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Date:   Wed, 24 Mar 2021 09:06:29 +0100
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        linux-fsdevel@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2 01/18] vfs: add miscattr ops
Message-ID: <20210324080629.tt4blkmx772efcby@wittgenstein>
References: <20210322144916.137245-1-mszeredi@redhat.com>
 <20210322144916.137245-2-mszeredi@redhat.com>
 <20210322223338.GD22094@magnolia>
MIME-Version: 1.0
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In-Reply-To: <20210322223338.GD22094@magnolia>
Precedence: bulk

On Mon, Mar 22, 2021 at 03:33:38PM -0700, Darrick J. Wong wrote:
> On Mon, Mar 22, 2021 at 03:48:59PM +0100, Miklos Szeredi wrote:
> > There's a substantial amount of boilerplate in filesystems handling
> > FS_IOC_[GS]ETFLAGS/ FS_IOC_FS[GS]ETXATTR ioctls.
> > 
> > Also due to userspace buffers being involved in the ioctl API this is
> > difficult to stack, as shown by overlayfs issues related to these ioctls.
> > 
> > Introduce a new internal API named "miscattr" (fsxattr can be confused with
> > xattr, xflags is inappropriate, since this is more than just flags).
> > 
> > There's significant overlap between flags and xflags and this API handles
> > the conversions automatically, so filesystems may choose which one to use.
> > 
> > In ->miscattr_get() a hint is provided to the filesystem whether flags or
> > xattr are being requested by userspace, but in this series this hint is
> > ignored by all filesystems, since generating all the attributes is cheap.
> > 
> > If a filesystem doesn't implemement the miscattr API, just fall back to
> > f_op->ioctl().  When all filesystems are converted, the fallback can be
> > removed.
> > 
> > 32bit compat ioctls are now handled by the generic code as well.
> > 
> > Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> > ---
> >  Documentation/filesystems/locking.rst |   5 +
> >  Documentation/filesystems/vfs.rst     |  15 ++
> >  fs/ioctl.c                            | 329 ++++++++++++++++++++++++++
> >  include/linux/fs.h                    |   4 +
> >  include/linux/miscattr.h              |  53 +++++
> >  5 files changed, 406 insertions(+)
> >  create mode 100644 include/linux/miscattr.h
> > 
> > diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst
> > index b7dcc86c92a4..a5aa2046d48f 100644
> > --- a/Documentation/filesystems/locking.rst
> > +++ b/Documentation/filesystems/locking.rst
> > @@ -80,6 +80,9 @@ prototypes::
> >  				struct file *, unsigned open_flag,
> >  				umode_t create_mode);
> >  	int (*tmpfile) (struct inode *, struct dentry *, umode_t);
> > +	int (*miscattr_set)(struct user_namespace *mnt_userns,
> > +			    struct dentry *dentry, struct miscattr *ma);
> > +	int (*miscattr_get)(struct dentry *dentry, struct miscattr *ma);
> >  
> >  locking rules:
> >  	all may block
> > @@ -107,6 +110,8 @@ fiemap:		no
> >  update_time:	no
> >  atomic_open:	shared (exclusive if O_CREAT is set in open flags)
> >  tmpfile:	no
> > +miscattr_get:	no or exclusive
> > +miscattr_set:	exclusive
> >  ============	=============================================
> >  
> >  
> > diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
> > index 2049bbf5e388..f125ce6c3b47 100644
> > --- a/Documentation/filesystems/vfs.rst
> > +++ b/Documentation/filesystems/vfs.rst
> > @@ -441,6 +441,9 @@ As of kernel 2.6.22, the following members are defined:
> >  				   unsigned open_flag, umode_t create_mode);
> >  		int (*tmpfile) (struct user_namespace *, struct inode *, struct dentry *, umode_t);
> >  	        int (*set_acl)(struct user_namespace *, struct inode *, struct posix_acl *, int);
> > +		int (*miscattr_set)(struct user_namespace *mnt_userns,
> > +				    struct dentry *dentry, struct miscattr *ma);
> > +		int (*miscattr_get)(struct dentry *dentry, struct miscattr *ma);
> >  	};
> >  
> >  Again, all methods are called without any locks being held, unless
> > @@ -588,6 +591,18 @@ otherwise noted.
