[PATCH 03/16] tmpfs: preserve explicitly disabled quota limits

["David Hildenbrand (Arm)" <david@kernel.org>]

From: Josef Bacik <josef@toxicpanda.com>

An administrator’s decision to disable a user’s or group’s quota
limits can be silently lost when tmpfs restores the mount defaults
after releasing and reacquiring the quota record. This can cause
later file creation or writes to fail with EDQUOT, even though the
administrator has explicitly disabled quota limits for the user or
group.

do_set_dqblk() sets DQ_FAKE_B when all block and inode limits are zero.
This means quota enforcement is disabled for that user or group.
shmem_is_empty_dquot() treats the flag as a reason to discard the entry
from the mount's quota tree. The next shmem_acquire_dquot() finds no
saved entry and recreates it with the mount's default hard limits.

If the cached dquot is reused, DQ_FAKE_B remains set: Q_GETQUOTA reports
the restored defaults, but enforcement is still disabled. Once that
dquot has been reclaimed, a new dquot enforces the restored defaults.

Remove the DQ_FAKE_B test. Only discard entries with zero usage, zero
soft limits and hard limits equal to the mount defaults. Explicitly
disabled limits are then preserved whenever the mount defaults are
nonzero.

Explicitly disabled limits now retain a quota-tree entry, just like other
limits that differ from the mount defaults.

Rename shmem_is_empty_dquot() to shmem_is_default_dquot() to describe
the check for default values.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: de4c0e7ca8b5 ("shmem: Add default quota limit mount options")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[ david: code cleanups; rewrite description ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem_quota.c | 30 +++++++++++++++---------------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/mm/shmem_quota.c b/mm/shmem_quota.c
index 43d3b4fd6cb1b..d191ebda3f00e 100644
--- a/mm/shmem_quota.c
+++ b/mm/shmem_quota.c
@@ -239,7 +239,7 @@ static int shmem_acquire_dquot(struct dquot *dquot)
 	return ret;
 }
 
-static bool shmem_is_empty_dquot(struct dquot *dquot)
+static bool shmem_is_default_dquot(struct dquot *dquot)
 {
 	struct shmem_sb_info *sbinfo = dquot->dq_sb->s_fs_info;
 	qsize_t bhardlimit;
@@ -253,21 +253,21 @@ static bool shmem_is_empty_dquot(struct dquot *dquot)
 		ihardlimit = sbinfo->qlimits.grpquota_ihardlimit;
 	}
 
-	if (test_bit(DQ_FAKE_B, &dquot->dq_flags) ||
-		(dquot->dq_dqb.dqb_curspace == 0 &&
-		 dquot->dq_dqb.dqb_curinodes == 0 &&
-		 dquot->dq_dqb.dqb_bsoftlimit == 0 &&
-		 dquot->dq_dqb.dqb_isoftlimit == 0 &&
-		 dquot->dq_dqb.dqb_bhardlimit == bhardlimit &&
-		 dquot->dq_dqb.dqb_ihardlimit == ihardlimit))
-		return true;
-
-	return false;
+	/*
+	 * Defaults are zero usage and soft limits, and the mount's hard limits.
+	 * DQ_FAKE_B only means that *all* limits are zero, which need not match
+	 * the mount defaults.
+	 */
+	return dquot->dq_dqb.dqb_curspace == 0 &&
+	       dquot->dq_dqb.dqb_curinodes == 0 &&
+	       dquot->dq_dqb.dqb_bsoftlimit == 0 &&
+	       dquot->dq_dqb.dqb_isoftlimit == 0 &&
+	       dquot->dq_dqb.dqb_bhardlimit == bhardlimit &&
+	       dquot->dq_dqb.dqb_ihardlimit == ihardlimit;
 }
 /*
- * Store limits from dquot in the tree unless it's fake. If it is fake
- * remove the id from the tree since there is no useful information in
- * there.
+ * Remove the ID from the tree if it has no usage and its limits match
+ * the mount defaults. Otherwise, save its limits for the next acquire.
  */
 static int shmem_release_dquot(struct dquot *dquot)
 {
@@ -302,7 +302,7 @@ static int shmem_release_dquot(struct dquot *dquot)
 	return -ENOENT;
 
 found:
-	if (shmem_is_empty_dquot(dquot)) {
+	if (shmem_is_default_dquot(dquot)) {
 		/* Remove entry from the tree */
 		rb_erase(&entry->node, info->dqi_priv);
 		kfree(entry);

-- 
2.43.0