Re: [PATCH] iov_iter: have iov_iter_advance decrement nr_segs appropriately

[Jeff Layton <jlayton@redhat.com>]

On Mon, 15 Aug 2011 14:07:26 -0400
Jeff Layton <jlayton@redhat.com> wrote:

> Currently, when you call iov_iter_advance, then the pointer to the kvec
> array can be incremented, but it does not decrement the nr_segs value in
> the iov_iter struct. The result is a iov_iter struct with a nr_segs
> value that goes beyond the end of the array.
> 
> While I'm not aware of anything that's specifically broken by this, it
> seems odd and a bit dangerous not to decrement that value. If someone
> were to trust the nr_segs value to be correct, then they could end up
> walking off the end of the array.
> 
> Changing this might also provide some micro-optimization when dealing
> with the last kvec in an array. Many of the other routines that deal
> with iov_iter have optimized codepaths when nr_segs == 1.
> 
> Cc: Nick Piggin <npiggin@suse.de>
> Signed-off-by: Jeff Layton <jlayton@redhat.com>
> ---
>  mm/filemap.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 


...and of course, I got the description wrong:

    s/kvec/iovec/

...I believe the patch itself is fine though.

> diff --git a/mm/filemap.c b/mm/filemap.c
> index 645a080..86508cc 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2113,6 +2113,7 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
>  	} else {
>  		const struct iovec *iov = i->iov;
>  		size_t base = i->iov_offset;
> +		unsigned long nr_segs = i->nr_segs;
>  
>  		/*
>  		 * The !iov->iov_len check ensures we skip over unlikely
> @@ -2128,11 +2129,13 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
>  			base += copy;
>  			if (iov->iov_len == base) {
>  				iov++;
> +				nr_segs--;
>  				base = 0;
>  			}
>  		}
>  		i->iov = iov;
>  		i->iov_offset = base;
> +		i->nr_segs = nr_segs;
>  	}
>  }
>  EXPORT_SYMBOL(iov_iter_advance);


-- 
Jeff Layton <jlayton@redhat.com>