netdev - [RFC] apparently bogus logics in unix_find

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Message-ID: <20190210042414.GH2217@ZenIV.linux.org.uk>
Date:   Sun, 10 Feb 2019 04:24:22 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     netdev@...r.kernel.org
Cc:     Solar Designer <solar@...nwall.com>,
        David Miller <davem@...emloft.net>
Subject: [RFC] apparently bogus logics in unix_find_other() since 2002

	In "net/unix/af_unix.c: Set ATIME on socket inode" (back in
2002) we'd grown something rather odd in unix_find_other().  In the
original patch it was
                u=unix_find_socket_byname(sunname, len, type, hash);
-               if (!u)
+               if (u) {
+                       struct dentry *dentry;
+                       dentry = u->protinfo.af_unix.dentry;
+                       if (dentry)
+                               UPDATE_ATIME(dentry->d_inode);
+               } else
                        goto fail;

These days the code is

                u = unix_find_socket_byname(net, sunname, len, type, hash);
                if (u) {
                        struct dentry *dentry;
                        dentry = unix_sk(u)->path.dentry;
                        if (dentry)
                                touch_atime(&unix_sk(u)->path);
                } else  
                        goto fail;

but the logics is the same.  It's the abstract address case - we have
'\0' in sunname->sun_path[0].  How in hell could that possibly have
non-NULL ->path.dentry and what would it be?

Note that unix_find_socket_byname() returns non-NULL u here only if
u->addr->name->sun_path[0] is equal to sunname->sun_path[0], i.e.
'\0'.  There are only two places where we ever might assign non-NULL
to ->path.dentry:
        if (sun_path[0]) {
                addr->hash = UNIX_HASH_SIZE;
                hash = d_backing_inode(path.dentry)->i_ino & (UNIX_HASH_SIZE - 1);
                spin_lock(&unix_table_lock);
                u->path = path;
                list = &unix_socket_table[hash];
        } else {
in unix_bind() (in which case u->addr->name->sun_path[0] will not be '\0') and
        /* copy address information from listening to new sock*/
        if (otheru->addr) {
                refcount_inc(&otheru->addr->refcnt);
                newu->addr = otheru->addr;
        }
        if (otheru->path.dentry) {
                path_get(&otheru->path);
                newu->path = otheru->path;
        }
in unix_stream_connect().  And once ->addr is non-NULL, it's never changed,
and ->addr->name contents is never modified afterwards.  So we would have
to had the same condition (non-NULL ->path.dentry with '\0' in
->addr->name->sun_path[0]) at earlier point on the listener socket.

Looks like that should be impossible; what am I missing here?  Incidentally,
how can the quoted fragment in in unix_stream_connect() be reached with NULL
otheru->addr?  After all, otheru is unix_sock of a listener; how could
we possibly have found it if it had NULL ->addr?

Confused...