Re: [RFC PATCH 2/3] fs: add RWF_ENCODED for writing compressed data

[Jann Horn <jannh@google.com>]

On Thu, Sep 19, 2019 at 8:54 AM Omar Sandoval <osandov@osandov.com> wrote:
> Btrfs can transparently compress data written by the user. However, we'd
> like to add an interface to write pre-compressed data directly to the
> filesystem. This adds support for so-called "encoded writes" via
> pwritev2().
>
> A new RWF_ENCODED flags indicates that a write is "encoded". If this
> flag is set, iov[0].iov_base points to a struct encoded_iov which
> contains metadata about the write: namely, the compression algorithm and
> the unencoded (i.e., decompressed) length of the extent. iov[0].iov_len
> must be set to sizeof(struct encoded_iov), which can be used to extend
> the interface in the future. The remaining iovecs contain the encoded
> extent.
>
> A similar interface for reading encoded data can be added to preadv2()
> in the future.
>
> Filesystems must indicate that they support encoded writes by setting
> FMODE_ENCODED_IO in ->file_open().
[...]
> +int import_encoded_write(struct kiocb *iocb, struct encoded_iov *encoded,
> +                        struct iov_iter *from)
> +{
> +       if (iov_iter_single_seg_count(from) != sizeof(*encoded))
> +               return -EINVAL;
> +       if (copy_from_iter(encoded, sizeof(*encoded), from) != sizeof(*encoded))
> +               return -EFAULT;
> +       if (encoded->compression == ENCODED_IOV_COMPRESSION_NONE &&
> +           encoded->encryption == ENCODED_IOV_ENCRYPTION_NONE) {
> +               iocb->ki_flags &= ~IOCB_ENCODED;
> +               return 0;
> +       }
> +       if (encoded->compression > ENCODED_IOV_COMPRESSION_TYPES ||
> +           encoded->encryption > ENCODED_IOV_ENCRYPTION_TYPES)
> +               return -EINVAL;
> +       if (!capable(CAP_SYS_ADMIN))
> +               return -EPERM;

How does this capable() check interact with io_uring? Without having
looked at this in detail, I suspect that when an encoded write is
requested through io_uring, the capable() check might be executed on
something like a workqueue worker thread, which is probably running
with a full capability set.