> >  	atomically creating, opening and unlinking a file in given
> >  	directory.
> >  
> > +``miscattr_get``
> 
> I wish this wasn't named "misc" because miscellaneous is vague.
> 
> fileattr_get, perhaps?
> 
> (FWIW I'm not /that/ passionate about starting a naming bikeshed, feel
> free to ignore.)
> 
> > +	called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
> > +	retrieve miscellaneous filesystem flags and attributes.  Also
> 
> "...miscellaneous *file* flags and attributes."
> 
> > +	called before the relevant SET operation to check what is being
> > +	changed (in this case with i_rwsem locked exclusive).  If unset,
> > +	then fall back to f_op->ioctl().
> > +
> > +``miscattr_set``
> > +	called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
> > +	change miscellaneous filesystem flags and attributes.  Callers hold
> 
> Same here.
> 
> > +	i_rwsem exclusive.  If unset, then fall back to f_op->ioctl().
> > +
> >  
> >  The Address Space Object
> >  ========================
> > diff --git a/fs/ioctl.c b/fs/ioctl.c
> > index 4e6cc0a7d69c..e5f3820809a4 100644
> > --- a/fs/ioctl.c
> > +++ b/fs/ioctl.c
> > @@ -19,6 +19,9 @@
> >  #include <linux/falloc.h>
> >  #include <linux/sched/signal.h>
> >  #include <linux/fiemap.h>
> > +#include <linux/mount.h>
> > +#include <linux/fscrypt.h>
> > +#include <linux/miscattr.h>
> >  
> >  #include "internal.h"
> >  
> > @@ -657,6 +660,311 @@ static int ioctl_file_dedupe_range(struct file *file,
> >  	return ret;
> >  }
> >  
> > +/**
> > + * miscattr_fill_xflags - initialize miscattr with xflags
> > + * @ma:		miscattr pointer
> > + * @xflags:	FS_XFLAG_* flags
> > + *
> > + * Set ->fsx_xflags, ->xattr_valid and ->flags (translated xflags).  All
> > + * other fields are zeroed.
> > + */
> > +void miscattr_fill_xflags(struct miscattr *ma, u32 xflags)
> > +{
> > +	memset(ma, 0, sizeof(*ma));
> > +	ma->xattr_valid = true;
> > +	ma->fsx_xflags = xflags;
> > +	if (ma->fsx_xflags & FS_XFLAG_IMMUTABLE)
> > +		ma->flags |= FS_IMMUTABLE_FL;
> 
> I wonder if maintaining redundant sets of flags in the same structure is
> going to bite us some day.
> 
> > +	if (ma->fsx_xflags & FS_XFLAG_APPEND)
> > +		ma->flags |= FS_APPEND_FL;
> > +	if (ma->fsx_xflags & FS_XFLAG_SYNC)
> > +		ma->flags |= FS_SYNC_FL;
> > +	if (ma->fsx_xflags & FS_XFLAG_NOATIME)
> > +		ma->flags |= FS_NOATIME_FL;
> > +	if (ma->fsx_xflags & FS_XFLAG_NODUMP)
> > +		ma->flags |= FS_NODUMP_FL;
> > +	if (ma->fsx_xflags & FS_XFLAG_DAX)
> > +		ma->flags |= FS_DAX_FL;
> > +	if (ma->fsx_xflags & FS_XFLAG_PROJINHERIT)
> > +		ma->flags |= FS_PROJINHERIT_FL;
> > +}
> > +EXPORT_SYMBOL(miscattr_fill_xflags);
> > +
> > +/**
> > + * miscattr_fill_flags - initialize miscattr with flags
> > + * @ma:		miscattr pointer
> > + * @flags:	FS_*_FL flags
> > + *
> > + * Set ->flags, ->flags_valid and ->fsx_xflags (translated flags).
> > + * All other fields are zeroed.
