linux-kernel - Re: [PATCH] vfs: Tighten up linkat(..., AT_EMPTY

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Message-ID: <CALCETrXtJBDqkvO3mHxDrHL3whCWX_B-LAXOnue7r90nyyWY_A@mail.gmail.com>
Date:	Wed, 21 Aug 2013 11:43:05 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"security@...nel.org" <security@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Willy Tarreau <w@....eu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Brad Spengler <spender@...ecurity.net>
Subject: Re: [PATCH] vfs: Tighten up linkat(..., AT_EMPTY_PATH)

On Wed, Aug 21, 2013 at 11:20 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> Can't really comment the patch, just a nit:
>
> On 08/21, Andy Lutomirski wrote:
>>
>> +static bool may_flink(const struct path *path)
>> +{
>> +     bool ret;
>> +     struct inode *inode = path->dentry->d_inode;
>> +
>> +     /*
>> +      * This is racy: I_LINKABLE could be cleared between this check
>> +      * and the actual link operation.
>
> OK,
>
>> +     spin_lock(&inode->i_lock);
>> +     ret = !!(inode->i_state & I_LINKABLE);
>> +     spin_unlock(&inode->i_lock);
>
> so why do we need to take a lock ?
>

We probably don't.  But other accesses to this field take that lock,
so it seemed safer.

(In principle, someone could take the lock, write I_LINKABLE, clear
it, and unlock, and we'd get confused if we didn't take the lock
ourselves.)

> Oleg.
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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