linux-kernel - Re: fs: Use non-const iov in aio_read/aio

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Message-ID: <20141103001634.GV7996@ZenIV.linux.org.uk>
Date:	Mon, 3 Nov 2014 00:16:34 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: Re: fs: Use non-const iov in aio_read/aio_write

On Mon, Nov 03, 2014 at 07:05:52AM +0800, Herbert Xu wrote:
> Currently the functions aio_read/aio_write use a const iov as
> input.  This is unnecessary as all their callers supply a
> stack-based or kmalloced iov which is never reused.  Conceptually
> this is fine because iovs supplied to aio_read/aio_write ultimately
> come from user-space so we always have to make a copy of them for
> the kernel.
> 
> This is also a joke because for as long (since 2.1.15) as we've
> had the const iov, the network stack (currently through do_sock_read
> and do_sock_write) has been casting the const away.  IOW if anybody
> did supply a const iov they would crash and burn if they ever
> entered the network stack.
> 
> The network stack needs a non-const iov because it iterates through
> the iov as it reads/writes data.
> 
> So we have two alternatives, either change the network stack to
> not touch the iovs or make the iovs non-const.
> 
> As there is no reason for the iovs to be const in the first place,
> I have taken the second choice and changed all aio_read/aio_write
> functions to use non-const iovs.

NAK with extreme prejudice.  The right way to deal with that is
to convert the socket side of things to iov_iter.  And give it a
consistent behaviour, while we are at it (some protocols do advance
the damn thing, so do not).  There are _very_ good reasons to have those
iovecs unchanged - if you look at the callers on the socket side, you'll
see a bunch that has to _copy_ iovec just to avoid it being buggered.
And you get rather suboptimal behaviour in memcpy_fromiovec() and friends,
exactly because you have to skip through the emptied elements.

IOW, no way in hell.
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