linux-kernel - Re: [f2fs-dev] [PATCH 2/2] f2fs: add F2FS_IOC_SET_COMPRESS

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Date:   Mon, 12 Oct 2020 23:11:50 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Daeho Jeong <daeho43@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH 2/2] f2fs: add F2FS_IOC_SET_COMPRESS_OPTION
 ioctl

On Tue, Oct 13, 2020 at 11:24:29AM +0900, Daeho Jeong wrote:
> +static int f2fs_ioc_set_compress_option(struct file *filp, unsigned long arg)
> +{
> +	struct inode *inode = file_inode(filp);
> +	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
> +	struct f2fs_comp_option option;
> +	int ret;
> +	int writecount;
> +
> +	if (!f2fs_sb_has_compression(sbi))
> +		return -EOPNOTSUPP;
> +
> +	if (!f2fs_compressed_file(inode) || IS_IMMUTABLE(inode))
> +		return -EINVAL;
> +
> +	if (f2fs_readonly(sbi->sb))
> +		return -EROFS;

f2fs_readonly() is redundant with mnt_want_write_file().

Also, shouldn't this require a writable file descriptor?  As-is, this ioctl can
be called on a file owned by another user, as long as the caller has read
access.

Note: if you change this to require a writable file descriptor, then
f2fs_readonly(), mnt_want_write_file(), and IS_IMMUTABLE() all would no longer
be needed.

> +
> +	if (copy_from_user(&option, (struct f2fs_comp_option __user *)arg,
> +				sizeof(option)))
> +		return -EFAULT;
> +
> +	if (option.log_cluster_size < MIN_COMPRESS_LOG_SIZE ||
> +			option.log_cluster_size > MAX_COMPRESS_LOG_SIZE ||
> +			option.algorithm >= COMPRESS_MAX)
> +		return -EINVAL;

What if f2fs_cops[options.algorithm] == NULL, e.g. COMPRESS_LZ4 without
CONFIG_F2FS_FS_LZ4?  Shouldn't the caller get an error then?

> +
> +	ret = mnt_want_write_file(filp);
> +	if (ret)
> +		return ret;
> +
> +	inode_lock(inode);
> +
> +	writecount = atomic_read(&inode->i_writecount);
> +	if ((filp->f_mode & FMODE_WRITE && writecount != 1) ||
> +			(!(filp->f_mode & FMODE_WRITE) && writecount)) {
> +		ret = -EBUSY;
> +		goto out;
> +	}

I don't think the check for i_writecount == 1 accomplishes anything because it
just means there are no *other* writable file descriptors.  It doesn't mean that
some other thread isn't concurrently trying to write to this same file
descriptor.  So the lock needs to be enough.  Is it?

- Eric