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Date:   Mon, 1 Jul 2019 12:40:49 +0200
From:   Christian Brauner <christian@brauner.io>
To:     David Howells <dhowells@redhat.com>
Cc:     viro@zeniv.linux.org.uk, raven@themaw.net, mszeredi@redhat.com,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 01/11] vfs: syscall: Add fsinfo() to query filesystem
 information [ver #15]
Message-ID: <20190701104048.c2t5aful2sabngmr@brauner.io>
References: <156173661696.14042.17822154531324224780.stgit@warthog.procyon.org.uk>
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On Fri, Jun 28, 2019 at 04:43:45PM +0100, David Howells wrote:
> Add a system call to allow filesystem information to be queried.  A request
> value can be given to indicate the desired attribute.  Support is provided
> for enumerating multi-value attributes.
> 
> ===============
> NEW SYSTEM CALL
> ===============
> 
> The new system call looks like:
> 
> 	int ret = fsinfo(int dfd,
> 			 const char *filename,
> 			 const struct fsinfo_params *params,
> 			 void *buffer,
> 			 size_t buf_size);
> 
> The params parameter optionally points to a block of parameters:
> 
> 	struct fsinfo_params {
> 		__u32	at_flags;
> 		__u32	request;
> 		__u32	Nth;
> 		__u32	Mth;
> 		__u64	__reserved[3];
> 	};
> 
> If params is NULL, it is assumed params->request should be
> fsinfo_attr_statfs, params->Nth should be 0, params->Mth should be 0 and
> params->at_flags should be 0.
> 
> If params is given, all of params->__reserved[] must be 0.
> 
> dfd, filename and params->at_flags indicate the file to query.  There is no
> equivalent of lstat() as that can be emulated with fsinfo() by setting
> AT_SYMLINK_NOFOLLOW in params->at_flags.  There is also no equivalent of
> fstat() as that can be emulated by passing a NULL filename to fsinfo() with
> the fd of interest in dfd.  AT_NO_AUTOMOUNT can also be used to an allow
> automount point to be queried without triggering it.
> 
> params->request indicates the attribute/attributes to be queried.  This can
> be one of:
> 
> 	FSINFO_ATTR_STATFS		- statfs-style info
> 	FSINFO_ATTR_FSINFO		- Information about fsinfo()
> 	FSINFO_ATTR_IDS			- Filesystem IDs
> 	FSINFO_ATTR_LIMITS		- Filesystem limits
> 	FSINFO_ATTR_SUPPORTS		- What's supported in statx(), IOC flags
> 	FSINFO_ATTR_CAPABILITIES	- Filesystem capabilities
> 	FSINFO_ATTR_TIMESTAMP_INFO	- Inode timestamp info
> 	FSINFO_ATTR_VOLUME_ID		- Volume ID (string)
> 	FSINFO_ATTR_VOLUME_UUID		- Volume UUID
> 	FSINFO_ATTR_VOLUME_NAME		- Volume name (string)
> 	FSINFO_ATTR_NAME_ENCODING	- Filename encoding (string)
> 	FSINFO_ATTR_NAME_CODEPAGE	- Filename codepage (string)
> 
> Some attributes (such as the servers backing a network filesystem) can have
> multiple values.  These can be enumerated by setting params->Nth and
> params->Mth to 0, 1, ... until ENODATA is returned.
> 
> buffer and buf_size point to the reply buffer.  The buffer is filled up to
> the specified size, even if this means truncating the reply.  The full size
> of the reply is returned.  In future versions, this will allow extra fields
> to be tacked on to the end of the reply, but anyone not expecting them will
> only get the subset they're expecting.  If either buffer of buf_size are 0,
> no copy will take place and the data size will be returned.
> 
> At the moment, this will only work on x86_64 and i386 as it requires the
> system call to be wired up.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: linux-api@vger.kernel.org
> ---
> 
>  arch/x86/entry/syscalls/syscall_32.tbl |    1 
>  arch/x86/entry/syscalls/syscall_64.tbl |    1 
>  fs/Kconfig                             |    7 
>  fs/Makefile                            |    1 
>  fs/fsinfo.c                            |  545 ++++++++++++++++++++++++++++++++
>  include/linux/fs.h                     |    5 
>  include/linux/fsinfo.h                 |   65 ++++
>  include/linux/syscalls.h               |    4 
>  include/uapi/asm-generic/unistd.h      |    4 
>  include/uapi/linux/fsinfo.h            |  219 +++++++++++++
>  kernel/sys_ni.c                        |    1 
>  samples/vfs/Makefile                   |    4 
>  samples/vfs/test-fsinfo.c              |  551 ++++++++++++++++++++++++++++++++
>  13 files changed, 1407 insertions(+), 1 deletion(-)
>  create mode 100644 fs/fsinfo.c
>  create mode 100644 include/linux/fsinfo.h
>  create mode 100644 include/uapi/linux/fsinfo.h
>  create mode 100644 samples/vfs/test-fsinfo.c
> 
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index ad968b7bac72..03decae51513 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -438,3 +438,4 @@
>  431	i386	fsconfig		sys_fsconfig			__ia32_sys_fsconfig
>  432	i386	fsmount			sys_fsmount			__ia32_sys_fsmount
>  433	i386	fspick			sys_fspick			__ia32_sys_fspick
> +434	i386	fsinfo			sys_fsinfo			__ia32_sys_fsinfo
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index b4e6f9e6204a..ea63df9a1020 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -355,6 +355,7 @@
>  431	common	fsconfig		__x64_sys_fsconfig
>  432	common	fsmount			__x64_sys_fsmount
>  433	common	fspick			__x64_sys_fspick
> +434	common	fsinfo			__x64_sys_fsinfo
>  
>  #
>  # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/fs/Kconfig b/fs/Kconfig
> index cbbffc8b9ef5..9e7d2f2c0111 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -15,6 +15,13 @@ config VALIDATE_FS_PARSER
>  	  Enable this to perform validation of the parameter description for a
>  	  filesystem when it is registered.
>  
> +config FSINFO

Hm, any reason why we would hide that syscalls under a config option?

