linux-kernel - Re: [PATCH RESEND] fs: sysfs: return EFBIG on write if offset is larger than binary file size

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Date:	Thu, 9 Oct 2014 14:46:31 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
Cc:	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH RESEND] fs: sysfs: return EFBIG on write if offset is
 larger than binary file size

Hello, Vladimir.

On Thu, Oct 09, 2014 at 08:41:55PM +0300, Vladimir Zapolskiy wrote:
> According to the user expectations common utilities like dd or sh
> redirection operator '>' should work correctly over binary files from
> sysfs. At the moment doing excessive write can not be ever completed
> (no error is returned), e.g. for 4-byte file:
> 
>   write(1, "\0\0\0\0\0\0\0\0", 8)         = 4
>   write(1, "\0\0\0\0", 4)                 = 0
>   write(1, "\0\0\0\0", 4)                 = 0
>   write(1, "\0\0\0\0", 4)                 = 0
>   write(1, "\0\0\0\0", 4)                 = 0
>   write(1, "\0\0\0\0", 4)                 = 0
>   .......
> 
> This is not a successful completion of write(2), so fix the problem by
> returning EFBIG as described in POSIX.1-2001:
> 
>   [EFBIG]
>     The file is a regular file, nbyte is greater than 0, and the
>     starting position is greater than or equal to the offset maximum
>     established in the open file description associated with fildes.
> 
> Note, the write(2) ABI is changed, however
> 1) write(2) behaviour is corrected in conformance to POSIX, the
>    existing userspace applications must be aware of possible errors on
>    a syscall,
> 2) the return value is changed on error path, so it is an exceptional
>    situation,
> 3) the change is related only to binary sysfs files, which is a very
>    small class of files, mainly representing non-volatile registers or
>    ram, eeproms etc, many of such files are read-only.
> 
> Presumably it is safe to apply the change, the described problem is
> definitely in the kernel and userspace utilities can not be changed to
> process 0 return value as an error, because it is just not an error
> according to POSIX.
> 
> Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Tejun Heo <tj@...nel.org>

This is a bit risky but the current behavior is problematic and as you
pointed out the danger of actual breakge is relatively low.  We might
as well give it a shot.

 Cautiously-acked-by: Tejun Heo <tj@...nel.org>

Please also cc stable.

Thanks.

-- 
tejun
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