linux-kernel - Re: [PATCH] dcache: fix locking bug in __dentry

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Message-ID: <52E26DDD.90702@redhat.com>
Date:	Fri, 24 Jan 2014 14:42:53 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dcache: fix locking bug in __dentry_path()

On 01/24/2014 01:24 PM, Al Viro wrote:
> On Fri, Jan 24, 2014 at 11:21:26AM +0100, Denys Vlasenko wrote:
>> If given buffer size is zero, we forget to rcu_read_unlock()
>> on error path.
> 
> Er...  Where could we ever get called with size zero?  IOW, the
> real question is whether that check makes any sense.  If nothing
> else, a much more obvious fix (again, assuming there is a call
> chain that could have triggered that in the first place), would
> be to take that
>         if (buflen < 1)
>                 goto Elong;
> on the very top of __dentry_path().  Note that buflen never
> change, so this check could bloody well be handled once - and
> earlier.

Yes. In fact the check is buggier than I thought:

        prepend(&end, &len, "\0", 1);
        if (buflen < 1)
                goto Elong;
        /* Get '/' right */
        retval = end-1;
        *retval = '/';   <=== !!!

It should be "buflen < 2". With buflen == 1, we touch buffer[-1].

> But again, the callers shouldn't be calling it that
> way...

I'll add a check in d_path() and post another patch.
I assume the assertion should be 'size >= 2', since
path "" should never be produced anyway, the shortest path
is "/"?

-- 
vda


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