> +	bool "Enable the fsinfo() system call"
> +	help
> +	  Enable the file system information querying system call to allow
> +	  comprehensive information to be retrieved about a filesystem,
> +	  superblock or mount object.
> +
>  if BLOCK
>  
>  config FS_IOMAP
> diff --git a/fs/Makefile b/fs/Makefile
> index c9aea23aba56..26eaeae4b9a1 100644
> --- a/fs/Makefile
> +++ b/fs/Makefile
> @@ -53,6 +53,7 @@ obj-$(CONFIG_SYSCTL)		+= drop_caches.o
>  
>  obj-$(CONFIG_FHANDLE)		+= fhandle.o
>  obj-$(CONFIG_FS_IOMAP)		+= iomap.o
> +obj-$(CONFIG_FSINFO)		+= fsinfo.o
>  
>  obj-y				+= quota/
>  
> diff --git a/fs/fsinfo.c b/fs/fsinfo.c
> new file mode 100644
> index 000000000000..09e743b16235
> --- /dev/null
> +++ b/fs/fsinfo.c
> @@ -0,0 +1,545 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Filesystem information query.
> + *
> + * Copyright (C) 2019 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells@redhat.com)
> + */
> +#include <linux/syscalls.h>
> +#include <linux/fs.h>
> +#include <linux/file.h>
> +#include <linux/mount.h>
> +#include <linux/namei.h>
> +#include <linux/statfs.h>
> +#include <linux/security.h>
> +#include <linux/uaccess.h>
> +#include <linux/fsinfo.h>
> +#include <uapi/linux/mount.h>
> +#include "internal.h"
> +
> +static u32 calc_mount_attrs(u32 mnt_flags)
> +{
> +	u32 attrs = 0;
> +
> +	if (mnt_flags & MNT_READONLY)
> +		attrs |= MOUNT_ATTR_RDONLY;
> +	if (mnt_flags & MNT_NOSUID)
> +		attrs |= MOUNT_ATTR_NOSUID;
> +	if (mnt_flags & MNT_NODEV)
> +		attrs |= MOUNT_ATTR_NODEV;
> +	if (mnt_flags & MNT_NOEXEC)
> +		attrs |= MOUNT_ATTR_NOEXEC;
> +	if (mnt_flags & MNT_NODIRATIME)
> +		attrs |= MOUNT_ATTR_NODIRATIME;
> +
> +	if (mnt_flags & MNT_NOATIME)
> +		attrs |= MOUNT_ATTR_NOATIME;
> +	else if (mnt_flags & MNT_RELATIME)
> +		attrs |= MOUNT_ATTR_RELATIME;
> +	else
> +		attrs |= MOUNT_ATTR_STRICTATIME;
> +	return attrs;
> +}
> +
> +/*
> + * Get basic filesystem stats from statfs.
> + */
> +static int fsinfo_generic_statfs(struct path *path, struct fsinfo_statfs *p)
> +{
> +	struct kstatfs buf;
> +	int ret;
> +
> +	ret = vfs_statfs(path, &buf);
> +	if (ret < 0)
> +		return ret;
> +
> +	p->f_blocks.hi	= 0;
> +	p->f_blocks.lo	= buf.f_blocks;
> +	p->f_bfree.hi	= 0;
> +	p->f_bfree.lo	= buf.f_bfree;
> +	p->f_bavail.hi	= 0;
> +	p->f_bavail.lo	= buf.f_bavail;
> +	p->f_files.hi	= 0;
> +	p->f_files.lo	= buf.f_files;
> +	p->f_ffree.hi	= 0;
> +	p->f_ffree.lo	= buf.f_ffree;
> +	p->f_favail.hi	= 0;
> +	p->f_favail.lo	= buf.f_ffree;
> +	p->f_bsize	= buf.f_bsize;
> +	p->f_frsize	= buf.f_frsize;
> +
> +	p->mnt_attrs	= calc_mount_attrs(path->mnt->mnt_flags);
> +	return sizeof(*p);
> +}
> +
> +static int fsinfo_generic_ids(struct path *path, struct fsinfo_ids *p)
> +{
> +	struct super_block *sb;
> +	struct kstatfs buf;
> +	int ret;
> +
> +	ret = vfs_statfs(path, &buf);
> +	if (ret < 0 && ret != -ENOSYS)
> +		return ret;
> +
> +	sb = path->dentry->d_sb;
> +	p->f_fstype	= sb->s_magic;
> +	p->f_dev_major	= MAJOR(sb->s_dev);
> +	p->f_dev_minor	= MINOR(sb->s_dev);
> +
> +	memcpy(&p->f_fsid, &buf.f_fsid, sizeof(p->f_fsid));
> +	strlcpy(p->f_fs_name, path->dentry->d_sb->s_type->name,
> +		sizeof(p->f_fs_name));
> +	return sizeof(*p);
> +}
> +
> +static int fsinfo_generic_limits(struct path *path, struct fsinfo_limits *lim)
> +{
> +	struct super_block *sb = path->dentry->d_sb;
> +
> +	lim->max_file_size.hi = 0;
> +	lim->max_file_size.lo = sb->s_maxbytes;
> +	lim->max_hard_links = sb->s_max_links;
> +	lim->max_uid = UINT_MAX;
> +	lim->max_gid = UINT_MAX;
> +	lim->max_projid = UINT_MAX;
> +	lim->max_filename_len = NAME_MAX;
> +	lim->max_symlink_len = PAGE_SIZE;
> +	lim->max_xattr_name_len = XATTR_NAME_MAX;
> +	lim->max_xattr_body_len = XATTR_SIZE_MAX;
> +	lim->max_dev_major = 0xffffff;
> +	lim->max_dev_minor = 0xff;
> +	return sizeof(*lim);
> +}
> +
> +static int fsinfo_generic_supports(struct path *path, struct fsinfo_supports *c)
> +{
> +	struct super_block *sb = path->dentry->d_sb;
> +
> +	c->stx_mask = STATX_BASIC_STATS;
> +	if (sb->s_d_op && sb->s_d_op->d_automount)
> +		c->stx_attributes |= STATX_ATTR_AUTOMOUNT;
> +	return sizeof(*c);
> +}
> +
> +static int fsinfo_generic_capabilities(struct path *path,
> +				       struct fsinfo_capabilities *c)
> +{
> +	struct super_block *sb = path->dentry->d_sb;
> +
> +	if (sb->s_mtd)
> +		fsinfo_set_cap(c, FSINFO_CAP_IS_FLASH_FS);
> +	else if (sb->s_bdev)
> +		fsinfo_set_cap(c, FSINFO_CAP_IS_BLOCK_FS);
> +
> +	if (sb->s_quota_types & QTYPE_MASK_USR)
> +		fsinfo_set_cap(c, FSINFO_CAP_USER_QUOTAS);
> +	if (sb->s_quota_types & QTYPE_MASK_GRP)
> +		fsinfo_set_cap(c, FSINFO_CAP_GROUP_QUOTAS);
> +	if (sb->s_quota_types & QTYPE_MASK_PRJ)
> +		fsinfo_set_cap(c, FSINFO_CAP_PROJECT_QUOTAS);
> +	if (sb->s_d_op && sb->s_d_op->d_automount)
> +		fsinfo_set_cap(c, FSINFO_CAP_AUTOMOUNTS);
> +	if (sb->s_id[0])
> +		fsinfo_set_cap(c, FSINFO_CAP_VOLUME_ID);
> +
> +	fsinfo_set_cap(c, FSINFO_CAP_HAS_ATIME);
> +	fsinfo_set_cap(c, FSINFO_CAP_HAS_CTIME);
> +	fsinfo_set_cap(c, FSINFO_CAP_HAS_MTIME);
> +	return sizeof(*c);
> +}
> +
> +static const struct fsinfo_timestamp_info fsinfo_default_timestamp_info = {
> +	.atime = {
> +		.minimum	= S64_MIN,
> +		.maximum	= S64_MAX,
> +		.gran_mantissa	= 1,
> +		.gran_exponent	= 0,
> +	},
> +	.mtime = {
> +		.minimum	= S64_MIN,
> +		.maximum	= S64_MAX,
> +		.gran_mantissa	= 1,
> +		.gran_exponent	= 0,
> +	},
> +	.ctime = {
> +		.minimum	= S64_MIN,
> +		.maximum	= S64_MAX,
> +		.gran_mantissa	= 1,
> +		.gran_exponent	= 0,
> +	},
> +	.btime = {
> +		.minimum	= S64_MIN,
> +		.maximum	= S64_MAX,
> +		.gran_mantissa	= 1,
> +		.gran_exponent	= 0,
> +	},
> +};
> +
> +static int fsinfo_generic_timestamp_info(struct path *path,
> +					 struct fsinfo_timestamp_info *ts)
> +{
> +	struct super_block *sb = path->dentry->d_sb;
> +	s8 exponent;
> +
> +	*ts = fsinfo_default_timestamp_info;
> +
> +

nit: redundant newline

> +	if (sb->s_time_gran < 1000000000) {
> +		if (sb->s_time_gran < 1000)
> +			exponent = -9;
> +		else if (sb->s_time_gran < 1000000)
> +			exponent = -6;
> +		else
> +			exponent = -3;
> +
> +		ts->atime.gran_exponent = exponent;
> +		ts->mtime.gran_exponent = exponent;
> +		ts->ctime.gran_exponent = exponent;
> +		ts->btime.gran_exponent = exponent;
> +	}
> +
> +	return sizeof(*ts);
> +}
> +
> +static int fsinfo_generic_volume_uuid(struct path *path,
> +				      struct fsinfo_volume_uuid *vu)
> +{
> +	struct super_block *sb = path->dentry->d_sb;
> +
> +	memcpy(vu, &sb->s_uuid, sizeof(*vu));
> +	return sizeof(*vu);
> +}
> +
> +static int fsinfo_generic_volume_id(struct path *path, char *buf)
> +{
> +	struct super_block *sb = path->dentry->d_sb;
> +	size_t len = strlen(sb->s_id);
> +
> +	memcpy(buf, sb->s_id, len + 1);
> +	return len;
> +}
> +
> +static int fsinfo_generic_name_encoding(struct path *path, char *buf)
> +{
> +	static const char encoding[] = "utf8";
> +
> +	memcpy(buf, encoding, sizeof(encoding) - 1);
> +	return sizeof(encoding) - 1;

Do we, not have any dumb helpers for scenarios like this?:

#define strlen_literal(x) (sizeof(""x"") - 1)
#define strlen_array(x) (sizeof(x) - 1)

Repeating sizeof(bla) - 1 seems like a good way to forget that -1 later
on :)

> +}
> +
> +/*
> + * Implement some queries generically from stuff in the superblock.
> + */
> +int generic_fsinfo(struct path *path, struct fsinfo_kparams *params)
> +{
> +#define _gen(X, Y) FSINFO_ATTR_##X: return fsinfo_generic_##Y(path, params->buffer)
> +
> +	switch (params->request) {
> +	case _gen(STATFS,		statfs);
> +	case _gen(IDS,			ids);
> +	case _gen(LIMITS,		limits);
> +	case _gen(SUPPORTS,		supports);
> +	case _gen(CAPABILITIES,		capabilities);
> +	case _gen(TIMESTAMP_INFO,	timestamp_info);
> +	case _gen(VOLUME_UUID,		volume_uuid);
> +	case _gen(VOLUME_ID,		volume_id);
> +	case _gen(NAME_ENCODING,	name_encoding);
> +	default:
> +		return -EOPNOTSUPP;
> +	}
> +}

[1]:
*grumble* *grumble*
Formal complaint about these code-generating macros again. :)
But fine. :)

> +EXPORT_SYMBOL(generic_fsinfo);
> +
> +/*
> + * Retrieve the filesystem info.  We make some stuff up if the operation is not
> + * supported.
> + */
> +static int vfs_fsinfo(struct path *path, struct fsinfo_kparams *params)
> +{
> +	struct dentry *dentry = path->dentry;
> +	int (*fsinfo)(struct path *, struct fsinfo_kparams *);
> +	int ret;
> +
> +	if (params->request == FSINFO_ATTR_FSINFO) {
> +		struct fsinfo_fsinfo *info = params->buffer;
> +
> +		info->max_attr	= FSINFO_ATTR__NR;
> +		info->max_cap	= FSINFO_CAP__NR;
> +		return sizeof(*info);
> +	}
> +
> +	fsinfo = dentry->d_sb->s_op->fsinfo;
> +	if (!fsinfo) {
> +		if (!dentry->d_sb->s_op->statfs)
> +			return -EOPNOTSUPP;
> +		fsinfo = generic_fsinfo;
> +	}
> +
> +	ret = security_sb_statfs(dentry);
> +	if (ret)
> +		return ret;
> +
> +	if (!params->overlarge)
> +		return fsinfo(path, params);
> +
> +	while (!signal_pending(current)) {
> +		params->usage = 0;
> +		ret = fsinfo(path, params);
> +		if (IS_ERR_VALUE((long)ret))
> +			return ret; /* Error */
> +		if ((unsigned int)ret <= params->buf_size)

if ((size_t)ret ...? Just for the sake of clarity if for nothing else.

