From: Michael Haggerty <mhagger@alum.mit.edu>
Date: Fri, 14 Jun 2013 07:17:49 GMT
Subject: Re: [PATCH 4/4] resolve_ref_unsafe(): close race condition reading loose refs
Message-ID: <51BAC39D.5030201@alum.mit.edu>
In-Reply-To: <87d2rqs9ma.fsf@linux-k42r.v.cablecom.net>

On 06/13/2013 10:22 AM, Thomas Rast wrote:
> Michael Haggerty <mhagger@alum.mit.edu> writes:
> 
>> One race is still possible and undetected: another process could
>> change the file from a regular file into a symlink between the call to
>> lstat and the call to open().  The open() call would silently follow
>> the symlink and not know that something is wrong.  I don't see a way
>> to detect this situation without the use of the O_NOFOLLOW option,
>> which is not portable and is not used elsewhere in our code base.
>>
>> However, we don't use symlinks anymore, so this situation is unlikely.
>> And it doesn't appear that treating a symlink as a regular file would
>> have grave consequences; after all, this is exactly how the code
>> handles non-relative symlinks.
> 
> You could fstat() the fd you got from open(), and verify that it is
> still the same inode/device.  That's wasting one syscall per ref for
> pretty much everyone, but perhaps if we really cared about this (and I
> gather from the above that we don't), we could conditionally use
> O_NOFOLLOW if available, otherwise do that fstat().

Yes, that would work.  For now I think I will not worry about it, but
I'll keep your trick in mind.

Thanks,
Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/