[RFC PATCH 2/2] quota_v2: persist DQF_ROOT_SQUASH

[Kitae Yoo <kitaeyoo777@gmail.com>]

Commit c119c5b9749e ("quota: Don't store flags for v2 quota format")
stopped persisting dqi_flags because no v2 flag was supported and
on-disk flags could contain unvalidated garbage. Now that
DQF_ROOT_SQUASH is settable on v2, losing it across quotaoff/quotaon or
remount would silently re-enable the CAP_SYS_RESOURCE exemption - a poor
property for an enforcement policy.

Store the flag in the existing on-disk dqi_flags field and mask on read
so only the supported flag is ever accepted from disk, which also keeps
pre-existing garbage bits out of the in-memory flags.

An older kernel rewriting quota info still clears the stored flag;
strict enforcement then needs to be set up again after booting back.

Signed-off-by: Kitae Yoo <kitaeyoo777@gmail.com>
---
 fs/quota/quota_v2.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/quota/quota_v2.c b/fs/quota/quota_v2.c
index a24fab5f9f..814db61681 100644
--- a/fs/quota/quota_v2.c
+++ b/fs/quota/quota_v2.c
@@ -142,8 +142,7 @@ static int v2_read_file_info(struct super_block *sb, int type)
 	}
 	info->dqi_bgrace = le32_to_cpu(dinfo.dqi_bgrace);
 	info->dqi_igrace = le32_to_cpu(dinfo.dqi_igrace);
-	/* No flags currently supported */
-	info->dqi_flags = 0;
+	info->dqi_flags = le32_to_cpu(dinfo.dqi_flags) & DQF_ROOT_SQUASH;
 	qinfo->dqi_sb = sb;
 	qinfo->dqi_type = type;
 	qinfo->dqi_blocks = le32_to_cpu(dinfo.dqi_blocks);
@@ -209,8 +208,7 @@ static int v2_write_file_info(struct super_block *sb, int type)
 	info->dqi_flags &= ~DQF_INFO_DIRTY;
 	dinfo.dqi_bgrace = cpu_to_le32(info->dqi_bgrace);
 	dinfo.dqi_igrace = cpu_to_le32(info->dqi_igrace);
-	/* No flags currently supported */
-	dinfo.dqi_flags = cpu_to_le32(0);
+	dinfo.dqi_flags = cpu_to_le32(info->dqi_flags & DQF_ROOT_SQUASH);
 	spin_unlock(&dq_data_lock);
 	dinfo.dqi_blocks = cpu_to_le32(qinfo->dqi_blocks);
 	dinfo.dqi_free_blk = cpu_to_le32(qinfo->dqi_free_blk);
-- 
2.50.1 (Apple Git-155)