> > + */
> > +void miscattr_fill_flags(struct miscattr *ma, u32 flags)
> > +{
> > +	memset(ma, 0, sizeof(*ma));
> > +	ma->flags_valid = true;
> > +	ma->flags = flags;
> > +	if (ma->flags & FS_SYNC_FL)
> > +		ma->fsx_xflags |= FS_XFLAG_SYNC;
> > +	if (ma->flags & FS_IMMUTABLE_FL)
> > +		ma->fsx_xflags |= FS_XFLAG_IMMUTABLE;
> > +	if (ma->flags & FS_APPEND_FL)
> > +		ma->fsx_xflags |= FS_XFLAG_APPEND;
> > +	if (ma->flags & FS_NODUMP_FL)
> > +		ma->fsx_xflags |= FS_XFLAG_NODUMP;
> > +	if (ma->flags & FS_NOATIME_FL)
> > +		ma->fsx_xflags |= FS_XFLAG_NOATIME;
> > +	if (ma->flags & FS_DAX_FL)
> > +		ma->fsx_xflags |= FS_XFLAG_DAX;
> > +	if (ma->flags & FS_PROJINHERIT_FL)
> > +		ma->fsx_xflags |= FS_XFLAG_PROJINHERIT;
> > +}
> > +EXPORT_SYMBOL(miscattr_fill_flags);
> > +
> > +/**
> > + * vfs_miscattr_get - retrieve miscellaneous inode attributes
> > + * @dentry:	the object to retrieve from
> > + * @ma:		miscattr pointer
> > + *
> > + * Call i_op->miscattr_get() callback, if exists.
> > + *
> > + * Returns 0 on success, or a negative error on failure.
> > + */
> > +int vfs_miscattr_get(struct dentry *dentry, struct miscattr *ma)
> > +{
> > +	struct inode *inode = d_inode(dentry);
> > +
> > +	if (d_is_special(dentry))
> > +		return -ENOTTY;
> > +
> > +	if (!inode->i_op->miscattr_get)
> > +		return -ENOIOCTLCMD;
> > +
> > +	return inode->i_op->miscattr_get(dentry, ma);
> > +}
> > +EXPORT_SYMBOL(vfs_miscattr_get);
> > +
> > +/**
> > + * fsxattr_copy_to_user - copy fsxattr to userspace.
> > + * @ma:		miscattr pointer
> > + * @ufa:	fsxattr user pointer
> > + *
> > + * Returns 0 on success, or -EFAULT on failure.
> > + */
> > +int fsxattr_copy_to_user(const struct miscattr *ma, struct fsxattr __user *ufa)
> > +{
> > +	struct fsxattr fa = {
> > +		.fsx_xflags	= ma->fsx_xflags,
> > +		.fsx_extsize	= ma->fsx_extsize,
> > +		.fsx_nextents	= ma->fsx_nextents,
> > +		.fsx_projid	= ma->fsx_projid,
> > +		.fsx_cowextsize	= ma->fsx_cowextsize,
> > +	};
> > +
> > +	if (copy_to_user(ufa, &fa, sizeof(fa)))
> > +		return -EFAULT;
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL(fsxattr_copy_to_user);
> > +
> > +static int fsxattr_copy_from_user(struct miscattr *ma,
> > +				  struct fsxattr __user *ufa)
> > +{
> > +	struct fsxattr fa;
> > +
> > +	if (copy_from_user(&fa, ufa, sizeof(fa)))
> > +		return -EFAULT;
> > +
> > +	miscattr_fill_xflags(ma, fa.fsx_xflags);
> > +	ma->fsx_extsize = fa.fsx_extsize;
> > +	ma->fsx_nextents = fa.fsx_nextents;
> > +	ma->fsx_projid = fa.fsx_projid;
> > +	ma->fsx_cowextsize = fa.fsx_cowextsize;
> > +
> > +	return 0;
> > +}
> > +
> > +/*
> > + * Generic function to check FS_IOC_FSSETXATTR/FS_IOC_SETFLAGS values and reject
> > + * any invalid configurations.
