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SubjectRe: [PATCH] reiserfs: Force type conversion in xattr_hash
On Thu, Apr 18, 2019 at 03:50:19PM -0700, Andrew Morton wrote:
> On Wed, 17 Apr 2019 17:22:00 +0530 Bharath Vedartham <linux.bhar@gmail.com> wrote:
>
> > This patch fixes the sparse warning:
> >
> > fs/reiserfs//xattr.c:453:28: warning: incorrect type in return
> > expression (different base types)
> > fs/reiserfs//xattr.c:453:28: expected unsigned int
> > fs/reiserfs//xattr.c:453:28: got restricted __wsum
> > fs/reiserfs//xattr.c:453:28: warning: incorrect type in return
> > expression (different base types)
> > fs/reiserfs//xattr.c:453:28: expected unsigned int
> > fs/reiserfs//xattr.c:453:28: got restricted __wsum
> >
> > csum_partial returns restricted integer __wsum whereas xattr_hash
> > expects a return type of __u32.
> >
> > ...
> >
> > --- a/fs/reiserfs/xattr.c
> > +++ b/fs/reiserfs/xattr.c
> > @@ -450,7 +450,7 @@ static struct page *reiserfs_get_page(struct inode *dir, size_t n)
> >
> > static inline __u32 xattr_hash(const char *msg, int len)
> > {
> > - return csum_partial(msg, len, 0);
> > + return (__force __u32)csum_partial(msg, len, 0);
> > }
> >
> > int reiserfs_commit_write(struct file *f, struct page *page,
>
> hm. Conversion from int to __u32 should be OK - why is sparse being so
> picky here?
>
> Why is the __force needed, btw?
The return type of csum_partial is __wsum which is a restricted integer
type.

__wsum is defined as:
typedef __u32 __bitwise __wsum;

Being a restricted integer type, sparse will complain whenever convert
the restricted type to another type without __force.

Interestingly enough if we look at the definition of csum_partial, more
specifically the first 2 lines:

__wsum csum_partial(const void *buff, int len, __wsum sum)
{
unsigned long result = do_csum(buff, len);

result += (__force u32)sum;

.....

sum is of type __wsum, result is of type unsigned long. __force is used
to suppress the sparse warning.

Thanks for your time!

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