vfs: 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. Cc: Nick Piggin <npiggin@suse.de> Signed-off-by: NJeff Layton <jlayton@redhat.com> Signed-off-by: NChristoph Hellwig <hch@lst.de>
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