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

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Message-ID: <40217483-1b8d-28ec-bbfc-8f979773b166@redhat.com>
Date:   Mon, 27 Sep 2021 12:09:01 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        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: Re: [PATCH] vboxsf: fix old signature detection

Hi,

On 9/27/21 11:40 AM, Arnd Bergmann wrote:
> 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) {
>                        ~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~

This actually seems to be a clang bug though, or at least a weird
interpretation (and different from gcc) of the C spec.

VBSF_MOUNT_SIGNATURE_BYTE_3 is defined as:

#define VBSF_MOUNT_SIGNATURE_BYTE_3 ('\375')

The C-spec:

http://port70.net/~nsz/c/c11/n1570.html#6.4.4.4p5

Says the following:

"The octal digits that follow the backslash in an octal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant. The numerical value of the octal integer so formed specifies the value of the desired character or wide character."

Character constants have a type of int, so 0375
clearly fits in the range of that.

I guess the problem is that gcc sees this as

const int VBSF_MOUNT_SIGNATURE_BYTE_3 = 0375;

Where as clang sees this as:

const int VBSF_MOUNT_SIGNATURE_BYTE_3 = (char)0375;

Which is a nice subtle incompatibility between the 2 :|


With that said, the patch is fine and I have no objections
against it:

Reviewed-by: Hans de Goede <hdegoede@...hat.com>

Although maybe it is better to actually remove any
ambiguity and just replace the defines with:

static const u8 VBSF_MOUNT_SIGNATURE_BYTE_0 = 0000;
static const u8 VBSF_MOUNT_SIGNATURE_BYTE_1 = 0377;
static const u8 VBSF_MOUNT_SIGNATURE_BYTE_2 = 0376;
static const u8 VBSF_MOUNT_SIGNATURE_BYTE_3 = 0375;

?

Regards,

Hans










> 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);
>