From: Colin Ian King <colin.king@xxxxxxxxxxxxx> Some file systems such as squashfs do not set the UUID in the superblock resulting in a zero'd UUID. In cases were two or more of these file systems are overlayed on the lower layer we can hit overlay corruption issues because identical zero'd overlayfs UUIDs are impossible to differentiate between. This can be fixed by creating an overlayfs UUID based on the file system from the superblock s_magic and s_dev fields. (This currently seems like enough information to be able create a UUID, but the could be scope to use other super block fields such as the pointer s_fs_info but may need some obfuscation). This issue can be reproduced with the following commands: mkdir -p /cdrom mount -t iso9660 -o ro,noatime /dev/sr0 /cdrom sleep 1 mkdir -p /cow mount -t tmpfs -o 'rw,noatime,mode=755' tmpfs /cow mkdir -p /cow/upper mkdir -p /cow/work modprobe -q -b overlay modprobe -q -b loop dev=$(losetup -f) mkdir -p /filesystem.squashfs losetup $dev /cdrom/casper/filesystem.squashfs mount -t squashfs -o ro,noatime $dev /filesystem.squashfs dev=$(losetup -f) mkdir -p /installer.squashfs losetup $dev /cdrom/casper/installer.squashfs mount -t squashfs -o ro,noatime $dev /installer.squashfs mkdir -p /root-tmp mount -t overlay -o 'upperdir=/cow/upper,lowerdir=/installer.squashfs:/filesystem.squashfs,workdir=/cow/work' /cow /root-tmp FILE=/root-tmp/etc/.pwd.lock echo foo > $FILE cat $FILE sync echo 3 > /proc/sys/vm/drop_caches cat $FILE The output from cat $FILE: cat: /root-tmp/etc/.pwd.lock: Input/output error dmesg reports: [ 42.415432] overlayfs: invalid origin (etc/.pwd.lock, ftype=8000, origin ftype=4000). BugLink: https://bugs.launchpad.net/bugs/1824407 Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx> --- fs/overlayfs/copy_up.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c index b801c6353100..a578db87936b 100644 --- a/fs/overlayfs/copy_up.c +++ b/fs/overlayfs/copy_up.c @@ -231,6 +231,7 @@ struct ovl_fh *ovl_encode_real_fh(struct dentry *real, bool is_upper) void *buf; int buflen = MAX_HANDLE_SZ; uuid_t *uuid = &real->d_sb->s_uuid; + static const uuid_t z_uuid; buf = kmalloc(buflen, GFP_KERNEL); if (!buf) @@ -272,7 +273,20 @@ struct ovl_fh *ovl_encode_real_fh(struct dentry *real, bool is_upper) if (is_upper) fh->flags |= OVL_FH_FLAG_PATH_UPPER; fh->len = fh_len; - fh->uuid = *uuid; + + if (uuid_equal(uuid, &z_uuid)) { + /* + * An zero'd uuid indicates the uuid in the super block was + * not set by the file system, so fake one instead + */ + struct super_block *sb = real->d_sb; + + memcpy(&fh->uuid.b[0], &sb->s_magic, 8); + memcpy(&fh->uuid.b[8], &sb->s_dev, 8); + } else { + fh->uuid = *uuid; + } + memcpy(fh->fid, buf, buflen); out: -- 2.20.1
