autofs: fix add autofs_parse_fd()

We are seeing systemd hang on its autofs direct mount at
/proc/sys/fs/binfmt_misc.

Historically this was due to a mismatch in the communication structure
size between a 64 bit kernel and a 32 bit user space and was fixed by
making the pipe communication record oriented.

During autofs v5 development I decided to stay with the existing usage
instead of changing to a packed structure for autofs <=> user space
communications which turned out to be a mistake on my part.

Problems arose and they were fixed by allowing for the 64 bit to 32
bit size difference in the automount(8) code.

Along the way systemd started to use autofs and eventually encountered
this problem too. systemd refused to compensate for the length
difference insisting it be fixed in the kernel. Fortunately Linus
implemented the packetized pipe which resolved the problem in a
straight forward and simple way.

In the autofs mount api conversion series I inadvertatly dropped the
packet pipe flag settings when adding the autofs_parse_fd() function.
This patch fixes that omission.

Fixes: 546694b8 ("autofs: add autofs_parse_fd()")
Signed-off-by: Ian Kent <raven@themaw.net>
Link: https://lore.kernel.org/r/20231023093359.64265-1-raven@themaw.netTested-by: Anders Roxell <anders.roxell@linaro.org>
Cc: Bill O'Donnell <bodonnel@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Naresh Kamboju <naresh.kamboju@linaro.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reported-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/autofs/autofs_i.h

@@ -221,15 +221,20 @@ static inline int autofs_check_pipe(struct file *pipe)
 	return 0;
 }
 
-static inline int autofs_prepare_pipe(struct file *pipe)
+static inline void autofs_set_packet_pipe_flags(struct file *pipe)
 {
-	int ret = autofs_check_pipe(pipe);
-	if (ret < 0)
-		return ret;
 	/* We want a packet pipe */
 	pipe->f_flags |= O_DIRECT;
 	/* We don't expect -EAGAIN */
 	pipe->f_flags &= ~O_NONBLOCK;
+}
+
+static inline int autofs_prepare_pipe(struct file *pipe)
+{
+	int ret = autofs_check_pipe(pipe);
+	if (ret < 0)
+		return ret;
+	autofs_set_packet_pipe_flags(pipe);
 	return 0;
 }
 

fs/autofs/inode.c

@@ -177,6 +177,8 @@ static int autofs_parse_fd(struct fs_context *fc, struct autofs_sb_info *sbi,
 		return -EBADF;
 	}
 
+	autofs_set_packet_pipe_flags(pipe);
+
 	if (sbi->pipe)
 		fput(sbi->pipe);