Re: [PATCH] net: ipv4: tcp.c: fix an assignment in an if condition

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Mar 12, 2022 at 05:27:44PM +0100, Alexander Vorwerk wrote:
> reported by checkpatch.pl

... nicely demonstrating why checkpatch.pl is a menace.

> Signed-off-by: Alexander Vorwerk <alexander.vorwerk@stud.uni-goettingen.de>
> ---
>  net/ipv4/tcp.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 28ff2a820f7c..7fa6e7e6ea80 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -959,10 +959,10 @@ static struct sk_buff *tcp_build_frag(struct sock *sk, int size_goal, int flags,
>  	struct sk_buff *skb = tcp_write_queue_tail(sk);
>  	struct tcp_sock *tp = tcp_sk(sk);
>  	bool can_coalesce;
> -	int copy, i;
> +	int copy = size_goal - skb->len;
> +	int i;
>  
> -	if (!skb || (copy = size_goal - skb->len) <= 0 ||
> -	    !tcp_skb_can_collapse_to(skb)) {
> +	if (!skb || copy <= 0 || !tcp_skb_can_collapse_to(skb)) {

	What is going to happen when you variant runs into
skb == NULL?  And if for some reason that cannot happen, where
is the explanation of that reason?

	IOW, this patch ends up quietly introducing a bug, with
no better rationale than "checkpatch.pl pointed me to that line".

NAKed-by: Al Viro <viro@zeniv.linux.org.uk>