Messages in this thread Patch in this message | | | Date | Mon, 14 Jan 2002 06:50:13 -0800 | From | "Adam J. Richter" <> | Subject | PATCH: linux-2.5.2-pre11/drivers/block/loop.c -- garbage reads |
| |
linux-2.5.2-pre11/drivers/block/loop.c returned incorrect data for reads from any loop device that was mounted on a block device, such as a disk partition. The bug is the problem that I asked about yesterday where do_bio_blockbacked in loop.c would transfer no data because bio->bi_idx was nonzero (or, more precisely, always equal to bio->bi_vcnt). Here is a test case that reproduces the problem:
#!/bin/sh # testdev can be any existant device that has a first sector # that is not all zeroes. We only need read access to it for this test. testdev=/dev/discs/disc0/part1 set -e dd count=1 if=$testdev of=/tmp/first_sector losetup /dev/loop/0 $testdev dd count=1 if=/dev/loop/0 of=/tmp/first_sector.loop losetup -d /dev/loop/0 cmp /tmp/first_sector /tmp/first_sector.loop
It turns out that the value of bio->bi_idx is correct. The "bio" variable in that routine refers to the I/O operation on the underlying device (not the loop device), and the I/O operation has completed when do_bio_blockbacked has been called for a READ. do_bio_blockbacked should be looking at rbh->bi_idx instead (rbh is the operation on /dev/loop/nnn, which has not yet completed). I have attached a patch below which does this, and I have verified that it fixes the test case and have used it in mounting a filesystem as well.
By the way, there are plenty of other problems with loop.c, such as the fact that it hangs my note book computer in "disk I/O" (presumably to /dev/loop/0) every time it boots, whereas I don't think I've seen that happen at all under 2.5.1-pre1 (although I have generally seen hangs about once every three weeks since I went to an encrypted root).
-- Adam J. Richter __ ______________ 4880 Stevens Creek Blvd, Suite 104 adam@yggdrasil.com \ / San Jose, California 95129-1034 +1 408 261-6630 | g g d r a s i l United States of America fax +1 408 261-6631 "Free Software For The Rest Of Us." --- linux-2.5.2-pre11/drivers/block/loop.c Mon Jan 14 05:48:01 2002 +++ linux/drivers/block/loop.c Mon Jan 14 06:13:33 2002 @@ -488,15 +488,15 @@ struct bio *rbh) { unsigned long IV = loop_get_iv(lo, rbh->bi_sector); - struct bio_vec *to; + struct bio_vec *from; char *vto, *vfrom; int ret = 0, i; - bio_for_each_segment(to, bio, i) { - vfrom = page_address(rbh->bi_io_vec[i].bv_page) + rbh->bi_io_vec[i].bv_offset; - vto = page_address(to->bv_page) + to->bv_offset; + bio_for_each_segment(from, rbh, i) { + vfrom = page_address(from->bv_page) + from->bv_offset; + vto = page_address(bio->bi_io_vec[i].bv_page) + bio->bi_io_vec[i].bv_offset; ret |= lo_do_transfer(lo, bio_data_dir(bio), vto, vfrom, - to->bv_len, IV); + from->bv_len, IV); } return ret; | |