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SubjectRe: [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

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