netdev - Re: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *

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Date:   Tue, 5 Oct 2021 23:09:08 +0200 (CEST)
From:   Jan Engelhardt <jengelh@...i.de>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, rcu <rcu@...r.kernel.org>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        coreteam <coreteam@...filter.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *


On Tuesday 2021-10-05 22:37, Steven Rostedt wrote:
>
>Really, thinking about abstraction, I don't believe there's anything wrong
>with returning a pointer of one type, and then typecasting it to a pointer
>of another type. Is there? As long as whoever uses the returned type does
>nothing with it.

Illegal.
https://en.cppreference.com/w/c/language/conversion
subsection "Pointer conversion"
"No other guarantees are offered"

>struct trace_pid_list *trace_pid_list_alloc(void)
>{
>	struct pid_list *pid_list;
>
>	pid_list = kmalloc(sizeof(*pid_list), GFP_KERNEL);
>	[..]
>
>	return (struct trace_pid_list *)pid_list;
>}

struct trace_pid_list { void *pid_list; };
struct trace_pid_list trace_pid_list_alloc(void)
{
	struct trace_pid_list t;
	t.pid_list = kmalloc(sizeof(t.orig), GFP_KERNEL);
	return t;
}
void freethat(struct strace_pid_list x)
{
	kfree(x.pid_list);
}

Might run afoul of -Waggregate-return in C.