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    Subject[PATCH] iov_iter: have iov_iter_advance decrement nr_segs appropriately
    Date
    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@suse.de>
    Signed-off-by: Jeff Layton <jlayton@redhat.com>
    ---
    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) {
    iov++;
    + nr_segs--;
    base = 0;
    }
    }
    i->iov = iov;
    i->iov_offset = base;
    + i->nr_segs = nr_segs;
    }
    }
    EXPORT_SYMBOL(iov_iter_advance);
    --
    1.7.6


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