libnvdimm/btt: Fix LBA masking during 'free list' population

The Linux BTT implementation assumes that log entries will never have
the 'zero' flag set, and indeed it never sets that flag for log entries
itself.

However, the UEFI spec is ambiguous on the exact format of the LBA field
of a log entry, specifically as to whether it should include the
additional flag bits or not. While a zero bit doesn't make sense in the
context of a log entry, other BTT implementations might still have it set.

If an implementation does happen to have it set, we would happily read
it in as the next block to write to for writes. Since a high bit is set,
it pushes the block number out of the range of an 'arena', and we fail
such a write with an EIO.

Follow the robustness principle, and tolerate such implementations by
stripping out the zero flag when populating the free list during
initialization. Additionally, use the same stripped out entries for
detection of incomplete writes and map restoration that happens at this
stage.

Add a sysfs file 'log_zero_flags' that indicates the ability to accept
such a layout to userspace applications. This enables 'ndctl
check-namespace' to recognize whether the kernel is able to handle zero
flags, or whether it should attempt a fix-up under the --repair option.

Cc: Dan Williams <dan.j.williams@intel.com>
Reported-by: Dexuan Cui <decui@microsoft.com>
Reported-by: Pedro d'Aquino Filocre F S Barbuda <pbarbuda@microsoft.com>
Tested-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/drivers/nvdimm/btt_devs.c b/drivers/nvdimm/btt_devs.c
index 795ad4ff35ca..b72a303176c7 100644
--- a/drivers/nvdimm/btt_devs.c
+++ b/drivers/nvdimm/btt_devs.c
@@ -159,11 +159,19 @@ static ssize_t size_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(size);
 
+static ssize_t log_zero_flags_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	return sprintf(buf, "Y\n");
+}
+static DEVICE_ATTR_RO(log_zero_flags);
+
 static struct attribute *nd_btt_attributes[] = {
 	&dev_attr_sector_size.attr,
 	&dev_attr_namespace.attr,
 	&dev_attr_uuid.attr,
 	&dev_attr_size.attr,
+	&dev_attr_log_zero_flags.attr,
 	NULL,
 };