linux-kernel - Re: [PATCH v2] uapi: fix __DECLARE_FLEX

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Message-ID: <202309080910.44BB7CEF@keescook>
Date:   Fri, 8 Sep 2023 09:11:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] uapi: fix __DECLARE_FLEX_ARRAY for C++

On Fri, Sep 08, 2023 at 07:03:17PM +0300, Alexey Dobriyan wrote:
> On Fri, Sep 08, 2023 at 08:53:12AM -0700, Kees Cook wrote:
> > On Fri, Sep 08, 2023 at 06:14:38PM +0300, Alexey Dobriyan wrote:
> > > __DECLARE_FLEX_ARRAY(T, member) macro expands to
> > > 
> > > 	struct {
> > > 		struct {} __empty_member;
> > > 		T member[];
> > > 	};
> > > 
> > > which is subtly wrong in C++ because sizeof(struct{}) is 1 not 0,
> > 
> > Ewwww. Isn't this a bug in C++?
> 
> Sort of, but it can't be fixed.
> 
> > > changing UAPI structures layouts.
> > > 
> > > This can be fixed by expanding to
> > > 
> > > 	T member[];
> > > 
> > > Now g++ doesn't like "T member[]" either throwing errors on code like
> > > this:
> > > 
> > > 	struct S {
> > > 		union {
> > > 			T1 member1[];
> > > 			T2 member2[];
> > > 		};
> > > 	};
> > > 
> > > or
> > > 
> > > 	struct S {
> > > 		T member[];
> > > 	};
> > > So use
> > > 
> > > 	T member[0];
> > > 
> > > which seems to work and does the right thing wrt structure layout.
> > 
> > It seems sad to leave C++ broken, but I guess we have to do this.
> > 
> > Acked-by: Kees Cook <keescook@...omium.org>
> > 
> > > Fix header guard while I'm at it.
> > 
> > Hm, when did that get broken? Maybe that should be fixed separately?
> 
> By your last commit?

:( Oops. I'm shocked this hasn't caused bigger problems.

> 
> > > Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> > 
> > Probably a Fixes: tag would be nice too.
> 
> OK
> 
> Fixes: c8248faf3ca2 ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
> Fixes: 3080ea5553cc ("stddef: Introduce DECLARE_FLEX_ARRAY() helper")

Okay, can you please split the patch so they can be backported
separately? Then I'll get them landed, etc.

Thanks for fixing these!

-Kees

-- 
Kees Cook