Re: [PATCH v2 06/13] iov: Add IOV_PUT_HEADER() to write header data back to iov_tail

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

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On Mon, Mar 09, 2026 at 10:47:37AM +0100, Laurent Vivier wrote:
> Add a counterpart to IOV_PEEK_HEADER() that writes header data back
> to an iov_tail after modification. If the header pointer matches the
> original iov buffer location, it only advances the offset. Otherwise,
> it copies the data using iov_from_buf().
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Although a couple of thoughts on possible improvements below.

> ---
>  iov.c | 24 ++++++++++++++++++++++++
>  iov.h | 14 +++++++++++++-
>  2 files changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/iov.c b/iov.c
> index cd48667226f3..296f24b61067 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -305,6 +305,30 @@ void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align)
>  	return v;
>  }
>  
> +/**
> + * iov_put_header_() - Write header back to an IOV tail
> + * @tail:	IOV tail to write header to
> + * @v:		Pointer to header data to write
> + * @len:	Length of header to write, in bytes
> + *
> + * Return: number of bytes written
> + */
> +/* cppcheck-suppress unusedFunction */
> +size_t iov_put_header_(struct iov_tail *tail, const void *v, size_t len)
> +{
> +	size_t l = len;
> +
> +	/* iov_peek_header_() already called iov_check_header() */
> +	if ((char *)tail->iov[0].iov_base + tail->off != v)
> +		l = iov_from_buf(tail->iov, tail->cnt, tail->off, v, len);

This assumes the tail is already pruned.  That will be the case
assuming this has a matching iov_peek_header_(), but it might be
better to explicitly document that assumption.

Better yet, is there a way we could enforce these come in pairs.
Something like:

	struct some_header *hdr;

	with_header(hdr, tail) {
		hdr->a = 123;
		hdr->b = hdr->c - 7;
	}

Where,
    #define with_header(hdr, tail)
	for ((typeof *(hdr)) store, hdr = IOV_PEEK_HEADER(tail,	store);
	     hdr;
	     IOV_PUT_HEADER(tail, hdr), hdr = NULL)


> +	ASSERT(l == len);
> +
> +	tail->off += l;
> +
> +	return l;
> +}
> +
>  /**
>   * iov_remove_header_() - Remove a header from an IOV tail
>   * @tail:	IOV tail to remove header from (modified)
> diff --git a/iov.h b/iov.h
> index d295d05b3bab..fe7c5163d2ab 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -90,6 +90,7 @@ bool iov_tail_prune(struct iov_tail *tail);
>  size_t iov_tail_size(struct iov_tail *tail);
>  bool iov_drop_header(struct iov_tail *tail, size_t len);
>  void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align);
> +size_t iov_put_header_(struct iov_tail *tail, const void *v, size_t len);
>  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);
> @@ -112,6 +113,16 @@ ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
>  					       sizeof(var_),		\
>  					       __alignof__(var_))))
>  
> +/**
> + * IOV_PUT_HEADER() - Write header back to an IOV tail
> + * @tail_:	IOV tail to write header to
> + * @var_:	Pointer to a variable containing the header data to write
> + *
> + * Return: number of bytes written
> + */
> +#define IOV_PUT_HEADER(tail_, var_)					\
> +	(iov_put_header_((tail_), (var_), sizeof(*var_)))
> +
>  /**
>   * IOV_REMOVE_HEADER() - Remove and return typed header from an IOV tail
>   * @tail_:	IOV tail to remove header from (modified)
> @@ -130,7 +141,8 @@ ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
>  	((__typeof__(var_) *)(iov_remove_header_((tail_), &(var_),	\
>  				    sizeof(var_), __alignof__(var_))))
>  
> -/** IOV_DROP_HEADER() - Remove a typed header from an IOV tail
> +/**
> + * IOV_DROP_HEADER() - Remove a typed header from an IOV tail
>   * @tail_:	IOV tail to remove header from (modified)
>   * @type_:	Data type of the header to remove
>   *
> -- 
> 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

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