Re: [PATCH 08/23] fs: don't change the address limit for ->write_iter in __kernel_write

[Al Viro]

On Tue, Jul 07, 2020 at 07:47:46PM +0200, Christoph Hellwig wrote:
> If we write to a file that implements ->write_iter there is no need
> to change the address limit if we send a kvec down.  Implement that
> case, and prefer it over using plain ->write with a changed address
> limit if available.

	You are flipping the priorities of ->write and ->write_iter
for kernel_write().  Now, there are 4 instances of file_operations
where we have both.  null_fops and zero_fops are fine either way -
->write() and ->write_iter() do the same thing there (and arguably
removing ->write might be the right thing; the only reason I hesistate
is that writing to /dev/null *is* critical for many things, including
the proper mail delivery ;-)

However, the other two (infinibarf and pcm) are different; there we
really have different semantics.  I don't believe anything writes into
either under KERNEL_DS, but having kernel_write() and vfs_write() with
subtly different semantics is asking for trouble down the road.

How about we remove ->write in null_fops/zero_fops and fail loudly if
*both* ->write() and ->write_iter() are present (in kernel_write(),
that is)?

There's a similar situation on the read side - there we have /dev/null
with both ->read() and ->read_iter() (and there "remove ->read" is
obviously the right thing to do) *and* we have pcm crap, with different
semantics for ->read() and ->read_iter().