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

From: Tejun Heo
Date: Thu Oct 09 2014 - 14:46:42 EST

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:
> 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@xxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>

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@xxxxxxxxxx>

Please also cc stable.


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