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Date:	Thu, 23 Oct 2014 15:56:33 +0300
From:	Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Tejun Heo <tj@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] fs: sysfs: return EFBIG on write if offset is
 larger than binary file size

Hello Greg,

On 09.10.2014 21:46, Tejun Heo wrote:
> 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.

have you had time to look at the problem? Understanding the risk of the
change, should the inconsistency with POSIX be documented in
Documentation/ABI/stable/syscalls instead?

With best wishes,
Vladimir
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