> +			return ret; /* It fitted */

Ok, a little confused here, tbh. params->buf_size is size_t and this
function returns an int. Forgot whether you mentioned this before,
buf_size exceed can't exceed INT_MAX?

> +		kvfree(params->buffer);
> +		params->buffer = NULL;
> +		params->buf_size = roundup(ret, PAGE_SIZE);
> +		if (params->buf_size > INT_MAX)
> +			return -ETOOSMALL;
> +		params->buffer = kvmalloc(params->buf_size, GFP_KERNEL);
> +		if (!params->buffer)
> +			return -ENOMEM;
> +	}
> +
> +	return -ERESTARTSYS;
> +}
> +
> +static int vfs_fsinfo_path(int dfd, const char __user *pathname,
> +			   struct fsinfo_kparams *params)
> +{
> +	struct path path;
> +	unsigned lookup_flags = LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT;
> +	int ret = -EINVAL;
> +
> +	if ((params->at_flags & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |
> +				 AT_EMPTY_PATH)) != 0)
> +		return -EINVAL;
> +
> +	if (params->at_flags & AT_SYMLINK_NOFOLLOW)
> +		lookup_flags &= ~LOOKUP_FOLLOW;
> +	if (params->at_flags & AT_NO_AUTOMOUNT)
> +		lookup_flags &= ~LOOKUP_AUTOMOUNT;
> +	if (params->at_flags & AT_EMPTY_PATH)
> +		lookup_flags |= LOOKUP_EMPTY;
> +
> +retry:
> +	ret = user_path_at(dfd, pathname, lookup_flags, &path);
> +	if (ret)
> +		goto out;
> +
> +	ret = vfs_fsinfo(&path, params);
> +	path_put(&path);
> +	if (retry_estale(ret, lookup_flags)) {
> +		lookup_flags |= LOOKUP_REVAL;
> +		goto retry;
> +	}
> +out:
> +	return ret;
> +}
> +
> +static int vfs_fsinfo_fd(unsigned int fd, struct fsinfo_kparams *params)
> +{
> +	struct fd f = fdget_raw(fd);
> +	int ret = -EBADF;
> +
> +	if (f.file) {
> +		ret = vfs_fsinfo(&f.file->f_path, params);
> +		fdput(f);
> +	}
> +	return ret;
> +}
> +
> +/*
> + * Return buffer information by requestable attribute.
> + *
> + * STRUCT	- a fixed-size structure with only one instance.
> + * STRUCT_N	- a sequence of STRUCTs, indexed by Nth
> + * STRUCT_NM	- a sequence of sequences of STRUCTs, indexed by Nth, Mth
> + * STRING	- a string with only one instance.
> + * STRING_N	- a sequence of STRING, indexed by Nth
> + * STRING_NM	- a sequence of sequences of STRING, indexed by Nth, Mth
> + * OPAQUE	- a blob that can be larger than 4K.
> + * STRUCT_ARRAY - an array of structs that can be larger than 4K
> + *
> + * If an entry is marked STRUCT, STRUCT_N or STRUCT_NM then if no buffer is
> + * supplied to sys_fsinfo(), sys_fsinfo() will handle returning the buffer size
> + * without calling vfs_fsinfo() and the filesystem.
> + *
> + * No struct may have more than 4K bytes.
> + */
> +struct fsinfo_attr_info {
> +	u8 type;
> +	u8 flags;
> +	u16 size;
> +};
> +
> +#define __FSINFO_STRUCT		0
> +#define __FSINFO_STRING		1
> +#define __FSINFO_OPAQUE		2
> +#define __FSINFO_STRUCT_ARRAY	3
> +#define __FSINFO_0		0
> +#define __FSINFO_N		0x0001
> +#define __FSINFO_NM		0x0002
> +
> +#define _Z(T, F, S) { .type = __FSINFO_##T, .flags = __FSINFO_##F, .size = S }
> +#define FSINFO_STRING(X)	 [FSINFO_ATTR_##X] = _Z(STRING, 0, 0)
> +#define FSINFO_STRUCT(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRUCT, 0, sizeof(struct fsinfo_##Y))
> +#define FSINFO_STRING_N(X)	 [FSINFO_ATTR_##X] = _Z(STRING, N, 0)
> +#define FSINFO_STRUCT_N(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRUCT, N, sizeof(struct fsinfo_##Y))
> +#define FSINFO_STRING_NM(X)	 [FSINFO_ATTR_##X] = _Z(STRING, NM, 0)
> +#define FSINFO_STRUCT_NM(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRUCT, NM, sizeof(struct fsinfo_##Y))
> +#define FSINFO_OPAQUE(X)	 [FSINFO_ATTR_##X] = _Z(OPAQUE, 0, 0)
> +#define FSINFO_STRUCT_ARRAY(X,Y) [FSINFO_ATTR_##X] = _Z(STRUCT_ARRAY, 0, sizeof(struct fsinfo_##Y))
> +
> +static const struct fsinfo_attr_info fsinfo_buffer_info[FSINFO_ATTR__NR] = {
> +	FSINFO_STRUCT		(STATFS,		statfs),
> +	FSINFO_STRUCT		(FSINFO,		fsinfo),
> +	FSINFO_STRUCT		(IDS,			ids),
> +	FSINFO_STRUCT		(LIMITS,		limits),
> +	FSINFO_STRUCT		(CAPABILITIES,		capabilities),
> +	FSINFO_STRUCT		(SUPPORTS,		supports),
> +	FSINFO_STRUCT		(TIMESTAMP_INFO,	timestamp_info),
> +	FSINFO_STRING		(VOLUME_ID),
> +	FSINFO_STRUCT		(VOLUME_UUID,		volume_uuid),
> +	FSINFO_STRING		(VOLUME_NAME),
> +	FSINFO_STRING		(NAME_ENCODING),
> +	FSINFO_STRING		(NAME_CODEPAGE),
> +};

See [1]. :)
Is it really wort it to have this code generating stuff in there?
I urge you to think about git grep users. For them this is an absolute
nightmare. :)
It's also annoying because one needs to expand the macro to review the
fsinfo() syscalls below that switches on a lot of the stuff you define
here.

> +
> +/**
> + * sys_fsinfo - System call to get filesystem information
> + * @dfd: Base directory to pathwalk from or fd referring to filesystem.
> + * @pathname: Filesystem to query or NULL.
> + * @_params: Parameters to define request (or NULL for enhanced statfs).
> + * @user_buffer: Result buffer.
> + * @user_buf_size: Size of result buffer.
> + *
> + * Get information on a filesystem.  The filesystem attribute to be queried is
> + * indicated by @_params->request, and some of the attributes can have multiple
> + * values, indexed by @_params->Nth and @_params->Mth.  If @_params is NULL,
> + * then the 0th fsinfo_attr_statfs attribute is queried.  If an attribute does
> + * not exist, EOPNOTSUPP is returned; if the Nth,Mth value does not exist,
> + * ENODATA is returned.
> + *
> + * On success, the size of the attribute's value is returned.  If
> + * @user_buf_size is 0 or @user_buffer is NULL, only the size is returned.  If
> + * the size of the value is larger than @user_buf_size, it will be truncated by
> + * the copy.  If the size of the value is smaller than @user_buf_size then the
> + * excess buffer space will be cleared.  The full size of the value will be
> + * returned, irrespective of how much data is actually placed in the buffer.
> + */
> +SYSCALL_DEFINE5(fsinfo,
> +		int, dfd, const char __user *, pathname,
> +		struct fsinfo_params __user *, params,
> +		void __user *, user_buffer, size_t, user_buf_size)
> +{
> +	struct fsinfo_attr_info info;
> +	struct fsinfo_params user_params;
> +	struct fsinfo_kparams kparams;
> +	unsigned int result_size;

Wouldn't it be better if this could be a size_t?

> +	int ret;
> +
> +	memset(&kparams, 0, sizeof(kparams));
> +
> +	if (params) {
> +		if (copy_from_user(&user_params, params, sizeof(user_params)))
> +			return -EFAULT;
> +		if (user_params.__reserved[0] ||
> +		    user_params.__reserved[1] ||
> +		    user_params.__reserved[2])
> +			return -EINVAL;
> +		if (user_params.request >= FSINFO_ATTR__NR)
> +			return -EOPNOTSUPP;
> +		kparams.at_flags = user_params.at_flags;
> +		kparams.request = user_params.request;
> +		kparams.Nth = user_params.Nth;
> +		kparams.Mth = user_params.Mth;
> +	} else {
> +		kparams.request = FSINFO_ATTR_STATFS;
> +	}
> +
> +	if (!user_buffer || !user_buf_size) {

Maybe we could be a little more strict and require both be set to their
respective zero values, i.e. only support reporting the size if
!user_buffer && user_buf_size = 0 for that to work. If only one of them
is set to their zero value we report EINVAL.

