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

From: Jann Horn <jannh@google.com>
To: Omar Sandoval <osandov@osandov.com>, Jens Axboe <axboe@kernel.dk>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-btrfs@vger.kernel.org, Dave Chinner <david@fromorbit.com>,
	Linux API <linux-api@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>,
	Andy Lutomirski <luto@kernel.org>
Subject: Re: [RFC PATCH 2/3] fs: add RWF_ENCODED for writing compressed data
Date: Thu, 19 Sep 2019 17:44:12 +0200	[thread overview]
Message-ID: <CAG48ez2GKv15Uj6Wzv0sG5v2bXyrSaCtRTw5Ok_ovja_CiO_fQ@mail.gmail.com> (raw)
In-Reply-To: <230a76e65372a8fb3ec62ce167d9322e5e342810.1568875700.git.osandov@fb.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.