From 5abebfdd02450fa1349daacf242e70b3736581e3 Mon Sep 17 00:00:00 2001 From: Kent Overstreet Date: Wed, 8 Feb 2012 22:07:18 +0100 Subject: [PATCH] bio: don't overflow in bio_get_nr_vecs() There were two places bio_get_nr_vecs() could overflow: First, it did a left shift to convert from sectors to bytes immediately before dividing by PAGE_SIZE. If PAGE_SIZE ever was less than 512 a great many things would break, so dividing by PAGE_SIZE >> 9 is safe and will generate smaller code too. The nastier overflow was in the DIV_ROUND_UP() (that's what the code was effectively doing, anyways). If n + d overflowed, the whole thing would return 0 which breaks things rather effectively. bio_get_nr_vecs() doesn't claim to give an exact value anyways, so the DIV_ROUND_UP() is silly; we could do a straight divide except if a device's queue_max_sectors was less than PAGE_SIZE we'd return 0. So we just add 1; this should always be safe - things will break badly if bio_get_nr_vecs() returns > BIO_MAX_PAGES (bio_alloc() will suddenly start failing) but it's queue_max_segments that must guard against this, if queue_max_sectors is preventing this from happen things are going to explode on architectures with different PAGE_SIZE. Signed-off-by: Kent Overstreet Cc: Tejun Heo Acked-by: Valdis Kletnieks Signed-off-by: Andrew Morton Signed-off-by: Jens Axboe --- fs/bio.c | 10 +++------- 1 file changed, 3 insertions(+), 7 deletions(-) diff --git a/fs/bio.c b/fs/bio.c index b1fe82cf88cf..b980ecde026a 100644 --- a/fs/bio.c +++ b/fs/bio.c @@ -505,13 +505,9 @@ EXPORT_SYMBOL(bio_clone); int bio_get_nr_vecs(struct block_device *bdev) { struct request_queue *q = bdev_get_queue(bdev); - int nr_pages; - - nr_pages = ((queue_max_sectors(q) << 9) + PAGE_SIZE - 1) >> PAGE_SHIFT; - if (nr_pages > queue_max_segments(q)) - nr_pages = queue_max_segments(q); - - return nr_pages; + return min_t(unsigned, + queue_max_segments(q), + queue_max_sectors(q) / (PAGE_SIZE >> 9) + 1); } EXPORT_SYMBOL(bio_get_nr_vecs); -- 2.30.2