> +		user_buf_size = 0;
> +		user_buffer = NULL;
> +	}
> +
> +	/* Allocate an appropriately-sized buffer.  We will truncate the
> +	 * contents when we write the contents back to userspace.
> +	 */
> +	info = fsinfo_buffer_info[kparams.request];
> +	if (kparams.Nth != 0 && !(info.flags & (__FSINFO_N | __FSINFO_NM)))
> +		return -ENODATA;
> +	if (kparams.Mth != 0 && !(info.flags & __FSINFO_NM))
> +		return -ENODATA;
> +
> +	switch (info.type) {
> +	case __FSINFO_STRUCT:
> +		kparams.buf_size = info.size;
> +		if (user_buf_size == 0)
> +			return info.size; /* We know how big the buffer should be */
> +		break;
> +
> +	case __FSINFO_STRING:
> +		kparams.buf_size = FSINFO_NORMAL_ATTR_MAX_SIZE;
> +		break;
> +
> +	case __FSINFO_OPAQUE:
> +	case __FSINFO_STRUCT_ARRAY:
> +		/* Opaque blob or array of struct elements.  We also create a
> +		 * buffer that can be used for scratch space.
> +		 */
> +		ret = -ENOMEM;
> +		kparams.scratch_buffer = kmalloc(FSINFO_SCRATCH_BUFFER_SIZE,
> +						GFP_KERNEL);
> +		if (!kparams.scratch_buffer)
> +			goto error;
> +		kparams.overlarge = true;
> +		kparams.buf_size = FSINFO_NORMAL_ATTR_MAX_SIZE;
> +		break;
> +
> +	default:
> +		return -ENOBUFS;
> +	}
> +
> +	/* We always allocate a buffer for a string, even if buf_size == 0 and
> +	 * we're not going to return any data.  This means that the filesystem
> +	 * code needn't care about whether the buffer actually exists or not.
> +	 */
> +	ret = -ENOMEM;
> +	kparams.buffer = kvzalloc(kparams.buf_size, GFP_KERNEL);
> +	if (!kparams.buffer)
> +		goto error_scratch;
> +
> +	if (pathname)
> +		ret = vfs_fsinfo_path(dfd, pathname, &kparams);
> +	else
> +		ret = vfs_fsinfo_fd(dfd, &kparams);
> +	if (ret < 0)
> +		goto error_buffer;
> +
> +	result_size = ret;
> +	if (result_size > user_buf_size)
> +		result_size = user_buf_size;
> +
> +	if (result_size > 0 &&
> +	    copy_to_user(user_buffer, kparams.buffer, result_size) != 0) {
> +		ret = -EFAULT;
> +		goto error_buffer;
> +	}
> +
> +	/* Clear any part of the buffer that we won't fill if we're putting a
> +	 * struct in there.  Strings, opaque objects and arrays are expected to
> +	 * be variable length.
> +	 */
> +	if (info.type == __FSINFO_STRUCT &&
> +	    user_buf_size > result_size &&
> +	    clear_user(user_buffer + result_size, user_buf_size - result_size) != 0) {
> +		ret = -EFAULT;
> +		goto error_buffer;
> +	}
> +
> +error_buffer:
> +	kvfree(kparams.buffer);
> +error_scratch:
> +	kfree(kparams.scratch_buffer);
> +error:
> +	return ret;
> +}
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index f7fdfe93e25d..50f58eac3e1f 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -66,6 +66,8 @@ struct fscrypt_info;
>  struct fscrypt_operations;
>  struct fs_context;
>  struct fs_parameter_description;
> +struct fsinfo_kparams;
> +enum fsinfo_attribute;
>  
>  extern void __init inode_init(void);
>  extern void __init inode_init_early(void);
> @@ -1922,6 +1924,9 @@ struct super_operations {
>  	int (*thaw_super) (struct super_block *);
>  	int (*unfreeze_fs) (struct super_block *);
>  	int (*statfs) (struct dentry *, struct kstatfs *);
> +#ifdef CONFIG_FSINFO
> +	int (*fsinfo) (struct path *, struct fsinfo_kparams *);
> +#endif
>  	int (*remount_fs) (struct super_block *, int *, char *);
>  	void (*umount_begin) (struct super_block *);
>  
> diff --git a/include/linux/fsinfo.h b/include/linux/fsinfo.h
> new file mode 100644
> index 000000000000..4c250136d693
> --- /dev/null
> +++ b/include/linux/fsinfo.h
> @@ -0,0 +1,65 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Filesystem information query
> + *
> + * Copyright (C) 2019 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells@redhat.com)
> + */
> +
> +#ifndef _LINUX_FSINFO_H
> +#define _LINUX_FSINFO_H
> +
> +#ifdef CONFIG_FSINFO
> +
> +#include <uapi/linux/fsinfo.h>
> +
> +#define FSINFO_NORMAL_ATTR_MAX_SIZE 4096
> +#define FSINFO_SCRATCH_BUFFER_SIZE 4096
> +
> +struct fsinfo_kparams {
> +	__u32			at_flags;	/* AT_SYMLINK_NOFOLLOW and similar */
> +	enum fsinfo_attribute	request;	/* What is being asking for */
> +	__u32			Nth;		/* Instance of it (some may have multiple) */
> +	__u32			Mth;		/* Subinstance */
> +	bool			overlarge;	/* T if the buffer may be resized */
> +	unsigned int		usage;		/* Amount of buffer used (if overlarge=T) */
> +	unsigned int		buf_size;	/* Size of ->buffer[] */
> +	void			*buffer;	/* Where to place the reply */
> +	char			*scratch_buffer; /* 4K scratch buffer (if overlarge=T) */
> +};
> +
> +extern int generic_fsinfo(struct path *, struct fsinfo_kparams *);
> +
> +static inline void fsinfo_set_cap(struct fsinfo_capabilities *c,
> +				  enum fsinfo_capability cap)
> +{
> +	c->capabilities[cap / 8] |= 1 << (cap % 8);
> +}
> +
> +static inline void fsinfo_clear_cap(struct fsinfo_capabilities *c,
> +				    enum fsinfo_capability cap)
> +{
> +	c->capabilities[cap / 8] &= ~(1 << (cap % 8));
> +}
> +
> +/**
> + * fsinfo_set_unix_caps - Set standard UNIX capabilities.

Hm, I'm not sure that "capabilities" is a good name here. This is
potentially misleading because of other uses of "capabilities" we
already have. Like, I don't want thes capabilities to pop up when I do
git grep capabilities. Just a short way until someone also speaks of
"fscaps" or "fsinfocaps" and then confusion is basically guaranteed. :)

Maybe "features" would be better?

