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

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Message-ID: <20210927130253.GH2083@kadam>
Date:   Mon, 27 Sep 2021 16:02:53 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Hans de Goede <hdegoede@...hat.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        linux-sparse@...r.kernel.org
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] vboxsf: fix old signature detection

GCC handles it the same way as Clang.  '\377' is -1 but in Sparse it's
255.  I've added the Sparse mailing list to the CC.

regards,
dan carpenter

On Mon, Sep 27, 2021 at 12:09:01PM +0200, Hans de Goede wrote:
> 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);
> >