Messages in this thread Patch in this message | | | Subject | Re: [PATCH] vboxsf: fix old signature detection | From | Hans de Goede <> | Date | Mon, 27 Sep 2021 12:09:01 +0200 |
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Hi,
On 9/27/21 11:40 AM, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@arndb.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@redhat.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@oracle.com> > Signed-off-by: Arnd Bergmann <arnd@arndb.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); >
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