From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [62.89.141.173])
	(using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
	(No client certificate requested)
	by smtp.subspace.kernel.org (Postfix) with ESMTPS id AA14C6FCD1
	for <llvm@lists.linux.dev>; Tue,  5 Dec 2023 21:57:35 +0000 (UTC)
Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zeniv.linux.org.uk
Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=ftp.linux.org.uk
Authentication-Results: smtp.subspace.kernel.org;
	dkim=pass (2048-bit key) header.d=linux.org.uk header.i=@linux.org.uk header.b="qlh/FNeD"
DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed;
	d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type:
	MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To:
	Content-Transfer-Encoding:Content-ID:Content-Description;
	bh=4R8XorxoFVOpovwERfSrloKkn2fMEhR5jZoBji1BSzM=; b=qlh/FNeDT4RM5b8OO4gTtNwyyw
	Yj566rfENDUFrZyrSODB7/RDqkMAoxkYS6Kqmvz/6ib39OUcGVHBY2AdXtFHtOrWZMpAmntVEYmeq
	Q4ox4UkGCyDPct/YjeoqTVzAak1qLHfUSr9eHYzJgCGeaSv/ofdJavh6WHUxRUviF/KOMiAeXuHxx
	cjvhQlTkXKiQ53swhQP37deCDEWQT51eQYoWRVjm+ZboLMI1xvqJT8iHwcV73gQGDV35Gccz1AUGz
	LGYZmNwp1qXNNKD2M9l634UMZw0J2f7EEHy8VVTMz28+ht0YI6Xh6xz2sEm6PBrlsNsU6Kl7peIep
	16Q8aXKQ==;
Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux))
	id 1rAdQ1-007Tjl-1a;
	Tue, 05 Dec 2023 21:57:29 +0000
Date: Tue, 5 Dec 2023 21:57:29 +0000
From: Al Viro <viro@zeniv.linux.org.uk>
To: tanzirh@google.com
Cc: Kees Cook <keescook@chromium.org>, Andy Shevchenko <andy@kernel.org>,
	linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
	Nick DeSaulniers <nnn@google.com>,
	Andrew Morton <akpm@linux-foundation.org>, llvm@lists.linux.dev
Subject: Re: [PATCH] lib/string: shrink lib/string.i via IWYU
Message-ID: <20231205215729.GG1674809@ZenIV>
References: <20231205-libstringheader-v1-1-7f9c573053a7@gmail.com>
 <20231205213807.GE1674809@ZenIV>
Precedence: bulk
X-Mailing-List: llvm@lists.linux.dev
List-Id: <llvm.lists.linux.dev>
List-Subscribe: <mailto:llvm+subscribe@lists.linux.dev>
List-Unsubscribe: <mailto:llvm+unsubscribe@lists.linux.dev>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20231205213807.GE1674809@ZenIV>
Sender: Al Viro <viro@ftp.linux.org.uk>

On Tue, Dec 05, 2023 at 09:38:07PM +0000, Al Viro wrote:

> It also breeds includes of asm/*.h, by the look of the output, which is
> not a good thing in general ;-/  E.g. #include <asm/uaccess.h> *anywhere*
> outside of linux/uaccess.h is a bad idea.

Let me elaborate a bit:

Consider e.g. a situation when we have linux/foo.h pulling asm/foo.h;
among the things declared there is foo_bar(), which is identical in
18 instances of asm/foo.h (out of current 21).

At some point we notice that; the obvious approach would be to have
3 unusual architectures have their asm/foo.h define _ARCH_HAS_ODD_FOO_BAR
and put the common variant in linux/foo.h under ifndef _ARCH_HAS_ODD_FOO_BAR.

Except that any file that pulls asm/foo.h will get broken by that.
And something like your tool might convert the users of linux/foo.h to
asm/foo.h - all it takes is using only the primitives that currently
happen to be in asm/foo.h on all architectures.

This is not an artifical example, BTW - look at the history of uaccess.h and
checksum.h...