> > + *
> > + * Note: must be called with inode lock held.
> > + */
> > +static int miscattr_set_prepare(struct inode *inode,
> > +			      const struct miscattr *old_ma,
> > +			      struct miscattr *ma)
> > +{
> > +	int err;
> > +
> > +	/*
> > +	 * The IMMUTABLE and APPEND_ONLY flags can only be changed by
> > +	 * the relevant capability.
> > +	 */
> > +	if ((ma->flags ^ old_ma->flags) & (FS_APPEND_FL | FS_IMMUTABLE_FL) &&
> > +	    !capable(CAP_LINUX_IMMUTABLE))
> > +		return -EPERM;
> > +
> > +	err = fscrypt_prepare_setflags(inode, old_ma->flags, ma->flags);
> > +	if (err)
> > +		return err;
> > +
> > +	/*
> > +	 * Project Quota ID state is only allowed to change from within the init
> > +	 * namespace. Enforce that restriction only if we are trying to change
> > +	 * the quota ID state. Everything else is allowed in user namespaces.
> > +	 */
> > +	if (current_user_ns() != &init_user_ns) {
> > +		if (old_ma->fsx_projid != ma->fsx_projid)
> > +			return -EINVAL;
> > +		if ((old_ma->fsx_xflags ^ ma->fsx_xflags) &
> > +				FS_XFLAG_PROJINHERIT)
> > +			return -EINVAL;
> > +	}
> > +
> > +	/* Check extent size hints. */
> > +	if ((ma->fsx_xflags & FS_XFLAG_EXTSIZE) && !S_ISREG(inode->i_mode))
> > +		return -EINVAL;
> > +
> > +	if ((ma->fsx_xflags & FS_XFLAG_EXTSZINHERIT) &&
> > +			!S_ISDIR(inode->i_mode))
> > +		return -EINVAL;
> > +
> > +	if ((ma->fsx_xflags & FS_XFLAG_COWEXTSIZE) &&
> > +	    !S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * It is only valid to set the DAX flag on regular files and
> > +	 * directories on filesystems.
> > +	 */
> > +	if ((ma->fsx_xflags & FS_XFLAG_DAX) &&
> > +	    !(S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode)))
> > +		return -EINVAL;
> > +
> > +	/* Extent size hints of zero turn off the flags. */
> > +	if (ma->fsx_extsize == 0)
> > +		ma->fsx_xflags &= ~(FS_XFLAG_EXTSIZE | FS_XFLAG_EXTSZINHERIT);
> > +	if (ma->fsx_cowextsize == 0)
> > +		ma->fsx_xflags &= ~FS_XFLAG_COWEXTSIZE;
> > +
> > +	return 0;
> > +}
> > +
> > +/**
> > + * vfs_miscattr_set - change miscellaneous inode attributes
> > + * @dentry:	the object to change
> > + * @ma:		miscattr pointer
> > + *
> > + * After verifying permissions, call i_op->miscattr_set() callback, if
> > + * exists.
> > + *
> > + * Verifying attributes involves retrieving current attributes with
> > + * i_op->miscattr_get(), this also allows initilaizing attributes that have
> > + * not been set by the caller to current values.  Inode lock is held
> > + * thoughout to prevent racing with another instance.
> > + *
> > + * Returns 0 on success, or a negative error on failure.
