linux-kernel - [PATCH] vboxsf: fix old signature detection

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Message-Id: <20210927094123.576521-1-arnd@kernel.org>
Date:   Mon, 27 Sep 2021 11:40:58 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Hans de Goede <hdegoede@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: [PATCH] vboxsf: fix old signature detection

From: Arnd Bergmann <arnd@...db.de>

The constant-out-of-range check in clang found an actual bug in
vboxsf, which leads to the detection of old mount signatures always
failing:

fs/vboxsf/super.c:394:21: error: result of comparison of constant -3 with expression of type 'unsigned char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
                       options[3] == VBSF_MOUNT_SIGNATURE_BYTE_3) {
                       ~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
fs/vboxsf/super.c:393:21: error: result of comparison of constant -2 with expression of type 'unsigned char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
                       options[2] == VBSF_MOUNT_SIGNATURE_BYTE_2 &&
                       ~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
fs/vboxsf/super.c:392:21: error: result of comparison of constant -1 with expression of type 'unsigned char' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
                       options[1] == VBSF_MOUNT_SIGNATURE_BYTE_1 &&
                       ~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~

The problem is that the pointer is of type 'unsigned char' but the
constant is a 'char'. My first idea was to change the type of the
pointer to 'char *', but I noticed that this was the original code
and it got changed after 'smatch' complained about this.

I don't know if there is a bug in smatch here, but it sounds to me
that clang's warning is correct. Forcing the constants to an unsigned
type should make the code behave consistently and avoid the warning
on both.

Fixes: 9d682ea6bcc7 ("vboxsf: Fix the check for the old binary mount-arguments struct")
Cc: Dan Carpenter <dan.carpenter@...cle.com>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 fs/vboxsf/super.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/vboxsf/super.c b/fs/vboxsf/super.c
index 4f5e59f06284..84e2236021de 100644
--- a/fs/vboxsf/super.c
+++ b/fs/vboxsf/super.c
@@ -21,10 +21,10 @@
 
 #define VBOXSF_SUPER_MAGIC 0x786f4256 /* 'VBox' little endian */
 
-#define VBSF_MOUNT_SIGNATURE_BYTE_0 ('\000')
-#define VBSF_MOUNT_SIGNATURE_BYTE_1 ('\377')
-#define VBSF_MOUNT_SIGNATURE_BYTE_2 ('\376')
-#define VBSF_MOUNT_SIGNATURE_BYTE_3 ('\375')
+#define VBSF_MOUNT_SIGNATURE_BYTE_0 (u8)('\000')
+#define VBSF_MOUNT_SIGNATURE_BYTE_1 (u8)('\377')
+#define VBSF_MOUNT_SIGNATURE_BYTE_2 (u8)('\376')
+#define VBSF_MOUNT_SIGNATURE_BYTE_3 (u8)('\375')
 
 static int follow_symlinks;
 module_param(follow_symlinks, int, 0444);
-- 
2.29.2