Re: [PATCH v8 2/3] iov: Introduce IOV_PUSH_HEADER() macro

[Jon Maloy <jmaloy@redhat.com>]

On 2026-04-16 12:09, Laurent Vivier wrote:
> Add iov_push_header_() and its typed wrapper IOV_PUSH_HEADER() to write
> a header into an iov_tail at the current offset and advance past it.
> 
> This is the write counterpart to IOV_PEEK_HEADER() / IOV_REMOVE_HEADER(),
> using iov_from_buf() to copy the header data across iovec boundaries.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>   iov.c | 23 +++++++++++++++++++++++
>   iov.h | 11 +++++++++++
>   2 files changed, 34 insertions(+)
> 
> diff --git a/iov.c b/iov.c
> index 28c6d40d2986..b1bcdc4649df 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -360,6 +360,29 @@ void *iov_peek_header_(struct iov_tail *tail, void *v, size_t len, size_t align)
>   	return v;
>   }
>   
> +/**
> + * iov_push_header_() - Write a new header 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_push_header_(struct iov_tail *tail, const void *v, size_t len)
> +{
> +	size_t l;
> +
> +	if (!iov_tail_prune(tail))
> +		return 0; /* No space */
> +
> +	l = iov_from_buf(tail->iov, tail->cnt, tail->off, v, len);
> +
> +	tail->off = tail->off + l;
> +
> +	return l;
> +}

A small observation: if iov_from_buf() returns less than ´len' because 
the tail has insufficient space, this function advances tail->off by 
that lenght and returns it. The caller in IOV_PUSH_HEADER gets back a
(value != sizeof(header)), but ignores it. This means a partial header 
write would go undetected.
Maybe a warning or even an assert() would be in place here?
/jon

> +
>   /**
>    * 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 3c63308e554f..4fdf14a85b19 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -93,6 +93,7 @@ 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_push_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);
> @@ -115,6 +116,16 @@ ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
>   					       sizeof(var_),		\
>   					       __alignof__(var_))))
>   
> +/**
> + * IOV_PUSH_HEADER() - Write a new header to an IOV tail
> + * @tail_:	IOV tail to write header to
> + * @var_:	A variable containing the header data to write
> + *
> + * Return: number of bytes written
> + */
> +#define IOV_PUSH_HEADER(tail_, var_)					\
> +	(iov_push_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)