Re: [PATCH 3/3] io_uring: add support for getdents

[Christian Brauner <brauner@kernel.org>]

On Tue, Jul 11, 2023 at 07:40:27PM +0800, Hao Xu wrote:
> From: Hao Xu <howeyxu@tencent.com>
> 
> This add support for getdents64 to io_uring, acting exactly like the
> syscall: the directory is iterated from it's current's position as
> stored in the file struct, and the file's position is updated exactly as
> if getdents64 had been called.
> 
> For filesystems that support NOWAIT in iterate_shared(), try to use it
> first; if a user already knows the filesystem they use do not support
> nowait they can force async through IOSQE_ASYNC in the sqe flags,
> avoiding the need to bounce back through a useless EAGAIN return.
> 
> Co-developed-by: Dominique Martinet <asmadeus@codewreck.org>
> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> Signed-off-by: Hao Xu <howeyxu@tencent.com>
> ---
>  include/uapi/linux/io_uring.h |  7 ++++
>  io_uring/fs.c                 | 60 +++++++++++++++++++++++++++++++++++
>  io_uring/fs.h                 |  3 ++
>  io_uring/opdef.c              |  8 +++++
>  4 files changed, 78 insertions(+)
> 
> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
> index 08720c7bd92f..6c0d521135a6 100644
> --- a/include/uapi/linux/io_uring.h
> +++ b/include/uapi/linux/io_uring.h
> @@ -65,6 +65,7 @@ struct io_uring_sqe {
>  		__u32		xattr_flags;
>  		__u32		msg_ring_flags;
>  		__u32		uring_cmd_flags;
> +		__u32		getdents_flags;
>  	};
>  	__u64	user_data;	/* data to be passed back at completion time */
>  	/* pack this to avoid bogus arm OABI complaints */
> @@ -235,6 +236,7 @@ enum io_uring_op {
>  	IORING_OP_URING_CMD,
>  	IORING_OP_SEND_ZC,
>  	IORING_OP_SENDMSG_ZC,
> +	IORING_OP_GETDENTS,
>  
>  	/* this goes last, obviously */
>  	IORING_OP_LAST,
> @@ -273,6 +275,11 @@ enum io_uring_op {
>   */
>  #define SPLICE_F_FD_IN_FIXED	(1U << 31) /* the last bit of __u32 */
>  
> +/*
> + * sqe->getdents_flags
> + */
> +#define IORING_GETDENTS_REWIND	(1U << 0)
> +
>  /*
>   * POLL_ADD flags. Note that since sqe->poll_events is the flag space, the
>   * command flags for POLL_ADD are stored in sqe->len.
> diff --git a/io_uring/fs.c b/io_uring/fs.c
> index f6a69a549fd4..77f00577e09c 100644
> --- a/io_uring/fs.c
> +++ b/io_uring/fs.c
> @@ -47,6 +47,13 @@ struct io_link {
>  	int				flags;
>  };
>  
> +struct io_getdents {
> +	struct file			*file;
> +	struct linux_dirent64 __user	*dirent;
> +	unsigned int			count;
> +	int				flags;
> +};
> +
>  int io_renameat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>  {
>  	struct io_rename *ren = io_kiocb_to_cmd(req, struct io_rename);
> @@ -291,3 +298,56 @@ void io_link_cleanup(struct io_kiocb *req)
>  	putname(sl->oldpath);
>  	putname(sl->newpath);
>  }
> +
> +int io_getdents_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
> +{
> +	struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents);
> +
> +	if (READ_ONCE(sqe->off) != 0)
> +		return -EINVAL;
> +
> +	gd->dirent = u64_to_user_ptr(READ_ONCE(sqe->addr));
> +	gd->count = READ_ONCE(sqe->len);
> +
> +	return 0;
> +}
> +
> +int io_getdents(struct io_kiocb *req, unsigned int issue_flags)
> +{
> +	struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents);
> +	struct file *file;
> +	unsigned long getdents_flags = 0;
> +	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
> +	bool should_lock = false;
> +	int ret;
> +
> +	if (force_nonblock) {
> +		if (!(req->file->f_mode & FMODE_NOWAIT))
> +			return -EAGAIN;
> +
> +		getdents_flags = DIR_CONTEXT_F_NOWAIT;

I mentioned this on the other patch but it seems really pointless to
have that extra flag. I would really like to hear a good reason for
this.

