new privimitive: iov_iter_revert()

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>

2 files changed, 68 insertions, 1 deletions

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 6e22b544d039..c146ebc69c53 100644
--- 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 *iov, unsigned long nr_segs, size_t to);
 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,
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index efb0b4d267a1..a75ea633b5c4 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -734,6 +734,68 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
 }
 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, int direction,
         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);