse the ROM attribute input

["Krzysztof Wilczyński" <kwilczynski@kernel.org>]

pci_write_rom() controls access to the ROM content through the
corresponding sysfs attribute, and treats the input as a request to
disable only when it matches the string "0\n" exactly:

  if ((off ==  0) && (*buf == '0') && (count == 2))

The count == 2 condition encodes the trailing newline that echo(1)
appends.  This was found when software in userspace wrote "0" without
a trailing newline aiming to disable access, which failed to match the
condition above and enabled access instead.  For example:

  $ echo 0 > rom     <- "0\n", count 2, access disabled
  $ echo -n 0 > rom  <- "0", count 1, access enabled
  $ echo > rom       <- "", count 1, access enabled (likely not be desirable)

Thus, parse the input with kstrtobool(), which handles common boolean
inputs such as "0", "1", "n", "y" or "off", "on", with or without a
trailing newline, so both of the above disable access, and update the
now stale comment.

As a side effect, input that does not parse as a boolean is rejected
with -EINVAL rather than enabling access.  The documented "0" and "1"
continue to work as before, and rejecting malformed input brings the
attribute in line with how sysfs attributes typically handle it.

Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
---
 drivers/pci/pci-sysfs.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index d37860841260..1ddd6be25d95 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -1309,18 +1309,19 @@ void __weak pci_remove_resource_files(struct pci_dev *dev) { return; }
  * @off: file offset
  * @count: number of byte in input
  *
- * writing anything except 0 enables it
+ * Writing a boolean value enables or disables the ROM display.
  */
 static ssize_t pci_write_rom(struct file *filp, struct kobject *kobj,
 			     const struct bin_attribute *bin_attr, char *buf,
 			     loff_t off, size_t count)
 {
 	struct pci_dev *pdev = to_pci_dev(kobj_to_dev(kobj));
+	bool enable;
 
-	if ((off ==  0) && (*buf == '0') && (count == 2))
-		pdev->rom_attr_enabled = 0;
-	else
-		pdev->rom_attr_enabled = 1;
+	if (kstrtobool(buf, &enable))
+		return -EINVAL;
+
+	pdev->rom_attr_enabled = enable;
 
 	return count;
 }
-- 
2.54.0