> + * @c: The capabilities mask to alter
> + */
> +static inline void fsinfo_set_unix_caps(struct fsinfo_capabilities *caps)
> +{
> +	fsinfo_set_cap(caps, FSINFO_CAP_UIDS);
> +	fsinfo_set_cap(caps, FSINFO_CAP_GIDS);
> +	fsinfo_set_cap(caps, FSINFO_CAP_DIRECTORIES);
> +	fsinfo_set_cap(caps, FSINFO_CAP_SYMLINKS);
> +	fsinfo_set_cap(caps, FSINFO_CAP_HARD_LINKS);
> +	fsinfo_set_cap(caps, FSINFO_CAP_DEVICE_FILES);
> +	fsinfo_set_cap(caps, FSINFO_CAP_UNIX_SPECIALS);
> +	fsinfo_set_cap(caps, FSINFO_CAP_SPARSE);
> +	fsinfo_set_cap(caps, FSINFO_CAP_HAS_ATIME);
> +	fsinfo_set_cap(caps, FSINFO_CAP_HAS_CTIME);
> +	fsinfo_set_cap(caps, FSINFO_CAP_HAS_MTIME);
> +}
> +
> +#endif /* CONFIG_FSINFO */
> +
> +#endif /* _LINUX_FSINFO_H */
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index e2870fe1be5b..958ac427ff37 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -50,6 +50,7 @@ struct stat64;
>  struct statfs;
>  struct statfs64;
>  struct statx;
> +struct fsinfo_params;
>  struct __sysctl_args;
>  struct sysinfo;
>  struct timespec;
> @@ -997,6 +998,9 @@ asmlinkage long sys_fspick(int dfd, const char __user *path, unsigned int flags)
>  asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
>  				       siginfo_t __user *info,
>  				       unsigned int flags);
> +asmlinkage long sys_fsinfo(int dfd, const char __user *pathname,
> +			   struct fsinfo_params __user *params,
> +			   void __user *buffer, size_t buf_size);
>  
>  /*
>   * Architecture-specific system calls
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index a87904daf103..50ddf5f25122 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -844,9 +844,11 @@ __SYSCALL(__NR_fsconfig, sys_fsconfig)
>  __SYSCALL(__NR_fsmount, sys_fsmount)
>  #define __NR_fspick 433
>  __SYSCALL(__NR_fspick, sys_fspick)
> +#define __NR_fsinfo 434
> +__SYSCALL(__NR_fsinfo, sys_fsinfo)
>  
>  #undef __NR_syscalls
> -#define __NR_syscalls 434
> +#define __NR_syscalls 435
>  
>  /*
>   * 32 bit systems traditionally used different
> diff --git a/include/uapi/linux/fsinfo.h b/include/uapi/linux/fsinfo.h
> new file mode 100644
> index 000000000000..cc7e13a9b95f
> --- /dev/null
> +++ b/include/uapi/linux/fsinfo.h
> @@ -0,0 +1,219 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +/* fsinfo() definitions.
> + *
> + * Copyright (C) 2019 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells@redhat.com)
> + */
> +#ifndef _UAPI_LINUX_FSINFO_H
> +#define _UAPI_LINUX_FSINFO_H
> +
> +#include <linux/types.h>
> +#include <linux/socket.h>
> +
> +/*
> + * The filesystem attributes that can be requested.  Note that some attributes
> + * may have multiple instances which can be switched in the parameter block.
> + */
> +enum fsinfo_attribute {
> +	FSINFO_ATTR_STATFS		= 0,	/* statfs()-style state */
> +	FSINFO_ATTR_FSINFO		= 1,	/* Information about fsinfo() */
> +	FSINFO_ATTR_IDS			= 2,	/* Filesystem IDs */
> +	FSINFO_ATTR_LIMITS		= 3,	/* Filesystem limits */
> +	FSINFO_ATTR_SUPPORTS		= 4,	/* What's supported in statx, iocflags, ... */
> +	FSINFO_ATTR_CAPABILITIES	= 5,	/* Filesystem capabilities (bits) */
> +	FSINFO_ATTR_TIMESTAMP_INFO	= 6,	/* Inode timestamp info */
> +	FSINFO_ATTR_VOLUME_ID		= 7,	/* Volume ID (string) */
> +	FSINFO_ATTR_VOLUME_UUID		= 8,	/* Volume UUID (LE uuid) */
> +	FSINFO_ATTR_VOLUME_NAME		= 9,	/* Volume name (string) */
> +	FSINFO_ATTR_NAME_ENCODING	= 10,	/* Filename encoding (string) */
> +	FSINFO_ATTR_NAME_CODEPAGE	= 11,	/* Filename codepage (string) */
> +	FSINFO_ATTR__NR
> +};
> +
> +/*
> + * Optional fsinfo() parameter structure.
> + *
> + * If this is not given, it is assumed that fsinfo_attr_statfs instance 0,0 is
> + * desired.
> + */
> +struct fsinfo_params {
> +	__u32	at_flags;	/* AT_SYMLINK_NOFOLLOW and similar flags */
> +	__u32	request;	/* What is being asking for (enum fsinfo_attribute) */
> +	__u32	Nth;		/* Instance of it (some may have multiple) */
> +	__u32	Mth;		/* Subinstance of Nth instance */
> +	__u64	__reserved[3];	/* Reserved params; all must be 0 */
> +};
> +
> +struct fsinfo_u128 {
> +#if defined(__BYTE_ORDER) ? __BYTE_ORDER == __BIG_ENDIAN : defined(__BIG_ENDIAN)
> +	__u64	hi;
> +	__u64	lo;
> +#elif defined(__BYTE_ORDER) ? __BYTE_ORDER == __LITTLE_ENDIAN : defined(__LITTLE_ENDIAN)
> +	__u64	lo;
> +	__u64	hi;
> +#endif
> +};
> +
> +/*
> + * Information struct for fsinfo(fsinfo_attr_statfs).
> + * - This gives extended filesystem information.
> + */
> +struct fsinfo_statfs {
> +	struct fsinfo_u128 f_blocks;	/* Total number of blocks in fs */
> +	struct fsinfo_u128 f_bfree;	/* Total number of free blocks */
> +	struct fsinfo_u128 f_bavail;	/* Number of free blocks available to ordinary user */
> +	struct fsinfo_u128 f_files;	/* Total number of file nodes in fs */
> +	struct fsinfo_u128 f_ffree;	/* Number of free file nodes */
> +	struct fsinfo_u128 f_favail;	/* Number of file nodes available to ordinary user */
> +	__u64	f_bsize;		/* Optimal block size */
> +	__u64	f_frsize;		/* Fragment size */
> +	__u64	mnt_attrs;		/* Mount attributes (MOUNT_ATTR_*) */
> +};
> +
> +/*
> + * Information struct for fsinfo(fsinfo_attr_ids).
> + *
> + * List of basic identifiers as is normally found in statfs().
> + */
> +struct fsinfo_ids {
> +	char	f_fs_name[15 + 1];	/* Filesystem name */
> +	__u64	f_fsid;			/* Short 64-bit Filesystem ID (as statfs) */
> +	__u64	f_sb_id;		/* Internal superblock ID for sbnotify()/mntnotify() */
> +	__u32	f_fstype;		/* Filesystem type from linux/magic.h [uncond] */
> +	__u32	f_dev_major;		/* As st_dev_* from struct statx [uncond] */
> +	__u32	f_dev_minor;
> +	__u32	__reserved[1];
> +};
> +
> +/*
> + * Information struct for fsinfo(fsinfo_attr_limits).
> + *
> + * List of supported filesystem limits.
> + */
> +struct fsinfo_limits {
> +	struct fsinfo_u128 max_file_size;	/* Maximum file size */
> +	struct fsinfo_u128 max_ino;		/* Maximum inode number */
> +	__u64	max_uid;			/* Maximum UID supported */
> +	__u64	max_gid;			/* Maximum GID supported */
> +	__u64	max_projid;			/* Maximum project ID supported */
> +	__u64	max_hard_links;			/* Maximum number of hard links on a file */
> +	__u64	max_xattr_body_len;		/* Maximum xattr content length */
> +	__u32	max_xattr_name_len;		/* Maximum xattr name length */
> +	__u32	max_filename_len;		/* Maximum filename length */
> +	__u32	max_symlink_len;		/* Maximum symlink content length */
> +	__u32	max_dev_major;			/* Maximum device major representable */
> +	__u32	max_dev_minor;			/* Maximum device minor representable */
> +	__u32	__reserved[1];
> +};
> +
> +/*
> + * Information struct for fsinfo(fsinfo_attr_supports).
> + *
> + * What's supported in various masks, such as statx() attribute and mask bits
> + * and IOC flags.
> + */
> +struct fsinfo_supports {
> +	__u64	stx_attributes;		/* What statx::stx_attributes are supported */
> +	__u32	stx_mask;		/* What statx::stx_mask bits are supported */
> +	__u32	ioc_flags;		/* What FS_IOC_* flags are supported */
> +	__u32	win_file_attrs;		/* What DOS/Windows FILE_* attributes are supported */
> +	__u32	__reserved[1];
> +};
> +
> +/*
> + * Information struct for fsinfo(fsinfo_attr_capabilities).
> + *
> + * Bitmask indicating filesystem capabilities where renderable as single bits.
> + */
> +enum fsinfo_capability {

Again, something other than "capability" might bet better, e.g.
"features".

