mm: iov_iter: have iov_iter_advance() decrement nr_segs appropriately

Currently, when you call iov_iter_advance, then the pointer to the iovec
array can be incremented, but it does not decrement the nr_segs value in
the iov_iter struct.  The result is a iov_iter struct with a nr_segs value
that goes beyond the end of the array.

While I'm not aware of anything that's specifically broken by this, it
seems odd and a bit dangerous not to decrement that value.  If someone
were to trust the nr_segs value to be correct, then they could end up
walking off the end of the array.

Changing this might also provide some micro-optimization when dealing with
the last iovec in an array.  Many of the other routines that deal with
iov_iter have optimized codepaths when nr_segs == 1.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index e496e6ab6f02279328988eb532dece0c117043fd..edd58959ec4a34cfd927c11a7dd15e41fbee0d82 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2115,6 +2115,7 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
        } else {
                const struct iovec *iov = i->iov;
                size_t base = i->iov_offset;
+               unsigned long nr_segs = i->nr_segs;
 
                /*
                 * The !iov->iov_len check ensures we skip over unlikely
@@ -2130,11 +2131,13 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
                        base += copy;
                        if (iov->iov_len == base) {
                                iov++;
+                               nr_segs--;
                                base = 0;
                        }
                }
                i->iov = iov;
                i->iov_offset = base;
+               i->nr_segs = nr_segs;
        }
 }
 EXPORT_SYMBOL(iov_iter_advance);