linux-kernel - Re: [PATCH 2/2] Perform check in iov_iter_fault_in_readable() by check_readable

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Message-ID: <871vqqkdf0.wl%vmayatsk@redhat.com>
Date:	Fri, 15 May 2009 09:56:03 +0200
From:	Vitaly Mayatskikh <v.mayatskih@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Vitaly Mayatskikh <v.mayatskih@...il.com>,
	Josef Bacik <josef@...hat.com>, sandeen@...hat.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] Perform check in iov_iter_fault_in_readable() by check_readable_bytes()

At Fri, 15 May 2009 08:56:00 +0200, Andi Kleen wrote:
 
> Can you describe how it makes it POSIX friendly?
> 
> My understanding was that EFAULT behaviour was undefined in POSIX.
> 
> The obvious hole in the patch is that all these checks are not race
> free -- they don't pin pages -- so if there's a parallel unmap even
> with your change they can still fail in the middle.


ptr = mmap(0, page_size, ....);
...
write(fd, ptr + page_size - 256, 512);

Write() will fail here, but it can write first 256 bytes. Previously,
all 512 bytes were proceeded, but last 256 bytes were zeroed, and
sys_write() returned 256. Not very nice too. 

-- 
wbr, Vitaly
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