> +	FSINFO_CAP_IS_KERNEL_FS		= 0,	/* fs is kernel-special filesystem */
> +	FSINFO_CAP_IS_BLOCK_FS		= 1,	/* fs is block-based filesystem */
> +	FSINFO_CAP_IS_FLASH_FS		= 2,	/* fs is flash filesystem */
> +	FSINFO_CAP_IS_NETWORK_FS	= 3,	/* fs is network filesystem */
> +	FSINFO_CAP_IS_AUTOMOUNTER_FS	= 4,	/* fs is automounter special filesystem */
> +	FSINFO_CAP_IS_MEMORY_FS		= 5,	/* fs is memory-based filesystem */
> +	FSINFO_CAP_AUTOMOUNTS		= 6,	/* fs supports automounts */
> +	FSINFO_CAP_ADV_LOCKS		= 7,	/* fs supports advisory file locking */
> +	FSINFO_CAP_MAND_LOCKS		= 8,	/* fs supports mandatory file locking */
> +	FSINFO_CAP_LEASES		= 9,	/* fs supports file leases */
> +	FSINFO_CAP_UIDS			= 10,	/* fs supports numeric uids */
> +	FSINFO_CAP_GIDS			= 11,	/* fs supports numeric gids */
> +	FSINFO_CAP_PROJIDS		= 12,	/* fs supports numeric project ids */
> +	FSINFO_CAP_STRING_USER_IDS	= 13,	/* fs supports string user identifiers */
> +	FSINFO_CAP_GUID_USER_IDS	= 14,	/* fs supports GUID user identifiers */
> +	FSINFO_CAP_WINDOWS_ATTRS	= 15,	/* fs has windows attributes */
> +	FSINFO_CAP_USER_QUOTAS		= 16,	/* fs has per-user quotas */
> +	FSINFO_CAP_GROUP_QUOTAS		= 17,	/* fs has per-group quotas */
> +	FSINFO_CAP_PROJECT_QUOTAS	= 18,	/* fs has per-project quotas */
> +	FSINFO_CAP_XATTRS		= 19,	/* fs has xattrs */
> +	FSINFO_CAP_JOURNAL		= 20,	/* fs has a journal */
> +	FSINFO_CAP_DATA_IS_JOURNALLED	= 21,	/* fs is using data journalling */
> +	FSINFO_CAP_O_SYNC		= 22,	/* fs supports O_SYNC */
> +	FSINFO_CAP_O_DIRECT		= 23,	/* fs supports O_DIRECT */
> +	FSINFO_CAP_VOLUME_ID		= 24,	/* fs has a volume ID */
> +	FSINFO_CAP_VOLUME_UUID		= 25,	/* fs has a volume UUID */
> +	FSINFO_CAP_VOLUME_NAME		= 26,	/* fs has a volume name */
> +	FSINFO_CAP_VOLUME_FSID		= 27,	/* fs has a volume FSID */
> +	FSINFO_CAP_IVER_ALL_CHANGE	= 28,	/* i_version represents data + meta changes */
> +	FSINFO_CAP_IVER_DATA_CHANGE	= 29,	/* i_version represents data changes only */
> +	FSINFO_CAP_IVER_MONO_INCR	= 30,	/* i_version incremented monotonically */
> +	FSINFO_CAP_DIRECTORIES		= 31,	/* fs supports (sub)directories */
> +	FSINFO_CAP_SYMLINKS		= 32,	/* fs supports symlinks */
> +	FSINFO_CAP_HARD_LINKS		= 33,	/* fs supports hard links */
> +	FSINFO_CAP_HARD_LINKS_1DIR	= 34,	/* fs supports hard links in same dir only */
> +	FSINFO_CAP_DEVICE_FILES		= 35,	/* fs supports bdev, cdev */
> +	FSINFO_CAP_UNIX_SPECIALS	= 36,	/* fs supports pipe, fifo, socket */
> +	FSINFO_CAP_RESOURCE_FORKS	= 37,	/* fs supports resource forks/streams */
> +	FSINFO_CAP_NAME_CASE_INDEP	= 38,	/* Filename case independence is mandatory */
> +	FSINFO_CAP_NAME_NON_UTF8	= 39,	/* fs has non-utf8 names */
> +	FSINFO_CAP_NAME_HAS_CODEPAGE	= 40,	/* fs has a filename codepage */
> +	FSINFO_CAP_SPARSE		= 41,	/* fs supports sparse files */
> +	FSINFO_CAP_NOT_PERSISTENT	= 42,	/* fs is not persistent */
> +	FSINFO_CAP_NO_UNIX_MODE		= 43,	/* fs does not support unix mode bits */
> +	FSINFO_CAP_HAS_ATIME		= 44,	/* fs supports access time */
> +	FSINFO_CAP_HAS_BTIME		= 45,	/* fs supports birth/creation time */
> +	FSINFO_CAP_HAS_CTIME		= 46,	/* fs supports change time */
> +	FSINFO_CAP_HAS_MTIME		= 47,	/* fs supports modification time */
> +	FSINFO_CAP__NR
> +};
> +
> +struct fsinfo_capabilities {
> +	__u8	capabilities[(FSINFO_CAP__NR + 7) / 8];
> +};
> +
> +struct fsinfo_timestamp_one {
> +	__s64	minimum;	/* Minimum timestamp value in seconds */
> +	__u64	maximum;	/* Maximum timestamp value in seconds */
> +	__u16	gran_mantissa;	/* Granularity(secs) = mant * 10^exp */
> +	__s8	gran_exponent;
> +	__u8	reserved[5];
> +};
> +
> +/*
> + * Information struct for fsinfo(fsinfo_attr_timestamp_info).
> + */
> +struct fsinfo_timestamp_info {
> +	struct fsinfo_timestamp_one	atime;	/* Access time */
> +	struct fsinfo_timestamp_one	mtime;	/* Modification time */
> +	struct fsinfo_timestamp_one	ctime;	/* Change time */
> +	struct fsinfo_timestamp_one	btime;	/* Birth/creation time */
> +};
> +
> +/*
> + * Information struct for fsinfo(fsinfo_attr_volume_uuid).
> + */
> +struct fsinfo_volume_uuid {
> +	__u8	uuid[16];
> +};
> +
> +/*
> + * Information struct for fsinfo(fsinfo_attr_fsinfo).
> + *
> + * This gives information about fsinfo() itself.
> + */
> +struct fsinfo_fsinfo {
> +	__u32	max_attr;	/* Number of supported attributes (fsinfo_attr__nr) */
> +	__u32	max_cap;	/* Number of supported capabilities (fsinfo_cap__nr) */
> +};
> +
> +#endif /* _UAPI_LINUX_FSINFO_H */
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 4d9ae5ea6caf..93927072396c 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -51,6 +51,7 @@ COND_SYSCALL_COMPAT(io_pgetevents);
>  COND_SYSCALL(io_uring_setup);
>  COND_SYSCALL(io_uring_enter);
>  COND_SYSCALL(io_uring_register);
> +COND_SYSCALL(fsinfo);
>  
>  /* fs/xattr.c */
>  
> diff --git a/samples/vfs/Makefile b/samples/vfs/Makefile
> index a3e4ffd4c773..d3cc8e9a4fd8 100644
> --- a/samples/vfs/Makefile
> +++ b/samples/vfs/Makefile
> @@ -1,10 +1,14 @@
>  # List of programs to build
>  hostprogs-y := \
> +	test-fsinfo \
>  	test-fsmount \
>  	test-statx
>  
>  # Tell kbuild to always build the programs
>  always := $(hostprogs-y)
>  
> +HOSTCFLAGS_test-fsinfo.o += -I$(objtree)/usr/include
> +HOSTLDLIBS_test-fsinfo += -lm
> +
>  HOSTCFLAGS_test-fsmount.o += -I$(objtree)/usr/include
>  HOSTCFLAGS_test-statx.o += -I$(objtree)/usr/include
> diff --git a/samples/vfs/test-fsinfo.c b/samples/vfs/test-fsinfo.c
> new file mode 100644
> index 000000000000..8cce1986df7e
> --- /dev/null
> +++ b/samples/vfs/test-fsinfo.c
> @@ -0,0 +1,551 @@
> +/* Test the fsinfo() system call
> + *
> + * Copyright (C) 2018 Red Hat, Inc. All Rights Reserved.
> + * Written by David Howells (dhowells@redhat.com)
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public Licence
> + * as published by the Free Software Foundation; either version
> + * 2 of the Licence, or (at your option) any later version.
> + */
> +
> +#define _GNU_SOURCE
> +#define _ATFILE_SOURCE

nit: Defining fsinfoat() implicitly or what's that supposed to do? If that's
the case wouldn't it be nicer to just explicitly declare fsinfoat()

> +#include <stdbool.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <stdint.h>
> +#include <string.h>
> +#include <unistd.h>
> +#include <ctype.h>
> +#include <errno.h>
> +#include <time.h>
> +#include <math.h>
> +#include <fcntl.h>
> +#include <sys/syscall.h>
> +#include <linux/fsinfo.h>
> +#include <linux/socket.h>
> +#include <sys/stat.h>
> +#include <arpa/inet.h>
> +
> +#ifndef __NR_fsinfo
> +#define __NR_fsinfo -1
> +#endif
> +
> +static bool debug = 0;
> +
> +static __attribute__((unused))
> +ssize_t fsinfo(int dfd, const char *filename, struct fsinfo_params *params,
> +	       void *buffer, size_t buf_size)
> +{
> +	return syscall(__NR_fsinfo, dfd, filename, params, buffer, buf_size);
> +}
> +
> +struct fsinfo_attr_info {
> +	unsigned char	type;
> +	unsigned char	flags;
> +	unsigned short	size;
> +};
> +
> +#define __FSINFO_STRUCT		0
> +#define __FSINFO_STRING		1
> +#define __FSINFO_OVER		2
> +#define __FSINFO_STRUCT_ARRAY	3
> +#define __FSINFO_0		0
> +#define __FSINFO_N		0x0001
> +#define __FSINFO_NM		0x0002
> +
> +#define _Z(T, F, S) { .type = __FSINFO_##T, .flags = __FSINFO_##F, .size = S }
> +#define FSINFO_STRING(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRING, 0, 0)
> +#define FSINFO_STRUCT(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRUCT, 0, sizeof(struct fsinfo_##Y))
> +#define FSINFO_STRING_N(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRING, N, 0)
> +#define FSINFO_STRUCT_N(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRUCT, N, sizeof(struct fsinfo_##Y))
> +#define FSINFO_STRING_NM(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRING, NM, 0)
> +#define FSINFO_STRUCT_NM(X,Y)	 [FSINFO_ATTR_##X] = _Z(STRUCT, NM, sizeof(struct fsinfo_##Y))
> +#define FSINFO_OVERLARGE(X,Y)	 [FSINFO_ATTR_##X] = _Z(OVER, 0, 0)
> +#define FSINFO_STRUCT_ARRAY(X,Y) [FSINFO_ATTR_##X] = _Z(STRUCT_ARRAY, 0, sizeof(struct fsinfo_##Y))

See [1] above but here it's less of an issue since this is a test file.
I missed that in the first review. :)

