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Date:   Tue, 5 Jul 2022 10:07:38 -0700
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Jeremy Bongio <bongiojp@gmail.com>
Cc:     Ted Tso <tytso@mit.edu>, linux-ext4@vger.kernel.org,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2] Add ioctls to get/set the ext4 superblock uuid.
Message-ID: <YsRv2iaVltGW8Yj2@magnolia>
References: <20220701201123.183468-1-bongiojp@gmail.com>
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In-Reply-To: <20220701201123.183468-1-bongiojp@gmail.com>
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On Fri, Jul 01, 2022 at 01:11:23PM -0700, Jeremy Bongio wrote:
> This fixes a race between changing the ext4 superblock uuid and operations
> like mounting, resizing, changing features, etc.
> 
> Reviewed-by: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Jeremy Bongio <bongiojp@gmail.com>
> ---
>  fs/ext4/ext4.h  | 13 ++++++++
>  fs/ext4/ioctl.c | 83 +++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 96 insertions(+)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 75b8d81b2469..0cf960cb591e 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -724,6 +724,8 @@ enum {
>  #define EXT4_IOC_GETSTATE		_IOW('f', 41, __u32)
>  #define EXT4_IOC_GET_ES_CACHE		_IOWR('f', 42, struct fiemap)
>  #define EXT4_IOC_CHECKPOINT		_IOW('f', 43, __u32)
> +#define EXT4_IOC_GETFSUUID		_IOR('f', 44, struct fsuuid)
> +#define EXT4_IOC_SETFSUUID		_IOW('f', 44, struct fsuuid)
>  
>  #define EXT4_IOC_SHUTDOWN _IOR ('X', 125, __u32)
>  
> @@ -753,6 +755,17 @@ enum {
>  						EXT4_IOC_CHECKPOINT_FLAG_ZEROOUT | \
>  						EXT4_IOC_CHECKPOINT_FLAG_DRY_RUN)
>  
> +/*
> + * Structure for EXT4_IOC_GETFSUUID/EXT4_IOC_SETFSUUID
> + */
> +struct fsuuid {
> +	__u32       fu_len;
> +	__u32       fu_flags;
> +	__u8 __user fu_uuid[];

__user is unnecessary here -- it applies to pointers, not to struct
members.

> +};
> +
> +#define EXT4_IOC_SETFSUUID_FLAG_BLOCKING 0x1

What does this do?

(Better yet, can you please write a manpage describing these new
ioctls?)

> +
>  #if defined(__KERNEL__) && defined(CONFIG_COMPAT)
>  /*
>   * ioctl commands in 32 bit emulation
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index cb01c1da0f9d..75069afc16ae 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -20,6 +20,7 @@
>  #include <linux/delay.h>
>  #include <linux/iversion.h>
>  #include <linux/fileattr.h>
> +#include <linux/uuid.h>
>  #include "ext4_jbd2.h"
>  #include "ext4.h"
>  #include <linux/fsmap.h>
> @@ -41,6 +42,15 @@ static void ext4_sb_setlabel(struct ext4_super_block *es, const void *arg)
>  	memcpy(es->s_volume_name, (char *)arg, EXT4_LABEL_MAX);
>  }
>  
> +/*
> + * Superblock modification callback function for changing file system
> + * UUID.
> + */
> +static void ext4_sb_setuuid(struct ext4_super_block *es, const void *arg)
> +{
> +	memcpy(es->s_uuid, (__u8 *)arg, UUID_SIZE);
> +}
> +
>  static
>  int ext4_update_primary_sb(struct super_block *sb, handle_t *handle,
>  			   ext4_update_sb_callback func,
> @@ -1131,6 +1141,73 @@ static int ext4_ioctl_getlabel(struct ext4_sb_info *sbi, char __user *user_label
>  	return 0;
>  }
>  
> +static int ext4_ioctl_getuuid(struct ext4_sb_info *sbi,
> +			struct fsuuid __user *ufsuuid)
> +{
> +	int ret = 0;
> +	__u8 uuid[UUID_SIZE];

Save some stack space and copy sbi->s_es->s_uuid into fsuuid.fu_uuid,
and then copy_to_user from the kernel stack object out to userspace's
object.