> > + */
> > +int vfs_miscattr_set(struct user_namespace *mnt_userns, struct dentry *dentry,
> > +		     struct miscattr *ma)
> > +{
> > +	struct inode *inode = d_inode(dentry);
> > +	struct miscattr old_ma = {};
> > +	int err;
> > +
> > +	if (d_is_special(dentry))
> > +		return -ENOTTY;
> > +
> > +	if (!inode->i_op->miscattr_set)
> > +		return -ENOIOCTLCMD;
> > +
> > +	if (!inode_owner_or_capable(mnt_userns, inode))
> > +		return -EPERM;
> > +
> > +	inode_lock(inode);
> > +	err = vfs_miscattr_get(dentry, &old_ma);
> > +	if (!err) {
> > +		/* initialize missing bits from old_ma */
> > +		if (ma->flags_valid) {
> > +			ma->fsx_xflags |= old_ma.fsx_xflags & ~FS_XFLAG_COMMON;
> > +			ma->fsx_extsize = old_ma.fsx_extsize;
> > +			ma->fsx_nextents = old_ma.fsx_nextents;
> > +			ma->fsx_projid = old_ma.fsx_projid;
> > +			ma->fsx_cowextsize = old_ma.fsx_cowextsize;
> > +		} else {
> > +			ma->flags |= old_ma.flags & ~FS_COMMON_FL;
> > +		}
> > +		err = miscattr_set_prepare(inode, &old_ma, ma);
> > +		if (!err)
> > +			err = inode->i_op->miscattr_set(mnt_userns, dentry, ma);
> > +	}
> > +	inode_unlock(inode);
> > +
> > +	return err;
> > +}
> > +EXPORT_SYMBOL(vfs_miscattr_set);
> > +
> > +static int ioctl_getflags(struct file *file, void __user *argp)
> > +{
> > +	struct miscattr ma = { .flags_valid = true }; /* hint only */
> > +	unsigned int flags;
> > +	int err;
> > +
> > +	err = vfs_miscattr_get(file_dentry(file), &ma);
> > +	if (!err) {
> > +		flags = ma.flags;
> > +		if (copy_to_user(argp, &flags, sizeof(flags)))
> > +			err = -EFAULT;
> > +	}
> > +	return err;
> > +}
> > +
> > +static int ioctl_setflags(struct file *file, void __user *argp)
> > +{
> > +	struct miscattr ma;
> > +	unsigned int flags;
> > +	int err;
> > +
> > +	if (copy_from_user(&flags, argp, sizeof(flags)))
> > +		return -EFAULT;
> > +
> > +	err = mnt_want_write_file(file);
> > +	if (!err) {
> > +		miscattr_fill_flags(&ma, flags);
> > +		err = vfs_miscattr_set(file_mnt_user_ns(file), file_dentry(file), &ma);
> > +		mnt_drop_write_file(file);
> > +	}
> > +	return err;
> > +}
> > +
> > +static int ioctl_fsgetxattr(struct file *file, void __user *argp)
> > +{
> > +	struct miscattr ma = { .xattr_valid = true }; /* hint only */
> > +	int err;
> > +
> > +	err = vfs_miscattr_get(file_dentry(file), &ma);
> > +	if (!err)
> > +		err = fsxattr_copy_to_user(&ma, argp);
> > +
> > +	return err;
> > +}
> > +
> > +static int ioctl_fssetxattr(struct file *file, void __user *argp)
> > +{
> > +	struct miscattr ma;
> > +	int err;
> > +
> > +	err = fsxattr_copy_from_user(&ma, argp);
> > +	if (!err) {
> > +		err = mnt_want_write_file(file);
> > +		if (!err) {
> > +			err = vfs_miscattr_set(file_mnt_user_ns(file), file_dentry(file), &ma);
> > +			mnt_drop_write_file(file);
> > +		}
> > +	}
> > +	return err;
> > +}
> > +
> >  /*
> >   * do_vfs_ioctl() is not for drivers and not intended to be EXPORT_SYMBOL()'d.
> >   * It's just a simple helper for sys_ioctl and compat_sys_ioctl.
> > @@ -727,6 +1035,18 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
> >  		return put_user(i_size_read(inode) - filp->f_pos,
> >  				(int __user *)argp);
> >  
> > +	case FS_IOC_GETFLAGS:
> > +		return ioctl_getflags(filp, argp);
> > +
> > +	case FS_IOC_SETFLAGS:
> > +		return ioctl_setflags(filp, argp);
> > +
> > +	case FS_IOC_FSGETXATTR:
> > +		return ioctl_fsgetxattr(filp, argp);
> > +
> > +	case FS_IOC_FSSETXATTR:
> > +		return ioctl_fssetxattr(filp, argp);
> > +
> >  	default:
> >  		if (S_ISREG(inode->i_mode))
> >  			return file_ioctl(filp, cmd, argp);
> > @@ -827,6 +1147,15 @@ COMPAT_SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd,
> >  		break;
> >  #endif
> >  
> > +	/*
> > +	 * These access 32-bit values anyway so no further handling is
> > +	 * necessary.