> +
> +static const struct fsinfo_attr_info fsinfo_buffer_info[FSINFO_ATTR__NR] = {
> +	FSINFO_STRUCT		(STATFS,		statfs),
> +	FSINFO_STRUCT		(FSINFO,		fsinfo),
> +	FSINFO_STRUCT		(IDS,			ids),
> +	FSINFO_STRUCT		(LIMITS,		limits),
> +	FSINFO_STRUCT		(CAPABILITIES,		capabilities),
> +	FSINFO_STRUCT		(SUPPORTS,		supports),
> +	FSINFO_STRUCT		(TIMESTAMP_INFO,	timestamp_info),
> +	FSINFO_STRING		(VOLUME_ID,		volume_id),
> +	FSINFO_STRUCT		(VOLUME_UUID,		volume_uuid),
> +	FSINFO_STRING		(VOLUME_NAME,		volume_name),
> +	FSINFO_STRING		(NAME_ENCODING,		name_encoding),
> +	FSINFO_STRING		(NAME_CODEPAGE,		name_codepage),
> +};
> +
> +#define FSINFO_NAME(X,Y) [FSINFO_ATTR_##X] = #Y
> +static const char *fsinfo_attr_names[FSINFO_ATTR__NR] = {
> +	FSINFO_NAME		(STATFS,		statfs),
> +	FSINFO_NAME		(FSINFO,		fsinfo),
> +	FSINFO_NAME		(IDS,			ids),
> +	FSINFO_NAME		(LIMITS,		limits),
> +	FSINFO_NAME		(CAPABILITIES,		capabilities),
> +	FSINFO_NAME		(SUPPORTS,		supports),
> +	FSINFO_NAME		(TIMESTAMP_INFO,	timestamp_info),
> +	FSINFO_NAME		(VOLUME_ID,		volume_id),
> +	FSINFO_NAME		(VOLUME_UUID,		volume_uuid),
> +	FSINFO_NAME		(VOLUME_NAME,		volume_name),
> +	FSINFO_NAME		(NAME_ENCODING,		name_encoding),
> +	FSINFO_NAME		(NAME_CODEPAGE,		name_codepage),
> +};
> +
> +union reply {
> +	char buffer[4096];
> +	struct fsinfo_statfs statfs;
> +	struct fsinfo_fsinfo fsinfo;
> +	struct fsinfo_ids ids;
> +	struct fsinfo_limits limits;
> +	struct fsinfo_supports supports;
> +	struct fsinfo_capabilities caps;
> +	struct fsinfo_timestamp_info timestamps;
> +	struct fsinfo_volume_uuid uuid;
> +};
> +
> +static void dump_hex(unsigned int *data, int from, int to)
> +{
> +	unsigned offset, print_offset = 1, col = 0;
> +
> +	from /= 4;
> +	to = (to + 3) / 4;
> +
> +	for (offset = from; offset < to; offset++) {
> +		if (print_offset) {
> +			printf("%04x: ", offset * 8);
> +			print_offset = 0;
> +		}
> +		printf("%08x", data[offset]);
> +		col++;
> +		if ((col & 3) == 0) {
> +			printf("\n");
> +			print_offset = 1;
> +		} else {
> +			printf(" ");
> +		}
> +	}
> +
> +	if (!print_offset)
> +		printf("\n");
> +}
> +
> +static void dump_attr_STATFS(union reply *r, int size)
> +{
> +	struct fsinfo_statfs *f = &r->statfs;
> +
> +	printf("\n");
> +	printf("\tblocks: n=%llu fr=%llu av=%llu\n",
> +	       (unsigned long long)f->f_blocks.lo,
> +	       (unsigned long long)f->f_bfree.lo,
> +	       (unsigned long long)f->f_bavail.lo);
> +
> +	printf("\tfiles : n=%llu fr=%llu av=%llu\n",
> +	       (unsigned long long)f->f_files.lo,
> +	       (unsigned long long)f->f_ffree.lo,
> +	       (unsigned long long)f->f_favail.lo);
> +	printf("\tbsize : %llu\n", f->f_bsize);
> +	printf("\tfrsize: %llu\n", f->f_frsize);
> +	printf("\tmntfl : %llx\n", (unsigned long long)f->mnt_attrs);
> +}
> +
> +static void dump_attr_FSINFO(union reply *r, int size)
> +{
> +	struct fsinfo_fsinfo *f = &r->fsinfo;
> +
> +	printf("max_attr=%u max_cap=%u\n", f->max_attr, f->max_cap);
> +}
> +
> +static void dump_attr_IDS(union reply *r, int size)
> +{
> +	struct fsinfo_ids *f = &r->ids;
> +
> +	printf("\n");
> +	printf("\tdev   : %02x:%02x\n", f->f_dev_major, f->f_dev_minor);
> +	printf("\tfs    : type=%x name=%s\n", f->f_fstype, f->f_fs_name);
> +	printf("\tfsid  : %llx\n", (unsigned long long)f->f_fsid);
> +}
> +
> +static void dump_attr_LIMITS(union reply *r, int size)
> +{
> +	struct fsinfo_limits *f = &r->limits;
> +
> +	printf("\n");
> +	printf("\tmax file size: %llx%016llx\n",
> +	       (unsigned long long)f->max_file_size.hi,
> +	       (unsigned long long)f->max_file_size.lo);
> +	printf("\tmax ino:       %llx%016llx\n",
> +	       (unsigned long long)f->max_ino.hi,
> +	       (unsigned long long)f->max_ino.lo);
> +	printf("\tmax ids      : u=%llx g=%llx p=%llx\n",
> +	       (unsigned long long)f->max_uid,
> +	       (unsigned long long)f->max_gid,
> +	       (unsigned long long)f->max_projid);
> +	printf("\tmax dev      : maj=%x min=%x\n",
> +	       f->max_dev_major, f->max_dev_minor);
> +	printf("\tmax links    : %llx\n",
> +	       (unsigned long long)f->max_hard_links);
> +	printf("\tmax xattr    : n=%x b=%llx\n",
> +	       f->max_xattr_name_len,
> +	       (unsigned long long)f->max_xattr_body_len);
> +	printf("\tmax len      : file=%x sym=%x\n",
> +	       f->max_filename_len, f->max_symlink_len);
> +}
> +
> +static void dump_attr_SUPPORTS(union reply *r, int size)
> +{
> +	struct fsinfo_supports *f = &r->supports;
> +
> +	printf("\n");
> +	printf("\tstx_attr=%llx\n", (unsigned long long)f->stx_attributes);
> +	printf("\tstx_mask=%x\n", f->stx_mask);
> +	printf("\tioc_flags=%x\n", f->ioc_flags);
> +	printf("\twin_fattrs=%x\n", f->win_file_attrs);
> +}
> +
> +#define FSINFO_CAP_NAME(C) [FSINFO_CAP_##C] = #C
> +static const char *fsinfo_cap_names[FSINFO_CAP__NR] = {
> +	FSINFO_CAP_NAME(IS_KERNEL_FS),
> +	FSINFO_CAP_NAME(IS_BLOCK_FS),
> +	FSINFO_CAP_NAME(IS_FLASH_FS),
> +	FSINFO_CAP_NAME(IS_NETWORK_FS),
> +	FSINFO_CAP_NAME(IS_AUTOMOUNTER_FS),
> +	FSINFO_CAP_NAME(IS_MEMORY_FS),
> +	FSINFO_CAP_NAME(AUTOMOUNTS),
> +	FSINFO_CAP_NAME(ADV_LOCKS),
> +	FSINFO_CAP_NAME(MAND_LOCKS),
> +	FSINFO_CAP_NAME(LEASES),
> +	FSINFO_CAP_NAME(UIDS),
> +	FSINFO_CAP_NAME(GIDS),
> +	FSINFO_CAP_NAME(PROJIDS),
> +	FSINFO_CAP_NAME(STRING_USER_IDS),
> +	FSINFO_CAP_NAME(GUID_USER_IDS),
> +	FSINFO_CAP_NAME(WINDOWS_ATTRS),
> +	FSINFO_CAP_NAME(USER_QUOTAS),
> +	FSINFO_CAP_NAME(GROUP_QUOTAS),
> +	FSINFO_CAP_NAME(PROJECT_QUOTAS),
> +	FSINFO_CAP_NAME(XATTRS),
> +	FSINFO_CAP_NAME(JOURNAL),
> +	FSINFO_CAP_NAME(DATA_IS_JOURNALLED),
> +	FSINFO_CAP_NAME(O_SYNC),
> +	FSINFO_CAP_NAME(O_DIRECT),
> +	FSINFO_CAP_NAME(VOLUME_ID),
> +	FSINFO_CAP_NAME(VOLUME_UUID),
> +	FSINFO_CAP_NAME(VOLUME_NAME),
> +	FSINFO_CAP_NAME(VOLUME_FSID),
> +	FSINFO_CAP_NAME(IVER_ALL_CHANGE),
> +	FSINFO_CAP_NAME(IVER_DATA_CHANGE),
> +	FSINFO_CAP_NAME(IVER_MONO_INCR),
> +	FSINFO_CAP_NAME(DIRECTORIES),
> +	FSINFO_CAP_NAME(SYMLINKS),
> +	FSINFO_CAP_NAME(HARD_LINKS),
> +	FSINFO_CAP_NAME(HARD_LINKS_1DIR),
> +	FSINFO_CAP_NAME(DEVICE_FILES),
> +	FSINFO_CAP_NAME(UNIX_SPECIALS),
> +	FSINFO_CAP_NAME(RESOURCE_FORKS),
> +	FSINFO_CAP_NAME(NAME_CASE_INDEP),
