linux-kernel - Re: [PATCH 3/5] fs: Add DEFINE

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Date:   Tue, 5 Dec 2023 12:38:35 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-fsdevel@...r.kernel.org, Tony Luck <tony.luck@...el.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 3/5] fs: Add DEFINE_FREE for struct inode

On Sat, Dec 02, 2023 at 01:34:32PM -0800, Kees Cook wrote:
> On Sat, Dec 02, 2023 at 09:28:46PM +0000, Al Viro wrote:
> > On Sat, Dec 02, 2023 at 01:22:13PM -0800, Kees Cook wrote:
> > > Allow __free(iput) markings for easier cleanup on inode allocations.
> > 
> > NAK.  That's a bloody awful idea for that particular data type, since
> > 	1) ERR_PTR(...) is not uncommon and passing it to iput() is a bug.
> 
> Ah, sounds like instead of "if (_T)", you'd rather see
> "if (!IS_ERR_OR_NULL(_T))" ?
> 
> > 	2) the common pattern is to have reference-consuming primitives,
> > with failure exits normally *not* having to do iput() at all.
> 
> This I'm not following. If I make a call to "new_inode(sb)" that I end
> up not using, I need to call "iput()" in it...

If we wanted to do this properly then we would need to emulate consume
or move semantics like Rust has. So a cleanup function for inodes based
on scope for example and then another primitive that transfers/moves
ownership of that refcount to the consumer. Usually this is emulate by
stuff like TAKE_POINTER() and similar stuff in userspace. But I'm not
sure how pleasant it would be to do this cleanly.