Re: [PATCH 01/12] iov: Add iov_tail_truncate() and iov_tail_zero_end()

[David Gibson <david@gibson.dropbear.id.au>]

[-- Attachment #1: Type: text/plain, Size: 3354 bytes --]

On Fri, Feb 27, 2026 at 03:03:19PM +0100, Laurent Vivier wrote:
> iov_tail_truncate() truncates a tail so it contains at most a given
> number of bytes from the current position, adjusting both the last
> iovec entry and the buffer count.
> 
> iov_tail_zero_end() zero-fills all backing buffer bytes beyond a given
> number of leading bytes from the current tail position and can be
> used to clear trailing padding in fixed-size frames.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  iov.c | 39 +++++++++++++++++++++++++++++++++++++++
>  iov.h |  2 ++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/iov.c b/iov.c
> index ad726daa4cd8..cb4d6fef5567 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -170,6 +170,45 @@ bool iov_tail_prune(struct iov_tail *tail)
>  	return !!tail->cnt;
>  }
>  
> +/**
> + * iov_tail_truncate() - Truncate tail to at most @size bytes
> + * @tail:	IO vector tail (modified in place, including backing iovecs)
> + * @size:	Maximum number of bytes to keep, relative to current tail offset
> + */
> +/* cppcheck-suppress unusedFunction */
> +void iov_tail_truncate(struct iov_tail *tail, size_t size)
> +{
> +	size_t i, off;
> +
> +	i = iov_skip_bytes(tail->iov, tail->cnt, tail->off + size, &off);
> +
> +	if (i < tail->cnt) {
> +		struct iovec *last = (struct iovec *)&tail->iov[i];

This cast makes me pretty nervous.  Up until now, a global property of
iov_tail has been that it never alters the underlying iovec array: the
tail is just a view into an immutable underlying vector.  Maybe it's
worth changing that, but we should do so explicitly if we do, which
would suggest to me removing the const from struct iov_tail, rather
than making a const-discarding cast here.

> +
> +		last->iov_len = off;
> +		tail->cnt = i + !!off;
> +	}
> +}
> +
> +/**
> + * iov_tail_zero_end() - Zero-fill tail bytes beyond @size
> + * @tail:	IO vector tail (backing buffers modified in place)
> + * @size:	Number of leading bytes to preserve
> + */
> +/* cppcheck-suppress unusedFunction */
> +void iov_tail_zero_end(struct iov_tail *tail, size_t size)
> +{
> +	size_t i, off;
> +
> +	i = iov_skip_bytes(tail->iov, tail->cnt, tail->off + size, &off);
> +
> +	for (; i < tail->cnt; i++) {
> +		memset((char *)tail->iov[i].iov_base + off, 0,
> +		       tail->iov[i].iov_len - off);
> +		off = 0;
> +	}
> +}
> +
>  /**
>   * iov_tail_size() - Calculate the total size of an IO vector tail
>   * @tail:	IO vector tail
> diff --git a/iov.h b/iov.h
> index d2184bfd12bd..a7b873d58134 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -89,6 +89,8 @@ void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align)
>  void *iov_remove_header_(struct iov_tail *tail, void *v, size_t len, size_t align);
>  ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
>  		       struct iov_tail *tail);
> +void iov_tail_truncate(struct iov_tail *tail, size_t size);
> +void iov_tail_zero_end(struct iov_tail *tail, size_t size);
>  
>  /**
>   * IOV_PEEK_HEADER() - Get typed pointer to a header from an IOV tail
> -- 
> 2.53.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]