Re: [PATCH 01/24] iov: add some functions to manage iovec

[Stefano Brivio <sbrivio@redhat.com>]

On Fri,  2 Feb 2024 15:11:28 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  Makefile |  4 +--
>  iov.c    | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  iov.h    | 46 +++++++++++++++++++++++++++++++++
>  3 files changed, 126 insertions(+), 2 deletions(-)
>  create mode 100644 iov.c
>  create mode 100644 iov.h
> 
> diff --git a/Makefile b/Makefile
> index af4fa87e7e13..c1138fb91d26 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -47,7 +47,7 @@ FLAGS += -DDUAL_STACK_SOCKETS=$(DUAL_STACK_SOCKETS)
>  PASST_SRCS = arch.c arp.c checksum.c conf.c dhcp.c dhcpv6.c flow.c icmp.c \
>  	igmp.c isolation.c lineread.c log.c mld.c ndp.c netlink.c packet.c \
>  	passt.c pasta.c pcap.c pif.c port_fwd.c tap.c tcp.c tcp_splice.c udp.c \
> -	util.c
> +	util.c iov.c
>  QRAP_SRCS = qrap.c
>  SRCS = $(PASST_SRCS) $(QRAP_SRCS)
>  
> @@ -56,7 +56,7 @@ MANPAGES = passt.1 pasta.1 qrap.1
>  PASST_HEADERS = arch.h arp.h checksum.h conf.h dhcp.h dhcpv6.h flow.h \
>  	flow_table.h icmp.h inany.h isolation.h lineread.h log.h ndp.h \
>  	netlink.h packet.h passt.h pasta.h pcap.h pif.h port_fwd.h siphash.h \
> -	tap.h tcp.h tcp_conn.h tcp_splice.h udp.h util.h
> +	tap.h tcp.h tcp_conn.h tcp_splice.h udp.h util.h iov.h
>  HEADERS = $(PASST_HEADERS) seccomp.h
>  
>  C := \#include <linux/tcp.h>\nstruct tcp_info x = { .tcpi_snd_wnd = 0 };
> diff --git a/iov.c b/iov.c
> new file mode 100644
> index 000000000000..38a8e7566021
> --- /dev/null
> +++ b/iov.c
> @@ -0,0 +1,78 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +
> +/* some parts copied from QEMU util/iov.c */

As David mentioned, we actually include authorship notices in passt,
and if you need an example of something with compatible licenses but
sourced from another project, see checksum.c.

> +
> +#include <sys/socket.h>

For consistency: newline between system and local includes.

> +#include "util.h"
> +#include "iov.h"
> +
> +size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,

If I understood the idea behind this function correctly, maybe
iov_fill_from_buf() would be more descriptive.

> +			 size_t offset, const void *buf, size_t bytes)
> +{
> +	size_t done;
> +	unsigned int i;

Customary newline between declarations and code.

> +	for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {
> +		if (offset < iov[i].iov_len) {
> +			size_t len = MIN(iov[i].iov_len - offset, bytes - done);
> +			memcpy((char *)iov[i].iov_base + offset, (char *)buf + done, len);
> +			done += len;
> +			offset = 0;
> +		} else {
> +			offset -= iov[i].iov_len;
> +		}
> +	}

I think the version you mentioned later in this thread (quoting here) is
much clearer. Some observations on it:

> for (i = 0; offset && i < iov_cnt && offset >= iov[i].iov_len ; i++)
>          offset -= iov[i].iov_len;

Do you actually need to check for offset to be non-zero? To me it looks like
offset >= iov[i].iov_len would suffice, and look more familiar.

What happens if offset is bigger than the sum of all iov[i].iov_len? To me it
looks like we would just go ahead and copy to the wrong position. But I just
had a quick look at the usage of this function, maybe it's safe to assume
that it can't ever happen.

> 
> for (done = 0; done < bytes && i < iov_cnt; i++) {

Rather than 'done', I would call this 'copied', so that we return the
bytes copied later, instead of the "done" ones. I think it's a bit clearer.

>      size_t len = MIN(iov[i].iov_len - offset, bytes - done);

Newline between declaration and code for consistency.

