Re: [PATCH] Fuse: Add backing file support for uring_cmd

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Feb 21, 2025 at 4:36 PM Moinak Bhattacharyya
<moinakb001@gmail.com> wrote:
>
> Sorry about that. Correctly-formatted patch follows. Should I send out a
> V2 instead?
>
> Add support for opening and closing backing files in the fuse_uring_cmd
> callback. Store backing_map (for open) and backing_id (for close) in the
> uring_cmd data.
> ---
>   fs/fuse/dev_uring.c       | 50 +++++++++++++++++++++++++++++++++++++++
>   include/uapi/linux/fuse.h |  6 +++++
>   2 files changed, 56 insertions(+)
>
> diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
> index ebd2931b4f2a..df73d9d7e686 100644
> --- a/fs/fuse/dev_uring.c
> +++ b/fs/fuse/dev_uring.c
> @@ -1033,6 +1033,40 @@ fuse_uring_create_ring_ent(struct io_uring_cmd *cmd,
>       return ent;
>   }
>
> +/*
> + * Register new backing file for passthrough, getting backing map from
> URING_CMD data
> + */
> +static int fuse_uring_backing_open(struct io_uring_cmd *cmd,
> +    unsigned int issue_flags, struct fuse_conn *fc)
> +{
> +    const struct fuse_backing_map *map = io_uring_sqe_cmd(cmd->sqe);
> +    int ret = fuse_backing_open(fc, map);
> +

I am not that familiar with io_uring, so I need to ask -
fuse_backing_open() does
fb->cred = prepare_creds();
to record server credentials
what are the credentials that will be recorded in the context of this
io_uring command?


> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    io_uring_cmd_done(cmd, ret, 0, issue_flags);
> +    return 0;
> +}
> +
> +/*
> + * Remove file from passthrough tracking, getting backing_id from
> URING_CMD data
> + */
> +static int fuse_uring_backing_close(struct io_uring_cmd *cmd,
> +    unsigned int issue_flags, struct fuse_conn *fc)
> +{
> +    const int *backing_id = io_uring_sqe_cmd(cmd->sqe);
> +    int ret = fuse_backing_close(fc, *backing_id);
> +
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    io_uring_cmd_done(cmd, ret, 0, issue_flags);
> +    return 0;
> +}
> +
>   /*
>    * Register header and payload buffer with the kernel and puts the
>    * entry as "ready to get fuse requests" on the queue
> @@ -1144,6 +1178,22 @@ int fuse_uring_cmd(struct io_uring_cmd *cmd,
> unsigned int issue_flags)
>               return err;
>           }
>           break;
> +    case FUSE_IO_URING_CMD_BACKING_OPEN:
> +        err = fuse_uring_backing_open(cmd, issue_flags, fc);
> +        if (err) {
> +            pr_info_once("FUSE_IO_URING_CMD_BACKING_OPEN failed err=%d\n",
> +                    err);
> +            return err;
> +        }
> +        break;
> +    case FUSE_IO_URING_CMD_BACKING_CLOSE:
> +        err = fuse_uring_backing_close(cmd, issue_flags, fc);
> +        if (err) {
> +            pr_info_once("FUSE_IO_URING_CMD_BACKING_CLOSE failed err=%d\n",
> +                    err);
> +            return err;
> +        }
> +        break;
>       default:
>           return -EINVAL;
>       }
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index 5e0eb41d967e..634265da1328 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -1264,6 +1264,12 @@ enum fuse_uring_cmd {
>
>       /* commit fuse request result and fetch next request */
>       FUSE_IO_URING_CMD_COMMIT_AND_FETCH = 2,
> +
> +    /* add new backing file for passthrough */
> +    FUSE_IO_URING_CMD_BACKING_OPEN = 3,
> +
> +    /* remove passthrough file by backing_id */
> +    FUSE_IO_URING_CMD_BACKING_CLOSE = 4,
>   };
>

An anecdote:
Why are we using FUSE_DEV_IOC_BACKING_OPEN
and not passing the backing fd directly in OPEN response?

The reason for that was security related - there was a concern that
an adversary would be able to trick some process into writing some fd
to /dev/fuse, whereas tricking some proces into doing an ioctl is not
so realistic.

AFAICT this concern does not exist when OPEN response is via
io_uring(?), so the backing_id indirection is not strictly needed,
but for the sake of uniformity with standard fuse protocol,
I guess we should maintain those commands in io_uring as well.

Thanks,
Amir.