[iov_iter] new privimitive: iov_iter_revert()

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

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 804e34c6f981de7402ba6a39bd0375b3a3dd0efe..f2d36a3d30052db827fab68e9278fc1b693068ca 100644 (file)
--- 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 e68604ae3cedf41ce98bc06de2142629fa115cbd..60abc44385b795de7449b3810e9e294b7d2c48fc 100644 (file)
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -786,6 +786,68 @@ 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.
  */
@@ -839,6 +901,7 @@ 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);