> > +	 */
> > +	case FS_IOC32_GETFLAGS:
> > +	case FS_IOC32_SETFLAGS:
> > +		cmd = (cmd == FS_IOC32_GETFLAGS) ?
> > +			FS_IOC_GETFLAGS : FS_IOC_SETFLAGS;
> > +		fallthrough;
> >  	/*
> >  	 * everything else in do_vfs_ioctl() takes either a compatible
> >  	 * pointer argument or no argument -- call it with a modified
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index ec8f3ddf4a6a..9e7f6a592a70 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -70,6 +70,7 @@ struct fsverity_info;
> >  struct fsverity_operations;
> >  struct fs_context;
> >  struct fs_parameter_spec;
> > +struct miscattr;
> >  
> >  extern void __init inode_init(void);
> >  extern void __init inode_init_early(void);
> > @@ -1963,6 +1964,9 @@ struct inode_operations {
> >  			struct dentry *, umode_t);
> >  	int (*set_acl)(struct user_namespace *, struct inode *,
> >  		       struct posix_acl *, int);
> > +	int (*miscattr_set)(struct user_namespace *mnt_userns,
> > +			    struct dentry *dentry, struct miscattr *ma);
> > +	int (*miscattr_get)(struct dentry *dentry, struct miscattr *ma);
> >  } ____cacheline_aligned;
> >  
> >  static inline ssize_t call_read_iter(struct file *file, struct kiocb *kio,
> > diff --git a/include/linux/miscattr.h b/include/linux/miscattr.h
> > new file mode 100644
> > index 000000000000..13683eb6ac78
> > --- /dev/null
> > +++ b/include/linux/miscattr.h
> > @@ -0,0 +1,53 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _LINUX_MISCATTR_H
> > +#define _LINUX_MISCATTR_H
> > +
> > +/* Flags shared betwen flags/xflags */
> > +#define FS_COMMON_FL \
> > +	(FS_SYNC_FL | FS_IMMUTABLE_FL | FS_APPEND_FL | \
> > +	 FS_NODUMP_FL |	FS_NOATIME_FL | FS_DAX_FL | \
> > +	 FS_PROJINHERIT_FL)
> > +
> > +#define FS_XFLAG_COMMON \
> > +	(FS_XFLAG_SYNC | FS_XFLAG_IMMUTABLE | FS_XFLAG_APPEND | \
> > +	 FS_XFLAG_NODUMP | FS_XFLAG_NOATIME | FS_XFLAG_DAX | \
> > +	 FS_XFLAG_PROJINHERIT)
> > +
> > +struct miscattr {
> > +	u32	flags;		/* flags (FS_IOC_GETFLAGS/FS_IOC_SETFLAGS) */
> > +	/* struct fsxattr: */
> > +	u32	fsx_xflags;	/* xflags field value (get/set) */
> 
> Hrmm... could we have /some/ note here that fsx_xflags comes from XFS
> and "flags" comes from ext*?
> 
> > +	u32	fsx_extsize;	/* extsize field value (get/set)*/
> > +	u32	fsx_nextents;	/* nextents field value (get)	*/
> > +	u32	fsx_projid;	/* project identifier (get/set) */
> > +	u32	fsx_cowextsize;	/* CoW extsize field value (get/set)*/
> > +	/* selectors: */
> > +	bool	flags_valid:1;
> > +	bool	xattr_valid:1;
> 
> What does this have to do with extended attributes?
> 
> OH, it has nothing to do with xattrs; this flag means that the fsx_*
> fields of miscattr have any significance.  Can this be named fsx_valid?

Yeah, that confused me too. If this is really related to fsx then naming
it fsx_valid would be potentially nicer.

Christian