linux-kernel - Re: [RFC][PATCH 10/13] make dump_emit() use vfs_write() instead of banging at ->f

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Message-ID: <CA+55aFz4JWB65bQRu7J6vQt7NPOoGKTxCYW4GQ8o5PHTZsYWKg@mail.gmail.com>
Date:	Tue, 8 Oct 2013 19:27:15 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Al Viro <viro@....linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 10/13] make dump_emit() use vfs_write() instead of
 banging at ->f_op->write directly

On Tue, Oct 8, 2013 at 7:06 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Oct 08, 2013 at 06:38:47PM -0700, Linus Torvalds wrote:
>> On Tue, Oct 8, 2013 at 6:18 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> >
>> > Point, but I would argue that we should yell very loud if we get 0 from
>> > vfs_write() for non-zero size.  I'm not sure if POSIX allows write(2)
>> > to return that, but a lot of userland code won't be expecting that and
>> > won't be able to cope...
>>
>> Actually POSIX very much allows zero returns. O_NDELAY is mentioned as
>> a possible cause, in addition to zero-sized writes themselves, of
>> course.
>
> Umm...  What it says is "If some data can be written without blocking the
> thread, write() shall write what it can and return the number of bytes
> written. Otherwise, it shall return -1 and set errno to EAGAIN."

Look closer.

  ".. most historical implementations return zero (with the O_NDELAY
flag set, which is the historical predecessor of O_NONBLOCK .."

>> Also, writing to (but not past) the end of a block device returns 0
>> for "end of device", iirc.
>
> What do you mean?  If the starting position is below the end of device,
> we get a non-zero length write, not exceeding the end.  If it's at
> the end of device, we get -ENOSPC.  It's out of scope for POSIX, but
> Linux is definitely acting that way...

Hmm. I'm pretty sure I've seen zero returns for EOF somewhere..

         Linus
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