linux-kernel - [PATCH 3/3] HID: avoid runtime call to strlen

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Date:   Tue, 30 Aug 2022 13:53:09 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nathan Chancellor <nathan@...nel.org>, Tom Rix <trix@...hat.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH 3/3] HID: avoid runtime call to strlen

While looking into a CONFIG_FORTIFY=y related bug, I noticed that
hid_allocate calls strlen() on a local C string variable. This variable
can only have literal string values. There is no benefit to having
FORTIFY have this be a checked strlen call, because these are literal
values.  By calling strlen() explicitly in the branches of a switch, the
compiler can evaluate strlen("literal value") at compile time, rather
than at runtime.

Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 drivers/hid/hid-input.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
index 48c1c02c69f4..9ad3cc88c26b 100644
--- a/drivers/hid/hid-input.c
+++ b/drivers/hid/hid-input.c
@@ -1922,12 +1922,15 @@ static struct hid_input *hidinput_allocate(struct hid_device *hid,
 		switch (application) {
 		case HID_GD_KEYBOARD:
 			suffix = "Keyboard";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_GD_KEYPAD:
 			suffix = "Keypad";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_GD_MOUSE:
 			suffix = "Mouse";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_DG_PEN:
 			/*
@@ -1938,36 +1941,44 @@ static struct hid_input *hidinput_allocate(struct hid_device *hid,
 			 * will have to change it and the test suite will not be happy.
 			 */
 			suffix = "Stylus";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_DG_STYLUS:
 			suffix = "Pen";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_DG_TOUCHSCREEN:
 			suffix = "Touchscreen";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_DG_TOUCHPAD:
 			suffix = "Touchpad";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_GD_SYSTEM_CONTROL:
 			suffix = "System Control";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_CP_CONSUMER_CONTROL:
 			suffix = "Consumer Control";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_GD_WIRELESS_RADIO_CTLS:
 			suffix = "Wireless Radio Control";
+			suffix_len = strlen(suffix);
 			break;
 		case HID_GD_SYSTEM_MULTIAXIS:
 			suffix = "System Multi Axis";
+			suffix_len = strlen(suffix);
 			break;
 		default:
+			suffix_len = 0;
 			break;
 		}
 	}
 
 	if (suffix) {
 		name_len = strlen(hid->name);
-		suffix_len = strlen(suffix);
 		if ((name_len < suffix_len) ||
 		    strcmp(hid->name + name_len - suffix_len, suffix)) {
 			hidinput->name = kasprintf(GFP_KERNEL, "%s %s",
-- 
2.37.2.672.g94769d06f0-goog