linux-kernel - [PATCH] driver core: Use scnprintf() instead of snprintf() in device show functions

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Date:   Sat, 20 Jun 2020 18:05:29 +0300
From:   Denis Efremov <efremov@...ux.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Denis Efremov <efremov@...ux.com>, linux-kernel@...r.kernel.org
Subject: [PATCH] driver core: Use scnprintf() instead of snprintf() in device show functions

>From Documentation/filesystems/sysfs.txt:
- show() must not use snprintf() when formatting the value to be
  returned to user space. If you can guarantee that an overflow
  will never happen you can use sprintf() otherwise you must use
  scnprintf().

The reason is that snprintf() returns the length the resulting string
would be, assuming the data will fit into the destination array.
scnprintf() returns the length of the string actually created in buf.

device_show_{ulong,int,bool}() functions are reference implementation
and should respect the kernel documentation.

Signed-off-by: Denis Efremov <efremov@...ux.com>
---

Actually, I don't know why snprintf() is used here initially. It looks
safe to use sprintf().

 drivers/base/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 67d39a90b45c..2815deb511fc 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1472,7 +1472,7 @@ ssize_t device_show_ulong(struct device *dev,
 			  char *buf)
 {
 	struct dev_ext_attribute *ea = to_ext_attr(attr);
-	return snprintf(buf, PAGE_SIZE, "%lx\n", *(unsigned long *)(ea->var));
+	return scnprintf(buf, PAGE_SIZE, "%lx\n", *(unsigned long *)(ea->var));
 }
 EXPORT_SYMBOL_GPL(device_show_ulong);
 
@@ -1502,7 +1502,7 @@ ssize_t device_show_int(struct device *dev,
 {
 	struct dev_ext_attribute *ea = to_ext_attr(attr);
 
-	return snprintf(buf, PAGE_SIZE, "%d\n", *(int *)(ea->var));
+	return scnprintf(buf, PAGE_SIZE, "%d\n", *(int *)(ea->var));
 }
 EXPORT_SYMBOL_GPL(device_show_int);
 
@@ -1523,7 +1523,7 @@ ssize_t device_show_bool(struct device *dev, struct device_attribute *attr,
 {
 	struct dev_ext_attribute *ea = to_ext_attr(attr);
 
-	return snprintf(buf, PAGE_SIZE, "%d\n", *(bool *)(ea->var));
+	return scnprintf(buf, PAGE_SIZE, "%d\n", *(bool *)(ea->var));
 }
 EXPORT_SYMBOL_GPL(device_show_bool);
 
-- 
2.26.2