linux-kernel - Re: [PATCH] io_uring/net: fix build warning for !CONFIG

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Message-Id: <9ef40a88-6243-4baf-8774-e4b72bfbb2f3@app.fastmail.com>
Date: Thu, 27 Feb 2025 14:58:55 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Pavel Begunkov" <asml.silence@...il.com>,
 "Arnd Bergmann" <arnd@...nel.org>, "Jens Axboe" <axboe@...nel.dk>
Cc: "Gabriel Krisman Bertazi" <krisman@...e.de>, "David Wei" <dw@...idwei.uk>,
 io-uring@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] io_uring/net: fix build warning for !CONFIG_COMPAT

On Thu, Feb 27, 2025, at 14:49, Pavel Begunkov wrote:
> On 2/27/25 13:20, Arnd Bergmann wrote:
>>        |                      ^~~
>> 
>> Since io_is_compat() turns into a compile-time 'false', the #ifdef
>> here is completely unnecessary, and removing it avoids the warning.
>
> I don't think __get_compat_msghdr() and other helpers are
> compiled for !COMPAT.

They are not defined without CONFIG_COMPAT. My point in the
message is that io_is_compat() turning into a compile-time
'false' value means that they also don't get called, because
compilers are really good at this type of dead code elimination.

> I'd just silence it like:
>
> if (io_is_compat(req->ctx)) {
> 	ret = -EFAULT;
> #ifdef CONFIG_COMPAT
> 	...
> #endif CONFIG_COMPAT
> }

That seems even less readable. If you want to be explicit
about it, you could use

     if (IS_ENABLED(CONFIG_COMPAT) && io_is_compat(req->ctx)) {

to replace the #ifdef, but as I wrote in the patch
description, the compile-time check is really redundant
because io_is_compat() is meant to do exactly that.

       Arnd