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From:   Manfred Spraul <manfred@colorfullife.com>
To:     LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     "Eric W . Biederman" <ebiederm@xmission.com>,
        Davidlohr Bueso <dave@stgolabs.net>, 1vier1@web.de,
        Manfred Spraul <manfred@colorfullife.com>,
        Waiman Long <longman@redhat.com>
Subject: [PATCH] ipc/util.c: Cleanup and improve sysvipc_find_ipc(), V3
Date:   Fri,  5 Aug 2022 13:57:33 +0200
Message-Id: <20220805115733.104763-1-manfred@colorfullife.com>
MIME-Version: 1.0
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Precedence: bulk

sysvipc_find_ipc() can be simplified further:

- It uses a for() loop to locate the next entry in the idr.
  This can be replaced with idr_get_next().

- It receives two parameters (pos - which is actually
  an idr index and not a position, and new_pos, which
  is really a position).
  One parameter is sufficient.

Link: https://lore.kernel.org/all/20210903052020.3265-3-manfred@colorfullife.com/
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
---

Resending the patch, it seems it was stuck:
- typo noticed by Waiman corrected.
- rediffed
- retested

@Andrew: Could you add it it linux-next/mm?
It is in my outbox for > 6 months now, and using a for() loop instead of
idr_get_next() is just wrong.
And the variable naming should be cleaned up as well
(idx -> index, without sequence number. id -> user space id, with
sequence number. The current function doesn't follow that)

  ipc/util.c | 53 ++++++++++++++++++++++++++++++++---------------------
 1 file changed, 32 insertions(+), 21 deletions(-)

diff --git a/ipc/util.c b/ipc/util.c
index a2208d0f26b2..05cb9de66735 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -782,28 +782,37 @@ struct pid_namespace *ipc_seq_pid_ns(struct seq_file *s)
 	return iter->pid_ns;
 }
 
-/*
- * This routine locks the ipc structure found at least at position pos.
+/**
+ * sysvipc_find_ipc - Find and lock the ipc structure based on seq pos
+ * @ids: ipc identifier set
+ * @pos: expected position
+ *
+ * The function finds an ipc structure, based on the sequence file
+ * position @pos. If there is no ipc structure at position @pos, then
+ * the successor is selected.
+ * If a structure is found, then it is locked (both rcu_read_lock() and
+ * ipc_lock_object()) and  @pos is set to the position needed to locate
+ * the found ipc structure.
+ * If nothing is found (i.e. EOF), @pos is not modified.
+ *
+ * The function returns the found ipc structure, or NULL at EOF.
  */
-static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
-					      loff_t *new_pos)
+static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t *pos)
 {
-	struct kern_ipc_perm *ipc = NULL;
-	int max_idx = ipc_get_maxidx(ids);
+	int tmpidx;
+	struct kern_ipc_perm *ipc;
 
-	if (max_idx == -1 || pos > max_idx)
-		goto out;
+	/* convert from position to idr index -> "-1" */
+	tmpidx = *pos - 1;
 
-	for (; pos <= max_idx; pos++) {
-		ipc = idr_find(&ids->ipcs_idr, pos);
-		if (ipc != NULL) {
-			rcu_read_lock();
-			ipc_lock_object(ipc);
-			break;
-		}
+	ipc = idr_get_next(&ids->ipcs_idr, &tmpidx);
+	if (ipc != NULL) {
+		rcu_read_lock();
+		ipc_lock_object(ipc);
+
+		/* convert from idr index to position  -> "+1" */
+		*pos = tmpidx + 1;
 	}
-out:
-	*new_pos = pos + 1;
 	return ipc;
 }
 
@@ -817,11 +826,13 @@ static void *sysvipc_proc_next(struct seq_file *s, void *it, loff_t *pos)
 	if (ipc && ipc != SEQ_START_TOKEN)
 		ipc_unlock(ipc);
 
-	return sysvipc_find_ipc(&iter->ns->ids[iface->ids], *pos, pos);
+	/* Next -> search for *pos+1 */
+	(*pos)++;
+	return sysvipc_find_ipc(&iter->ns->ids[iface->ids], pos);
 }
 
 /*
- * File positions: pos 0 -> header, pos n -> ipc id = n - 1.
+ * File positions: pos 0 -> header, pos n -> ipc idx = n - 1.
  * SeqFile iterator: iterator value locked ipc pointer or SEQ_TOKEN_START.
  */
 static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
@@ -846,8 +857,8 @@ static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
 	if (*pos == 0)
 		return SEQ_START_TOKEN;
 
-	/* Find the (pos-1)th ipc */
-	return sysvipc_find_ipc(ids, *pos - 1, pos);
+	/* Otherwise return the correct ipc structure */
+	return sysvipc_find_ipc(ids, pos);
 }
 
 static void sysvipc_proc_stop(struct seq_file *s, void *it)
-- 
2.37.1