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

[Laurent Vivier <lvivier@redhat.com>]

On 3/24/26 03:41, 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.

I tried but it's not possible to put a declaration and an initialization in the first part 
of the for().

for instance

#define with_header(hdr_, tail_)\
for (__typeof__(*hdr_) store_, hdr_ = IOV_PEEK_HEADER(tail_, store_);\
      hdr_;\
      IOV_PUT_HEADER(tail_, hdr_), hdr_ = NULL)

Then :

struct ethhdr *eh;
struct iov_tail *tail;

with_header(eh, tail)

becomes:

for (struct ethhdr store_, eh = IOV_PEEK_HEADER(tail, store_);
      eh;
      IOV_PUT_HEADER(store, eh), eh = NULL)


And

struct ethhdr store_, eh = IOV_PEEK_HEADER(tail, store_);

declares store_, but also declares eh, that is already declared.

We could do something like:

#define with_header(hdr_, tail_)\
for (__typeof__(*hdr_) store_, *tmp_hdr_ = (hdr_ = IOV_PEEK_HEADER(tail_, store_));\
      hdr_;\
      IOV_PUT_HEADER(tail_, hdr_), hdr_ = NULL)

But I preferred to introduce the type rather than a dummy variable.

Moreover, doing as I did, the variable is local to the body of the for() and cannot be 
used outside.

Thanks,
Laurent
> 
>> + * @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)
>> +
>>   #endif /* IOVEC_H */
>> -- 
>> 2.53.0
>>
>