> +	struct fsuuid fsuuid;
> +
> +	if (copy_from_user(&fsuuid, ufsuuid, sizeof(fsuuid)))
> +		return -EFAULT;
> +
> +	if (fsuuid.fu_len != UUID_SIZE)
> +		return -EINVAL;

This function needs to check that fsuuid.fu_flags doesn't contain any
unknown bitflags.

> +
> +	lock_buffer(sbi->s_sbh);
> +	memcpy(uuid, sbi->s_es->s_uuid, UUID_SIZE);
> +	unlock_buffer(sbi->s_sbh);
> +
> +	if (copy_to_user(&ufsuuid->fu_uuid[0], uuid, UUID_SIZE))
> +		ret = -EFAULT;
> +	return ret;

	if (copy_to_user(...))
		return -EFAULT;

	return 0;

?

> +}
> +
> +static int ext4_ioctl_setuuid(struct file *filp,
> +			const struct fsuuid __user *ufsuuid)
> +{
> +	int ret = 0;
> +	struct super_block *sb = file_inode(filp)->i_sb;
> +	struct fsuuid fsuuid;
> +	__u8 uuid[UUID_SIZE];
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +
> +	/*
> +	 * If any checksums (group descriptors or metadata) are being used
> +	 * then the checksum seed feature is required to change the UUID.
> +	 */
> +	if (((ext4_has_feature_gdt_csum(sb) || ext4_has_metadata_csum(sb))
> +			&& !ext4_has_feature_csum_seed(sb))
> +		|| ext4_has_feature_stable_inodes(sb))
> +		return -EOPNOTSUPP;
> +
> +	if (copy_from_user(&fsuuid, ufsuuid, sizeof(fsuuid)))
> +		return -EFAULT;
> +
> +	if (fsuuid.fu_len != UUID_SIZE)
> +		return -EINVAL;

This function needs to check that fsuuid.fu_flags doesn't contain any
unknown bits.

> +
> +	if (copy_from_user(uuid, &ufsuuid->fu_uuid[0], UUID_SIZE))
> +		return -EFAULT;
> +
> +	ret = mnt_want_write_file(filp);
> +	if (ret)
> +		return ret;
> +
> +	do {
> +		if (ret == -EBUSY)
> +			msleep(1000);
> +		ret = ext4_update_superblocks_fn(sb, ext4_sb_setuuid, &uuid);
> +	} while (ret == -EBUSY &&
> +		fsuuid.fu_flags & EXT4_IOC_SETFSUUID_FLAG_BLOCKING);

So... I guess by default, userspace gets NOWAIT mode?  That's a little
strange, usually kernel convention is blocking mode by default, with
nowait selectable via function flags.

Also, what's the intended use case here?  Why would we want to set a
uuid but only if the superblock(s) aren't busy?

--D

> +
> +	mnt_drop_write_file(filp);
> +
> +	return ret;
> +}
> +
>  static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>  {
>  	struct inode *inode = file_inode(filp);
> @@ -1509,6 +1586,10 @@ static long __ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>  		return ext4_ioctl_setlabel(filp,
>  					   (const void __user *)arg);
>  
> +	case EXT4_IOC_GETFSUUID:
> +		return ext4_ioctl_getuuid(EXT4_SB(sb), (void __user *)arg);
> +	case EXT4_IOC_SETFSUUID:
> +		return ext4_ioctl_setuuid(filp, (const void __user *)arg);
>  	default:
>  		return -ENOTTY;
>  	}
> @@ -1586,6 +1667,8 @@ long ext4_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>  	case EXT4_IOC_CHECKPOINT:
>  	case FS_IOC_GETFSLABEL:
>  	case FS_IOC_SETFSLABEL:
> +	case EXT4_IOC_GETFSUUID:
> +	case EXT4_IOC_SETFSUUID:
>  		break;
>  	default:
>  		return -ENOIOCTLCMD;
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog
>