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Date: Tue, 5 Jul 2022 10:07:38 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Jeremy Bongio <bongiojp@...il.com>
Cc: Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org,
linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] Add ioctls to get/set the ext4 superblock uuid.
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@....edu>
> Signed-off-by: Jeremy Bongio <bongiojp@...il.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
>
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