linux-ext4 - Re: [PATCH v2] ext4: implement support for get/set fs label

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Date:   Mon, 29 Nov 2021 13:28:09 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Lukas Czerner <lczerner@...hat.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH v2] ext4: implement support for get/set fs label

On Nov 12, 2021, at 1:20 AM, Lukas Czerner <lczerner@...hat.com> wrote:
> 
> Implement support for FS_IOC_GETFSLABEL and FS_IOC_SETFSLABEL ioctls for
> online reading and setting of file system label.
> 
> ext4_ioctl_getlabel() is simple, just get the label from the primary
> superblock bh. This might not be the first sb on the file system if
> 'sb=' mount option is used.
> 
> In ext4_ioctl_setlabel() we update what ext4 currently views as a
> primary superblock and then proceed to update backup superblocks. There
> are two caveats:
> - the primary superblock might not be the first superblock and so it
>   might not be the one used by userspace tools if read directly
>   off the disk.
> - because the primary superblock might not be the first superblock we
>   potentialy have to update it as part of backup superblock update.
>   However the first sb location is a bit more complicated than the rest
>   so we have to account for that.
> 
> Tested with generic/492 with various configurations. I also checked the
> behavior with 'sb=' mount options, including very large file systems
> with and without sparse_super/sparse_super2.
> 
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> ---

One minor issue/question inline.

> +static int ext4_ioctl_setlabel(struct file *filp, const char __user *user_label)
> +{
> +	size_t len;
> +	handle_t *handle;
> +	ext4_group_t ngroups;
> +	ext4_fsblk_t sb_block;
> +	struct buffer_head *bh;
> +	int ret = 0, ret2, grp;
> +	unsigned long offset = 0;
> +	char new_label[EXT4_LABEL_MAX + 1];
> +	struct super_block *sb = file_inode(filp)->i_sb;
> +	struct ext4_sb_info *sbi = EXT4_SB(sb);
> +	struct ext4_super_block *es = sbi->s_es;
> +
> +	/* Sanity check, this should never happen */
> +	BUILD_BUG_ON(sizeof(es->s_volume_name) < EXT4_LABEL_MAX);
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +	/*
> +	 * Copy the maximum length allowed for ext4 label with one more to
> +	 * find the required terminating null byte in order to test the
> +	 * label length. The on disk label doesn't need to be null terminated.
> +	 */
> +	if (copy_from_user(new_label, user_label, EXT4_LABEL_MAX + 1))
> +		return -EFAULT;
> +
> +	len = strnlen(new_label, EXT4_LABEL_MAX + 1);
> +	if (len > EXT4_LABEL_MAX)
> +		return -EINVAL;
> +
> +	ret = mnt_want_write_file(filp);
> +	if (ret)
> +		return ret;
> +
> +	handle = ext4_journal_start_sb(sb, EXT4_HT_MISC, EXT4_MAX_TRANS_DATA);
> +	if (IS_ERR(handle)) {
> +		ret = PTR_ERR(handle);
> +		goto err_out;
> +	}
> +	/* Update the primary superblock first */
> +	ret = ext4_journal_get_write_access(handle, sb,
> +					    sbi->s_sbh,
> +					    EXT4_JTR_NONE);
> +	if (ret)
> +		goto err_journal;
> +
> +	lock_buffer(sbi->s_sbh);
> +	memset(es->s_volume_name, 0, sizeof(es->s_volume_name));
> +	memcpy(es->s_volume_name, new_label, len);

(minor) this introduces a very small window where s_volume_name is unset.
Since "new_label" is already a temporary buffer of the correct size, it
would be better IMHO to zero it out, copy the new label from userspace
into it, and then copy EXT4_LABEL_MAX bytes of new_label to s_volume_name.

It still isn't perfect, but reduces the window significantly.

> +	/* Update backup superblocks */
> +	ngroups = ext4_get_groups_count(sb);
> +	for (grp = 0; grp < ngroups; grp++) {

		:
		:

> +		ext4_debug("update backup superblock %llu\n", sb_block);
> +		BUFFER_TRACE(bh, "get_write_access");
> +		ret = ext4_journal_get_write_access(handle, sb,
> +						    bh,
> +						    EXT4_JTR_NONE);
> +		if (ret) {
> +			brelse(bh);
> +			break;
> +		}
> +
> +		es = (struct ext4_super_block *) (bh->b_data + offset);
> +		lock_buffer(bh);
> +		if (ext4_has_metadata_csum(sb) &&
> +		    es->s_checksum != ext4_superblock_csum(sb, es)) {
> +			ext4_msg(sb, KERN_ERR, "Invalid checksum for backup "
> +				 "superblock %llu\n", sb_block);
> +			unlock_buffer(bh);
> +			brelse(bh);
> +			ret = -EFSBADCRC;
> +			break;
> +		}
> +		memset(es->s_volume_name, 0, sizeof(es->s_volume_name));
> +		memcpy(es->s_volume_name, new_label, len);

Same here.

The rest looks fine.

Cheers, Andreas






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