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SubjectRe: [PATCH v2 0/2] iter revert problems
On Sat, Aug 21, 2021 at 03:24:28PM +0100, Pavel Begunkov wrote:
> On 8/12/21 9:40 PM, Pavel Begunkov wrote:
> > For the bug description see 2/2. As mentioned there the current problems
> > is because of generic_write_checks(), but there was also a similar case
> > fixed in 5.12, which should have been triggerable by normal
> > write(2)/read(2) and others.
> >
> > It may be better to enforce reexpands as a long term solution, but for
> > now this patchset is quickier and easier to backport.
>
> We need to do something with this, hopefully soon.

I still don't like that approach ;-/ If anything, I would rather do
something like this, and to hell with one extra word on stack in
several functions; at least that way the semantics is easy to describe.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/fs/io_uring.c b/fs/io_uring.c
index d94fb5835a20..5501f8b3af3b 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3420,6 +3420,7 @@ static int io_write(struct io_kiocb *req, unsigned int issue_flags)
} else {
copy_iov:
/* some cases will consume bytes even on error returns */
+ iov_iter_reexpand(iter, iter->count + iter->truncated);
iov_iter_revert(iter, io_size - iov_iter_count(iter));
ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
return ret ?: -EAGAIN;
diff --git a/include/linux/uio.h b/include/linux/uio.h
index 82c3c3e819e0..5265024e8b90 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -47,6 +47,7 @@ struct iov_iter {
};
loff_t xarray_start;
};
+ size_t truncated;
};

static inline enum iter_type iov_iter_type(const struct iov_iter *i)
@@ -254,8 +255,10 @@ static inline void iov_iter_truncate(struct iov_iter *i, u64 count)
* conversion in assignement is by definition greater than all
* values of size_t, including old i->count.
*/
- if (i->count > count)
+ if (i->count > count) {
+ i->truncated += i->count - count;
i->count = count;
+ }
}

/*
@@ -264,6 +267,7 @@ static inline void iov_iter_truncate(struct iov_iter *i, u64 count)
*/
static inline void iov_iter_reexpand(struct iov_iter *i, size_t count)
{
+ i->truncated -= count - i->count;
i->count = count;
}

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