Re: [PATCH v4 4/5] iov: Add IOV_PUT_HEADER() and with_header() to write header data back to iov_tail

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

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On Tue, Mar 24, 2026 at 01:41:18PM +1100, David Gibson wrote:
> On Mon, Mar 23, 2026 at 03:31:50PM +0100, Laurent Vivier wrote:
> > Add iov_put_header_() and its wrapper macro IOV_PUT_HEADER() as a
> > counterpart to IOV_PEEK_HEADER(). This writes header data back to an
> > iov_tail after modification. If the header pointer matches the
> > original iov buffer location, the data was already modified in place
> > and no copy is needed. Otherwise, it copies the data back using
> > iov_from_buf().
> > 
> > Add with_header(), a for-loop macro that combines IOV_PEEK_HEADER()
> > and IOV_PUT_HEADER() to allow modifying a header in place within a
> > block scope.
> > 
> > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > ---
> >  iov.c | 22 ++++++++++++++++++++++
> >  iov.h | 25 ++++++++++++++++++++++++-
> >  2 files changed, 46 insertions(+), 1 deletion(-)
> > 
> > diff --git a/iov.c b/iov.c
> > index 8134b8c9f988..7fc9c3c78a32 100644
> > --- a/iov.c
> > +++ b/iov.c
> > @@ -308,6 +308,28 @@ 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_(const 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);
> > +
> > +	assert(l == len);
> > +
> > +	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..4ce425ccdbe5 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_(const 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
> >   *
> > @@ -138,4 +150,15 @@ ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
> >   */
> >  #define IOV_DROP_HEADER(tail_, type_) 	iov_drop_header((tail_), sizeof(type_))
> >  
> > +/**
> > + * with_header() - Execute a block on a given header
> > + * @type:	Data type of the header to modify
> 
> We already use __typeof__ in IOV_PEEK_HEADER(), so we should be able
> to use that to avoid explicitly passing the type.
> 
> > + * @hdr_:	Variable name to receive the header pointer
> > + * @tail_:	IOV tail to peek/put the header from/to
> > + */
> > +#define with_header(type_, hdr_, tail_)					\
> > +	for (type_ store_, *hdr_ = IOV_PEEK_HEADER(tail_, store_); 	\
> > +	     hdr_;							\
> > +	     IOV_PUT_HEADER(tail_, hdr_), hdr_ = NULL)
> > +

I know I suggested this, but looking at it now, I'm wondering if the
fact that you _must not_ alter the tail in the block is too
non-obvious a constraint :/.  In particular this means you can never
work with multiple headers at once, say:
	with_header(iphdr) {
		with_header(udphdr) {
			...
		}
	}

-- 
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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