> +	}
> +
> +	file = req->file;
> +	if (file && (file->f_mode & FMODE_ATOMIC_POS)) {
> +		if (file_count(file) > 1)

Assume we have a regular non-threaded process that just opens an fd to a
file. The process registers an async readdir request via that fd for the
file with io_uring and goes to do other stuff while waiting for the
result.

Some time later, io_uring gets to io_getdents() and the task is still
single threaded and the file hasn't been shared in the meantime. So
io_getdents() doesn't take the lock and starts the readdir() call.

Concurrently, the process that registered the io_uring request was free
to do other stuff and issued a synchronous readdir() system call which
calls fdget_pos(). Since the fdtable still isn't shared it doesn't
increment f_count and doesn't acquire the mutex. Now there's another
concurrent readdir() going on.

(Similar thing can happen if the process creates a thread for example.)

Two readdir() requests now proceed concurrently which is not intended.
Now to verify that this race can't happen with io_uring:

* regular fds:
  It seems that io_uring calls fget() on each regular file descriptor
  when an async request is registered. So that means that io_uring
  always hold its own explicit reference here.
  So as long as the original task is alive or another thread is alive
  f_count is guaranteed to be > 1 and so the mutex would always be
  acquired.

  If the registering process dies right before io_uring gets to the
  io_getdents() request no other process can steal the fd anymore and in
  that case the readdir call would not lock. But that's fine.

* fixed fds:
  I don't know the reference counting rules here. io_uring would need to
  ensure that it's impossible for two async readdir requests via a fixed
  fd to race because f_count is == 1.

  Iiuc, if a process registers a file it opened as a fixed file and
  immediately closes the fd afterwards - without anyone else holding a
  reference to that file - and only uses the fixed fd going forward, the
  f_count of that file in io_uring's fixed file table is always 1.

  So one could issue any number of concurrent readdir requests with no
  mutual exclusion. So for fixed files there definitely is a race, no?

All of that could ofc be simplified if we could just always acquire the
mutex in fdget_pos() and other places and drop that file_count(file) > 1
optimization everywhere. But I have no idea if the optimization for not
acquiring the mutex if f_count == 1 is worth it?

I hope I didn't confuse myself here.

Jens, do yo have any input here?

> +			should_lock = true;
> +	}
> +	if (should_lock) {
> +		if (!force_nonblock)
> +			mutex_lock(&file->f_pos_lock);
> +		else if (!mutex_trylock(&file->f_pos_lock))
> +			return -EAGAIN;
> +	}

Open-coding this seems extremely brittle with an invitation for subtle
bugs.

> +
> +	ret = vfs_getdents(file, gd->dirent, gd->count, getdents_flags);
> +	if (should_lock)
> +		mutex_unlock(&file->f_pos_lock);
> +
> +	if (ret == -EAGAIN && force_nonblock)
> +		return -EAGAIN;
> +
> +	io_req_set_res(req, ret, 0);
> +	return 0;
> +}
> +
> diff --git a/io_uring/fs.h b/io_uring/fs.h
> index 0bb5efe3d6bb..f83a6f3a678d 100644
> --- a/io_uring/fs.h
> +++ b/io_uring/fs.h
> @@ -18,3 +18,6 @@ int io_symlinkat(struct io_kiocb *req, unsigned int issue_flags);
>  int io_linkat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
>  int io_linkat(struct io_kiocb *req, unsigned int issue_flags);
>  void io_link_cleanup(struct io_kiocb *req);
> +
> +int io_getdents_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe);
> +int io_getdents(struct io_kiocb *req, unsigned int issue_flags);
> diff --git a/io_uring/opdef.c b/io_uring/opdef.c
> index 3b9c6489b8b6..1bae6b2a8d0b 100644
> --- a/io_uring/opdef.c
> +++ b/io_uring/opdef.c
> @@ -428,6 +428,11 @@ const struct io_issue_def io_issue_defs[] = {
>  		.prep			= io_eopnotsupp_prep,
>  #endif
>  	},
> +	[IORING_OP_GETDENTS] = {
> +		.needs_file		= 1,
> +		.prep			= io_getdents_prep,
> +		.issue			= io_getdents,
> +	},
>  };
>  
>  
> @@ -648,6 +653,9 @@ const struct io_cold_def io_cold_defs[] = {
>  		.fail			= io_sendrecv_fail,
>  #endif
>  	},
> +	[IORING_OP_GETDENTS] = {
> +		.name			= "GETDENTS",
> +	},
>  };
>  
>  const char *io_uring_get_opcode(u8 opcode)
> -- 
> 2.25.1
>