linux-kernel - Re: [GIT PULL] percpu fix for v5.9-rc6

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Message-ID: <20200918223957.GA2964553@rani.riverdale.lan>
Date:   Fri, 18 Sep 2020 18:39:57 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Matthew Wilcox <willy@...radead.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [GIT PULL] percpu fix for v5.9-rc6

On Fri, Sep 18, 2020 at 02:18:20PM -0700, Linus Torvalds wrote:
> On Fri, Sep 18, 2020 at 2:00 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > You could just assert that offsetof(typeof(s),flex) == sizeof(s), no?
> 
> No, because the whole point is that I want that "sizeof(s)" to *WARN*.
> 
> It's a nonsensical thing to do. That 's' has no statically known size.
> 
> The C standard is being very confused here, in that it tries to claim
> that the flexible arrays are somehow fundamentally different from a
> zero-sized one. But then it acts as if they are exactly the same wrt
> sizeof() and structure copies.
> 
> It should warn, exactly because right now it causes potential bugs
> like the one that started this thread.
> 
> You can't have both "zero-sized arrays are bad and shouldn't be used"
> and "flexible arrays are good, and work exactly like zero-sized
> arrays".
> 
> Either zero-sized arrays are bad or they aren't. And if they are bad,
> then flexible arrays shouldn't work *exactly* like them apart from
> some UBSAN warnings.
> 
> See my point?
> 
>              Linus

Ouch, offsetof() and sizeof() will give different results in the
presence of alignment padding.

https://godbolt.org/z/rqnxTK

I think, grepping at random, that at least struct scsi_vpd is like this,
size is 24 but data[] starts at offset 20.

	struct scsi_vpd {
		struct rcu_head	rcu;
		int		len;
		unsigned char	data[];
	};