netdev - Re: [PATCH net v1] netlink: optimize the NMLSG

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Message-ID: <20240912055857.55226-1-kuniyu@amazon.com>
Date: Wed, 11 Sep 2024 22:58:57 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <atlas.yu@...onical.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>, <stephen@...workplumber.org>
Subject: Re: [PATCH net v1] netlink: optimize the NMLSG_OK macro

From: Atlas Yu <atlas.yu@...onical.com>
Date: Thu, 12 Sep 2024 10:40:18 +0800
> When nlmsg_len >= sizeof(hdr) and nlmsg_len <= len are true, we can set
> up an inequation: len >= nlmsg_len >= sizeof(hdr), which makes checking
> len >= sizeof(hdr) useless.
> 
> gcc -O3 cannot optimize ok1 to generate the same instructions as ok2 on
> x86_64 (not investigated on other architectures).
>   int ok1(int a, int b, int c) { return a >= b && c >= b && c <= a; }
>   int ok2(int a, int b, int c) { return c >= b && c <= a; }
> 
> Signed-off-by: Atlas Yu <atlas.yu@...onical.com>
> ---
>  include/uapi/linux/netlink.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
> index f87aaf28a649..85dcfa6b33af 100644
> --- a/include/uapi/linux/netlink.h
> +++ b/include/uapi/linux/netlink.h
> @@ -104,8 +104,7 @@ struct nlmsghdr {
>  #define NLMSG_NEXT(nlh,len)	 ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
>  				  (struct nlmsghdr *)(((char *)(nlh)) + \
>  				  NLMSG_ALIGN((nlh)->nlmsg_len)))
> -#define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
> -			   (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
> +#define NLMSG_OK(nlh,len) ((nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \

If len < sizeof(struct nlmsghdr)
(more specifically, offsetof(struct nlmsghdr, nlmsg_len)),
you can't access nlmsg_len; it returns a garbage.


>  			   (nlh)->nlmsg_len <= (len))
>  #define NLMSG_PAYLOAD(nlh,len) ((nlh)->nlmsg_len - NLMSG_SPACE((len)))
>  
> -- 
> 2.43.0
>