[PATCH] XArray: Fix a math problem in xa_is_err()

[PATCH] XArray: Fix a math problem in xa_is_err()

[Dan Carpenter]

There is a math problem here which leads to a lot of static checker
warnings for me:

net/sunrpc/clnt.c:451 rpc_new_client() error: (-4096) too low for ERR_PTR

Error values are from -1 to -4095 or from 0xffffffff to 0xfffff001 in
hexadecimal.  (I am assuming a 32 bit system for simplicity).  We are
using the lowest two bits to hold some internal XArray data so the
error is shifted two spaces to the left.  0xfffff001 << 2 is 0xffffc004.
And finally we want to check that BIT(1) is set so we add 2 which gives
us 0xffffc006.

In other words, we should be checking that "entry >= 0xffffc006", but
the check is actually testing if "entry >= 0xffffc002".

Fixes: 76b4e5299565 ("XArray: Permit storing 2-byte-aligned pointers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
 include/linux/xarray.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index 12244aa98a69..4208042f939a 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -177,7 +177,8 @@ static inline bool xa_is_internal(const void *entry)
 static inline bool xa_is_err(const void *entry)
 {
 	return unlikely(xa_is_internal(entry) &&
-			(unsigned long)entry >= -((MAX_ERRNO << 2) + 2));
+			(unsigned long)entry >=
+				(((unsigned long)(-MAX_ERRNO << 2) + 2)));
 }
 
 /**
-- 
2.17.1

Re: [PATCH] XArray: Fix a math problem in xa_is_err()

[Matthew Wilcox]

On Tue, Jan 15, 2019 at 10:52:41PM +0300, Dan Carpenter wrote:
> @@ -177,7 +177,8 @@ static inline bool xa_is_internal(const void *entry)
>  static inline bool xa_is_err(const void *entry)
>  {
>  	return unlikely(xa_is_internal(entry) &&
> -			(unsigned long)entry >= -((MAX_ERRNO << 2) + 2));
> +			(unsigned long)entry >=
> +				(((unsigned long)(-MAX_ERRNO << 2) + 2)));
>  }

Ugh all the brackets, I'm not surprised I got it wrong.  How about this
instead; does it make your static checker happy?

diff --git a/include/linux/xarray.h b/include/linux/xarray.h
index 7da665f5cb20..5d9d318bcf7a 100644
--- a/include/linux/xarray.h
+++ b/include/linux/xarray.h
@@ -177,7 +177,7 @@ static inline bool xa_is_internal(const void *entry)
 static inline bool xa_is_err(const void *entry)
 {
        return unlikely(xa_is_internal(entry) &&
-                       (unsigned long)entry >= -((MAX_ERRNO << 2) + 2));
+                       entry >= xa_mk_internal(-MAX_ERRNO));
 }
 
 /**

(passes sparse & gcc, but ...)

Re: [PATCH] XArray: Fix a math problem in xa_is_err()

[Dan Carpenter]

On Tue, Jan 15, 2019 at 01:07:26PM -0800, Matthew Wilcox wrote:
> On Tue, Jan 15, 2019 at 10:52:41PM +0300, Dan Carpenter wrote:
> > @@ -177,7 +177,8 @@ static inline bool xa_is_internal(const void *entry)
> >  static inline bool xa_is_err(const void *entry)
> >  {
> >  	return unlikely(xa_is_internal(entry) &&
> > -			(unsigned long)entry >= -((MAX_ERRNO << 2) + 2));
> > +			(unsigned long)entry >=
> > +				(((unsigned long)(-MAX_ERRNO << 2) + 2)));
> >  }
> 
> Ugh all the brackets, I'm not surprised I got it wrong.  How about this
> instead; does it make your static checker happy?
> 

I wasn't a super fan of that expression either...  I think your code
will work, and if not then it's the checker which needs fixing.  Thanks!

regards,
dan carpenter