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Subject: [PATCH v4 1/2] exportfs: fix the fallback implementation of the
 get_name export operation
From: Chuck Lever <cel@kernel.org>
To: jlayton@redhat.com, amir73il@gmail.com
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>,
 Jeff Layton <jlayton@kernel.org>, Chuck Lever <chuck.lever@oracle.com>,
 linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
 trondmy@hammerspace.com, viro@zeniv.linux.org.uk, brauner@kernel.org
Date: Thu, 04 Jan 2024 15:55:26 -0500
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From: Trond Myklebust <trond.myklebust@hammerspace.com>

The fallback implementation for the get_name export operation uses
readdir() to try to match the inode number to a filename. That filename
is then used together with lookup_one() to produce a dentry.
A problem arises when we match the '.' or '..' entries, since that
causes lookup_one() to fail. This has sometimes been seen to occur for
filesystems that violate POSIX requirements around uniqueness of inode
numbers, something that is common for snapshot directories.

This patch just ensures that we skip '.' and '..' rather than allowing a
match.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/linux-nfs/CAOQ4uxiOZobN76OKB-VBNXWeFKVwLW_eK5QtthGyYzWU9mjb7Q@mail.gmail.com/
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/exportfs/expfs.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 3ae0154c5680..84af58eaf2ca 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -255,7 +255,9 @@ static bool filldir_one(struct dir_context *ctx, const char *name, int len,
 		container_of(ctx, struct getdents_callback, ctx);
 
 	buf->sequence++;
-	if (buf->ino == ino && len <= NAME_MAX) {
+	/* Ignore the '.' and '..' entries */
+	if ((len > 2 || name[0] != '.' || (len == 2 && name[1] != '.')) &&
+	    buf->ino == ino && len <= NAME_MAX) {
 		memcpy(buf->name, name, len);
 		buf->name[len] = '\0';
 		buf->found = 1;