[PATCH] iov_iter: have iov_iter_advance decrement nr_segs appropriately

From: Jeff Layton
Date: Mon Aug 15 2011 - 14:07:54 EST

Currently, when you call iov_iter_advance, then the pointer to the kvec
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 kvec in an array. Many of the other routines that deal
with iov_iter have optimized codepaths when nr_segs == 1.

Cc: Nick Piggin <npiggin@xxxxxxx>
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
mm/filemap.c | 3 +++
1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 645a080..86508cc 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2113,6 +2113,7 @@ 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
@@ -2128,11 +2129,13 @@ void iov_iter_advance(struct iov_iter *i, size_t bytes)
base += copy;
if (iov->iov_len == base) {
+ nr_segs--;
base = 0;
i->iov = iov;
i->iov_offset = base;
+ i->nr_segs = nr_segs;

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