linux-kernel - RE: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum

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Date:   Wed, 22 Jul 2020 09:27:32 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Al Viro' <viro@...IV.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: RE: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead
 of 0 as initial sum

From: Al Viro
> Sent: 21 July 2020 21:26
> Preparation for the change of calling conventions; right now all
> callers pass 0 as initial sum.  Passing 0xffffffff instead yields
> the values comparable mod 0xffff and guarantees that 0 will not
> be returned on success.
> 
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> ---
>  lib/iov_iter.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 7405922caaec..d5b7e204fea6 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1451,7 +1451,7 @@ size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
>  		int err = 0;
>  		next = csum_and_copy_from_user(v.iov_base,
>  					       (to += v.iov_len) - v.iov_len,
> -					       v.iov_len, 0, &err);
> +					       v.iov_len, ~0U, &err);
>  		if (!err) {
>  			sum = csum_block_add(sum, next, off);
>  			off += v.iov_len;

Can't you remove the csum_block_add() by passing the
old 'sum' in instead of the ~0U ?
You'll need to keep track of whether the buffer fragment
is odd/even aligned.
After an odd length fragment a bswap32() or 8 bit rotate will
fix things (and maybe one right at the end).

	David

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