Re: [PATCH] netlink.7: fix alignment issue in example

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Antonin,

On 12/27/19 10:15 PM, Antonin Décimo wrote:
> PVS-Studio reports that in
> 
>     char buf[8192];
>     /* ... */
>     nh = (struct nlmsghdr *) buf,
> 
> the pointer 'buf' is cast to a more strictly aligned pointer
> type. This is undefined behaviour. One possible solution to make sure
> that buf is correctly aligned is to declare buf as an array of struct
> nlmsghdr. Other solutions include allocating the array on the heap,
> use an union, or stdalign features.
> With this patch, the buffer still contains 8192 bytes.
> 
> This was raised on Stack Overflow:
> https://stackoverflow.com/questions/57745580/netlink-receive-buffer-alignment

Thanks. Patch applied.

Cheers,

Michael

> ---
>  man7/netlink.7 | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/man7/netlink.7 b/man7/netlink.7
> index 81d4249..853dee6 100644
> --- a/man7/netlink.7
> +++ b/man7/netlink.7
> @@ -533,8 +533,9 @@ And the last example is about reading netlink message.
>  .in +4n
>  .EX
>  int len;
> -char buf[8192];     /* 8192 to avoid message truncation on
> -                       platforms with page size > 4096 */
> +/* 8192 to avoid message truncation on platforms with
> +   page size > 4096 */
> +struct nlmsghdr buf[8192/sizeof(struct nlmsghdr)];
>  struct iovec iov = { buf, sizeof(buf) };
>  struct sockaddr_nl sa;
>  struct msghdr msg;
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/