• Richard Weinberger's avatar
    UBI: Fix static volume checks when Fastmap is used · 1900149c
    Richard Weinberger authored
    Ezequiel reported that he's facing UBI going into read-only
    mode after power cut. It turned out that this behavior happens
    only when updating a static volume is interrupted and Fastmap is
    used.
    
    A possible trace can look like:
    ubi0 warning: ubi_io_read_vid_hdr [ubi]: no VID header found at PEB 2323, only 0xFF bytes
    ubi0 warning: ubi_eba_read_leb [ubi]: switch to read-only mode
    CPU: 0 PID: 833 Comm: ubiupdatevol Not tainted 4.6.0-rc2-ARCH #4
    Hardware name: SAMSUNG ELECTRONICS CO., LTD. 300E4C/300E5C/300E7C/NP300E5C-AD8AR, BIOS P04RAP 10/15/2012
    0000000000000286 00000000eba949bd ffff8800c45a7b38 ffffffff8140d841
    ffff8801964be000 ffff88018eaa4800 ffff8800c45a7bb8 ffffffffa003abf6
    ffffffff850e2ac0 8000000000000163 ffff8801850e2ac0 ffff8801850e2ac0
    Call Trace:
    [<ffffffff8140d841>] dump_stack+0x63/0x82
    [<ffffffffa003abf6>] ubi_eba_read_leb+0x486/0x4a0 [ubi]
    [<ffffffffa00453b3>] ubi_check_volume+0x83/0xf0 [ubi]
    [<ffffffffa0039d97>] ubi_open_volume+0x177/0x350 [ubi]
    [<ffffffffa00375d8>] vol_cdev_open+0x58/0xb0 [ubi]
    [<ffffffff8124b08e>] chrdev_open+0xae/0x1d0
    [<ffffffff81243bcf>] do_dentry_open+0x1ff/0x300
    [<ffffffff8124afe0>] ? cdev_put+0x30/0x30
    [<ffffffff81244d36>] vfs_open+0x56/0x60
    [<ffffffff812545f4>] path_openat+0x4f4/0x1190
    [<ffffffff81256621>] do_filp_open+0x91/0x100
    [<ffffffff81263547>] ? __alloc_fd+0xc7/0x190
    [<ffffffff812450df>] do_sys_open+0x13f/0x210
    [<ffffffff812451ce>] SyS_open+0x1e/0x20
    [<ffffffff81a99e32>] entry_SYSCALL_64_fastpath+0x1a/0xa4
    
    UBI checks static volumes for data consistency and reads the
    whole volume upon first open. If the volume is found erroneous
    users of UBI cannot read from it, but another volume update is
    possible to fix it. The check is performed by running
    ubi_eba_read_leb() on every allocated LEB of the volume.
    For static volumes ubi_eba_read_leb() computes the checksum of all
    data stored in a LEB. To verify the computed checksum it has to read
    the LEB's volume header which stores the original checksum.
    If the volume header is not found UBI treats this as fatal internal
    error and switches to RO mode. If the UBI device was attached via a
    full scan the assumption is correct, the volume header has to be
    present as it had to be there while scanning to get known as mapped.
    If the attach operation happened via Fastmap the assumption is no
    longer correct. When attaching via Fastmap UBI learns the mapping
    table from Fastmap's snapshot of the system state and not via a full
    scan. It can happen that a LEB got unmapped after a Fastmap was
    written to the flash. Then UBI can learn the LEB still as mapped and
    accessing it returns only 0xFF bytes. As UBI is not a FTL it is
    allowed to have mappings to empty PEBs, it assumes that the layer
    above takes care of LEB accounting and referencing.
    UBIFS does so using the LEB property tree (LPT).
    For static volumes UBI blindly assumes that all LEBs are present and
    therefore special actions have to be taken.
    
    The described situation can happen when updating a static volume is
    interrupted, either by a user or a power cut.
    The volume update code first unmaps all LEBs of a volume and then
    writes LEB by LEB. If the sequence of operations is interrupted UBI
    detects this either by the absence of LEBs, no volume header present
    at scan time, or corrupted payload, detected via checksum.
    In the Fastmap case the former method won't trigger as no scan
    happened and UBI automatically thinks all LEBs are present.
    Only by reading data from a LEB it detects that the volume header is
    missing and incorrectly treats this as fatal error.
    To deal with the situation ubi_eba_read_leb() from now on checks
    whether we attached via Fastmap and handles the absence of a
    volume header like a data corruption error.
    This way interrupted static volume updates will correctly get detected
    also when Fastmap is used.
    
    Cc: <stable@vger.kernel.org>
    Reported-by: default avatarEzequiel Garcia <ezequiel@vanguardiasur.com.ar>
    Tested-by: default avatarEzequiel Garcia <ezequiel@vanguardiasur.com.ar>
    Signed-off-by: default avatarRichard Weinberger <richard@nod.at>
    1900149c
fastmap.c 39.6 KB