>      memcpy((char *)iov[i].iov_base + offset, (char *)buf + done, len);
>      done += len;
> }
> 
> +	return done;
> +}
> +
> +size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
> +		       size_t offset, void *buf, size_t bytes)
> +{
> +	size_t done;
> +	unsigned int i;
> +	for (i = 0, done = 0; (offset || done < bytes) && i < iov_cnt; i++) {

Same here, I think this would be easier to understand if you split it
into two loops.

> +		if (offset < iov[i].iov_len) {
> +			size_t len = MIN(iov[i].iov_len - offset, bytes - done);
> +			memcpy((char *)buf + done, (char *)iov[i].iov_base + offset, len);
> +			done += len;
> +			offset = 0;
> +		} else {
> +			offset -= iov[i].iov_len;
> +		}
> +	}

Newline here would be appreciated for consistency.

> +	return done;
> +}
> +
> +size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt)
> +{
> +	size_t len;
> +	unsigned int i;
> +
> +	len = 0;

...then it could be initialised in the declaration.

> +	for (i = 0; i < iov_cnt; i++) {
> +		len += iov[i].iov_len;
> +	}
> +	return len;
> +}
> +
> +unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
> +		  const struct iovec *iov, unsigned int iov_cnt,
> +		  size_t offset, size_t bytes)
> +{
> +	size_t len;
> +	unsigned int i, j;
> +	for (i = 0, j = 0;
> +		 i < iov_cnt && j < dst_iov_cnt && (offset || bytes); i++) {

Same here, if possible, split.

> +		if (offset >= iov[i].iov_len) {
> +			offset -= iov[i].iov_len;
> +			continue;
> +		}
> +		len = MIN(bytes, iov[i].iov_len - offset);
> +
> +		dst_iov[j].iov_base = (char *)iov[i].iov_base + offset;
> +		dst_iov[j].iov_len = len;
> +		j++;
> +		bytes -= len;
> +		offset = 0;
> +	}
> +	return j;
> +}
> diff --git a/iov.h b/iov.h
> new file mode 100644
> index 000000000000..31fbf6d0e1cf
> --- /dev/null
> +++ b/iov.h
> @@ -0,0 +1,46 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +
> +/* some parts copied from QEMU include/qemu/iov.h */
> +
> +#ifndef IOVEC_H
> +#define IOVEC_H
> +
> +#include <unistd.h>
> +#include <string.h>
> +
> +size_t iov_from_buf_full(const struct iovec *iov, unsigned int iov_cnt,
> +			 size_t offset, const void *buf, size_t bytes);
> +size_t iov_to_buf_full(const struct iovec *iov, const unsigned int iov_cnt,
> +		       size_t offset, void *buf, size_t bytes);
> +
> +static inline size_t iov_from_buf(const struct iovec *iov,
> +				  unsigned int iov_cnt, size_t offset,
> +				  const void *buf, size_t bytes)
> +{

Is there a particular reason to include these two in a header? The
compiler will inline as needed if they are in a source file.

> +	if (__builtin_constant_p(bytes) && iov_cnt &&
> +		offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
> +		memcpy((char *)iov[0].iov_base + offset, buf, bytes);
> +		return bytes;
> +	} else {

We generally avoid else-after-return in passt -- clang-tidy should also
warn about it (but I haven't tried yet).

> +		return iov_from_buf_full(iov, iov_cnt, offset, buf, bytes);
> +	}
> +}
> +
> +static inline size_t iov_to_buf(const struct iovec *iov,
> +				const unsigned int iov_cnt, size_t offset,
> +				void *buf, size_t bytes)
> +{
> +	if (__builtin_constant_p(bytes) && iov_cnt &&
> +		offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
> +		memcpy(buf, (char *)iov[0].iov_base + offset, bytes);
> +		return bytes;
> +	} else {
> +		return iov_to_buf_full(iov, iov_cnt, offset, buf, bytes);
> +	}
> +}
> +
> +size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt);
> +unsigned iov_copy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
> +		  const struct iovec *iov, unsigned int iov_cnt,
> +		  size_t offset, size_t bytes);
> +#endif

-- 
Stefano