linux-kernel - [PATCH v2 4/4] autofs: add comment about autofs_mountpoint

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Message-ID: <158561607160.23197.3228166849408166685.stgit@mickey.themaw.net>
Date:   Tue, 31 Mar 2020 08:54:31 +0800
From:   Ian Kent <raven@...maw.net>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     autofs mailing list <autofs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH v2 4/4] autofs: add comment about autofs_mountpoint_changed()

The function autofs_mountpoint_changed() is unusual, add a comment
about two cases for which it is used.

Signed-off-by: Ian Kent <raven@...maw.net>
---
 fs/autofs/root.c |   21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/fs/autofs/root.c b/fs/autofs/root.c
index b3f748e4df08..5efb7fa1ce2b 100644
--- a/fs/autofs/root.c
+++ b/fs/autofs/root.c
@@ -280,9 +280,24 @@ static struct dentry *autofs_mountpoint_changed(struct path *path)
 	struct dentry *dentry = path->dentry;
 	struct autofs_sb_info *sbi = autofs_sbi(dentry->d_sb);
 
-	/*
-	 * If this is an indirect mount the dentry could have gone away
-	 * as a result of an expire and a new one created.
+	/* If this is an indirect mount the dentry could have gone away
+	 * and a new one created.
+	 *
+	 * This is unusual and I can't remember the case for which it
+	 * was originally added now. But an example of how this can
+	 * happen is an autofs indirect mount that has the "browse"
+	 * option set and also has the "symlink" option in the autofs
+	 * map entry. In this case the daemon will remove the browse
+	 * directory and create a symlink as the mount (pointing to a
+	 * local path) leaving the struct path stale.
+	 *
+	 * Another not so obvious case is when a mount in an autofs
+	 * indirect mount that uses the "nobrowse" option is being
+	 * expired and the mount has been umounted but the mount point
+	 * directory remains when a stat family system call is made.
+	 * In this case the mount point is removed (by the daemon) and
+	 * a new mount triggered leading to a stale dentry in the struct
+	 * path of the waiting process.
 	 */
 	if (autofs_type_indirect(sbi->type) && d_unhashed(dentry)) {
 		struct dentry *parent = dentry->d_parent;