Re: null_handle_cmd() doesn't initialize data when reading

[Alexander Potapenko <glider@google.com>]

>
> Can you describe again the problem you are seeing please ? I can't find the
> first email of this thread and forgot what the problem is.
>

Yes, sorry.

The original message was:
>> I'm debugging an issue in nullb driver reported by KMSAN at QEMU startup.
>> There are numerous reports like the one below when checking nullb for
>> different partition types.
>> Basically, read_dev_sector() allocates a cache page which is then
>> wrapped into a bio and passed to the device driver, but never
>> initialized.

>> I've tracked the problem down to a call to null_handle_cmd(cmd,
>> /*sector*/0, /*nr_sectors*/8, /*op*/0).
>> Turns out all the if-branches in this function are skipped, so neither
>> of null_handle_throttled(), null_handle_flush(),
>> null_handle_badblocks(), null_handle_memory_backed(),
>> null_handle_zoned() is executed, and we proceed directly to
>> nullb_complete_cmd().

>> As a result, the pages read from the nullb device are never
>> initialized, at least at boot time.
>> How can we fix this?

Today null_handle_cmd() looks different, but the problem is still manifesting.
KMSAN reports look as follows:

=====================================================
BUG: KMSAN: uninit-value in adfspart_check_ICS+0xb8e/0xde0
CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.7.0-rc4+ #4177
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:77
 dump_stack+0x1c9/0x220 lib/dump_stack.c:118
 kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
 __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
 adfspart_check_ICS+0xb8e/0xde0 block/partitions/acorn.c:364
 check_partition block/partitions/core.c:140
 blk_add_partitions+0x86a/0x2560 block/partitions/core.c:571
 bdev_disk_changed+0x5c2/0xa30 fs/block_dev.c:1543
 __blkdev_get+0x1195/0x2280 fs/block_dev.c:1646
 blkdev_get+0x219/0x6b0 fs/block_dev.c:1748
 register_disk block/genhd.c:763
 __device_add_disk+0x15b5/0x20a0 block/genhd.c:853
 device_add_disk+0x90/0xa0 block/genhd.c:871
 add_disk ./include/linux/genhd.h:294
 null_gendisk_register drivers/block/null_blk_main.c:1628
 null_add_dev+0x2eaa/0x35d0 drivers/block/null_blk_main.c:1803
 null_init+0x6c5/0xd8d drivers/block/null_blk_main.c:1888
 do_one_initcall+0x4c9/0x930 init/main.c:1160
...
Uninit was created at:
 kmsan_save_stack_with_flags+0x3c/0x90 mm/kmsan/kmsan.c:144
 kmsan_internal_alloc_meta_for_pages mm/kmsan/kmsan_shadow.c:280
 kmsan_alloc_page+0xb9/0x180 mm/kmsan/kmsan_shadow.c:304
 __alloc_pages_nodemask+0xc0e/0x5dd0 mm/page_alloc.c:4848
 __alloc_pages ./include/linux/gfp.h:504
 alloc_page_interleave mm/mempolicy.c:2161
 alloc_pages_current+0x2e7/0x990 mm/mempolicy.c:2293
 alloc_pages ./include/linux/gfp.h:540
 __page_cache_alloc+0x95/0x310 mm/filemap.c:959
 do_read_cache_page+0x293/0x1510 mm/filemap.c:2752
 read_cache_page+0xf3/0x110 mm/filemap.c:2867
 read_mapping_page ./include/linux/pagemap.h:397
 read_part_sector+0x156/0x560 block/partitions/core.c:643
 adfspart_check_ICS+0xa0/0xde0 block/partitions/acorn.c:360
 check_partition block/partitions/core.c:140
 blk_add_partitions+0x86a/0x2560 block/partitions/core.c:571
 bdev_disk_changed+0x5c2/0xa30 fs/block_dev.c:1543
 __blkdev_get+0x1195/0x2280 fs/block_dev.c:1646
 blkdev_get+0x219/0x6b0 fs/block_dev.c:1748
 register_disk block/genhd.c:763
 __device_add_disk+0x15b5/0x20a0 block/genhd.c:853
 device_add_disk+0x90/0xa0 block/genhd.c:871
 add_disk ./include/linux/genhd.h:294
 null_gendisk_register drivers/block/null_blk_main.c:1628
 null_add_dev+0x2eaa/0x35d0 drivers/block/null_blk_main.c:1803
 null_init+0x6c5/0xd8d drivers/block/null_blk_main.c:1888
 do_one_initcall+0x4c9/0x930 init/main.c:1160
=====================================================