Re: [PATCH 4/4] btrfs: don't read from userspace twice in btrfs_uring_encoded_read()

[Jens Axboe <axboe@kernel.dk>]

On 11/20/24 9:02 AM, Mark Harmstone wrote:
> If we return -EAGAIN the first time because we need to block,
> btrfs_uring_encoded_read() will get called twice. Take a copy of args
> the first time, to prevent userspace from messing around with it.
> 
> Signed-off-by: Mark Harmstone <maharmstone@fb.com>
> ---
>  fs/btrfs/ioctl.c | 74 ++++++++++++++++++++++++++++++++----------------
>  1 file changed, 49 insertions(+), 25 deletions(-)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 488dcd022dea..97f7812cbf7c 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -4873,7 +4873,7 @@ static int btrfs_uring_encoded_read(struct io_uring_cmd *cmd, unsigned int issue
>  {
>  	size_t copy_end_kernel = offsetofend(struct btrfs_ioctl_encoded_io_args, flags);
>  	size_t copy_end;
> -	struct btrfs_ioctl_encoded_io_args args = { 0 };
> +	struct btrfs_ioctl_encoded_io_args *args;
>  	int ret;
>  	u64 disk_bytenr, disk_io_size;
>  	struct file *file;
> @@ -4888,6 +4888,9 @@ static int btrfs_uring_encoded_read(struct io_uring_cmd *cmd, unsigned int issue
>  	struct extent_state *cached_state = NULL;
>  	u64 start, lockend;
>  	void __user *sqe_addr;
> +	struct io_kiocb *req = cmd_to_io_kiocb(cmd);
> +	struct io_uring_cmd_data *data = req->async_data;
> +	bool need_copy = false;
>  
>  	if (!capable(CAP_SYS_ADMIN)) {
>  		ret = -EPERM;
> @@ -4899,34 +4902,55 @@ static int btrfs_uring_encoded_read(struct io_uring_cmd *cmd, unsigned int issue
>  	io_tree = &inode->io_tree;
>  	sqe_addr = u64_to_user_ptr(READ_ONCE(cmd->sqe->addr));
>  
> +	if (!data->op_data) {
> +		data->op_data = kzalloc(sizeof(*args), GFP_NOFS);
> +		if (!data->op_data) {
> +			ret = -ENOMEM;
> +			goto out_acct;
> +		}
> +
> +		need_copy = true;
> +	}

I'd probably get rid of this need_copy variable and just do the copy
here? Might look cleaner with an btrfs_alloc_copy_foo() helper, as you
could just do:

ret = btrfs_alloc_copy_foo(...);
if (unlikely(ret))
	return ret;

and hide all that ugly compat business outside the meat of this
function.

More of a style thing, the change itself looks fine.

-- 
Jens Axboe