Patch "[iov_iter] new privimitive: iov_iter_revert()" has been added to the 4.9-stable tree

Patch "[iov_iter] new privimitive: iov_iter_revert()" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    [iov_iter] new privimitive: iov_iter_revert()

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     new-privimitive-iov_iter_revert.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 27c0e3748e41ca79171ffa3e97415a20af6facd0 Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Fri, 17 Feb 2017 18:42:24 -0500
Subject: [iov_iter] new privimitive: iov_iter_revert()

From: Al Viro <viro@zeniv.linux.org.uk>

commit 27c0e3748e41ca79171ffa3e97415a20af6facd0 upstream.

opposite to iov_iter_advance(); the caller is responsible for never
using it to move back past the initial position.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 include/linux/uio.h |    6 ++++
 lib/iov_iter.c      |   63 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+), 1 deletion(-)

--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -39,7 +39,10 @@ struct iov_iter {
 	};
 	union {
 		unsigned long nr_segs;
-		int idx;
+		struct {
+			int idx;
+			int start_idx;
+		};
 	};
 };
 
@@ -81,6 +84,7 @@ unsigned long iov_shorten(struct iovec *
 size_t iov_iter_copy_from_user_atomic(struct page *page,
 		struct iov_iter *i, unsigned long offset, size_t bytes);
 void iov_iter_advance(struct iov_iter *i, size_t bytes);
+void iov_iter_revert(struct iov_iter *i, size_t bytes);
 int iov_iter_fault_in_readable(struct iov_iter *i, size_t bytes);
 size_t iov_iter_single_seg_count(const struct iov_iter *i);
 size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -734,6 +734,68 @@ void iov_iter_advance(struct iov_iter *i
 }
 EXPORT_SYMBOL(iov_iter_advance);
 
+void iov_iter_revert(struct iov_iter *i, size_t unroll)
+{
+	if (!unroll)
+		return;
+	i->count += unroll;
+	if (unlikely(i->type & ITER_PIPE)) {
+		struct pipe_inode_info *pipe = i->pipe;
+		int idx = i->idx;
+		size_t off = i->iov_offset;
+		while (1) {
+			size_t n = off - pipe->bufs[idx].offset;
+			if (unroll < n) {
+				off -= (n - unroll);
+				break;
+			}
+			unroll -= n;
+			if (!unroll && idx == i->start_idx) {
+				off = 0;
+				break;
+			}
+			if (!idx--)
+				idx = pipe->buffers - 1;
+			off = pipe->bufs[idx].offset + pipe->bufs[idx].len;
+		}
+		i->iov_offset = off;
+		i->idx = idx;
+		pipe_truncate(i);
+		return;
+	}
+	if (unroll <= i->iov_offset) {
+		i->iov_offset -= unroll;
+		return;
+	}
+	unroll -= i->iov_offset;
+	if (i->type & ITER_BVEC) {
+		const struct bio_vec *bvec = i->bvec;
+		while (1) {
+			size_t n = (--bvec)->bv_len;
+			i->nr_segs++;
+			if (unroll <= n) {
+				i->bvec = bvec;
+				i->iov_offset = n - unroll;
+				return;
+			}
+			unroll -= n;
+		}
+	} else { /* same logics for iovec and kvec */
+		const struct iovec *iov = i->iov;
+		while (1) {
+			size_t n = (--iov)->iov_len;
+			i->nr_segs++;
+			if (unroll <= n) {
+				i->iov = iov;
+				i->iov_offset = n - unroll;
+				return;
+			}
+			unroll -= n;
+		}
+	}
+}
+EXPORT_SYMBOL(iov_iter_revert);
+
 /*
  * Return the count of just the current iov_iter segment.
  */
@@ -787,6 +849,7 @@ void iov_iter_pipe(struct iov_iter *i, i
 	i->idx = (pipe->curbuf + pipe->nrbufs) & (pipe->buffers - 1);
 	i->iov_offset = 0;
 	i->count = count;
+	i->start_idx = i->idx;
 }
 EXPORT_SYMBOL(iov_iter_pipe);
 


Patches currently in stable-queue which might be from viro@zeniv.linux.org.uk are

queue-4.9/make-skb_copy_datagram_msg-et.al.-preserve-msg_iter-on-error.patch
queue-4.9/new-privimitive-iov_iter_revert.patch
queue-4.9/x86-pmem-fix-broken-__copy_user_nocache-cache-bypass-assumptions.patch