> +	FSINFO_CAP_NAME(NAME_NON_UTF8),
> +	FSINFO_CAP_NAME(NAME_HAS_CODEPAGE),
> +	FSINFO_CAP_NAME(SPARSE),
> +	FSINFO_CAP_NAME(NOT_PERSISTENT),
> +	FSINFO_CAP_NAME(NO_UNIX_MODE),
> +	FSINFO_CAP_NAME(HAS_ATIME),
> +	FSINFO_CAP_NAME(HAS_BTIME),
> +	FSINFO_CAP_NAME(HAS_CTIME),
> +	FSINFO_CAP_NAME(HAS_MTIME),
> +};
> +
> +static void dump_attr_CAPABILITIES(union reply *r, int size)
> +{
> +	struct fsinfo_capabilities *f = &r->caps;
> +	int i;
> +
> +	for (i = 0; i < sizeof(f->capabilities); i++)
> +		printf("%02x", f->capabilities[i]);
> +	printf("\n");
> +	for (i = 0; i < FSINFO_CAP__NR; i++)
> +		if (f->capabilities[i / 8] & (1 << (i % 8)))
> +			printf("\t- %s\n", fsinfo_cap_names[i]);
> +}
> +
> +static void print_time(struct fsinfo_timestamp_one *t, char stamp)
> +{
> +	printf("\t%ctime : gran=%gs range=%llx-%llx\n",
> +	       stamp,
> +	       t->gran_mantissa * pow(10., t->gran_exponent),
> +	       (long long)t->minimum,
> +	       (long long)t->maximum);
> +}
> +
> +static void dump_attr_TIMESTAMP_INFO(union reply *r, int size)
> +{
> +	struct fsinfo_timestamp_info *f = &r->timestamps;
> +
> +	printf("\n");
> +	print_time(&f->atime, 'a');
> +	print_time(&f->mtime, 'm');
> +	print_time(&f->ctime, 'c');
> +	print_time(&f->btime, 'b');
> +}
> +
> +static void dump_attr_VOLUME_UUID(union reply *r, int size)
> +{
> +	struct fsinfo_volume_uuid *f = &r->uuid;
> +
> +	printf("%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x"
> +	       "-%02x%02x%02x%02x%02x%02x\n",
> +	       f->uuid[ 0], f->uuid[ 1],
> +	       f->uuid[ 2], f->uuid[ 3],
> +	       f->uuid[ 4], f->uuid[ 5],
> +	       f->uuid[ 6], f->uuid[ 7],
> +	       f->uuid[ 8], f->uuid[ 9],
> +	       f->uuid[10], f->uuid[11],
> +	       f->uuid[12], f->uuid[13],
> +	       f->uuid[14], f->uuid[15]);
> +}
> +
> +/*
> + *
> + */
> +typedef void (*dumper_t)(union reply *r, int size);
> +
> +#define FSINFO_DUMPER(N) [FSINFO_ATTR_##N] = dump_attr_##N
> +static const dumper_t fsinfo_attr_dumper[FSINFO_ATTR__NR] = {
> +	FSINFO_DUMPER(STATFS),
> +	FSINFO_DUMPER(FSINFO),
> +	FSINFO_DUMPER(IDS),
> +	FSINFO_DUMPER(LIMITS),
> +	FSINFO_DUMPER(SUPPORTS),
> +	FSINFO_DUMPER(CAPABILITIES),
> +	FSINFO_DUMPER(TIMESTAMP_INFO),
> +	FSINFO_DUMPER(VOLUME_UUID),
> +};
> +
> +static void dump_fsinfo(enum fsinfo_attribute attr,
> +			struct fsinfo_attr_info about,
> +			union reply *r, int size)
> +{
> +	dumper_t dumper = fsinfo_attr_dumper[attr];
> +	unsigned int len;
> +
> +	if (!dumper) {
> +		printf("<no dumper>\n");
> +		return;
> +	}
> +
> +	len = about.size;
> +	if (about.type == __FSINFO_STRUCT && size < len) {
> +		printf("<short data %u/%u>\n", size, len);
> +		return;
> +	}
> +
> +	dumper(r, size);
> +}
> +
> +/*
> + * Try one subinstance of an attribute.
> + */
> +static int try_one(const char *file, struct fsinfo_params *params, bool raw)
> +{
> +	struct fsinfo_attr_info about;
> +	union reply *r;
> +	size_t buf_size = 4096;
> +	char *p;
> +	int ret;
> +
> +	for (;;) {
> +		r = malloc(buf_size);
> +		if (!r) {
> +			perror("malloc");
> +			exit(1);
> +		}
> +		memset(r->buffer, 0xbd, buf_size);
> +
> +		errno = 0;
> +		ret = fsinfo(AT_FDCWD, file, params, r->buffer, buf_size);
> +		if (params->request >= FSINFO_ATTR__NR) {
> +			if (ret == -1 && errno == EOPNOTSUPP)
> +				exit(0);
> +			fprintf(stderr, "Unexpected error for too-large command %u: %m\n",
> +				params->request);
> +			exit(1);
> +		}
> +		if (ret == -1)
> +			break;
> +
> +		if (ret <= buf_size)
> +			break;
> +		buf_size = (ret + 4096 - 1) & ~(4096 - 1);
> +	}
> +
> +	if (debug)
> +		printf("fsinfo(%s,%s,%u,%u) = %d: %m\n",
> +		       file, fsinfo_attr_names[params->request],
> +		       params->Nth, params->Mth, ret);
> +
> +	about = fsinfo_buffer_info[params->request];
> +	if (ret == -1) {
> +		if (errno == ENODATA) {
> +			if (!(about.flags & (__FSINFO_N | __FSINFO_NM)) &&
> +			    params->Nth == 0 && params->Mth == 0) {
> +				fprintf(stderr,
> +					"Unexpected ENODATA (%u[%u][%u])\n",
> +					params->request, params->Nth, params->Mth);
> +				exit(1);
> +			}
> +			return (params->Mth == 0) ? 2 : 1;
> +		}
> +		if (errno == EOPNOTSUPP) {
> +			if (params->Nth > 0 || params->Mth > 0) {
> +				fprintf(stderr,
> +					"Should return -ENODATA (%u[%u][%u])\n",
> +					params->request, params->Nth, params->Mth);
> +				exit(1);
> +			}
> +			//printf("\e[33m%s\e[m: <not supported>\n",
> +			//       fsinfo_attr_names[attr]);
> +			return 2;
> +		}
> +		perror(file);
> +		exit(1);
> +	}
> +
> +	if (raw) {
> +		if (ret > 4096)
> +			ret = 4096;
> +		dump_hex((unsigned int *)r->buffer, 0, ret);
> +		return 0;
> +	}
> +
> +	switch (about.flags & (__FSINFO_N | __FSINFO_NM)) {
> +	case 0:
> +		printf("\e[33m%s\e[m: ",
> +		       fsinfo_attr_names[params->request]);
> +		break;
> +	case __FSINFO_N:
> +		printf("\e[33m%s[%u]\e[m: ",
> +		       fsinfo_attr_names[params->request],
> +		       params->Nth);
> +		break;
> +	case __FSINFO_NM:
> +		printf("\e[33m%s[%u][%u]\e[m: ",
> +		       fsinfo_attr_names[params->request],
> +		       params->Nth, params->Mth);
> +		break;
> +	}
> +
> +	switch (about.type) {
> +	case __FSINFO_STRUCT:
> +		dump_fsinfo(params->request, about, r, ret);
> +		return 0;
> +
> +	case __FSINFO_STRING:
> +		if (ret >= 4096) {
> +			ret = 4096;
> +			r->buffer[4092] = '.';
> +			r->buffer[4093] = '.';
> +			r->buffer[4094] = '.';
> +			r->buffer[4095] = 0;
> +		} else {
> +			r->buffer[ret] = 0;
> +		}
> +		for (p = r->buffer; *p; p++) {
> +			if (!isprint(*p)) {
> +				printf("<non-printable>\n");
> +				continue;
> +			}
> +		}
> +		printf("%s\n", r->buffer);
> +		return 0;
> +
> +	case __FSINFO_OVER:
> +		return 0;
> +
> +	case __FSINFO_STRUCT_ARRAY:
> +		dump_fsinfo(params->request, about, r, ret);
> +		return 0;
> +
> +	default:
> +		fprintf(stderr, "Fishy about %u %u,%u,%u\n",
> +			params->request, about.type, about.flags, about.size);
> +		exit(1);
> +	}
> +}
> +
> +/*
> + *
> + */
> +int main(int argc, char **argv)
> +{
> +	struct fsinfo_params params = {
> +		.at_flags = AT_SYMLINK_NOFOLLOW,
> +	};
> +	unsigned int attr;
> +	int raw = 0, opt, Nth, Mth;
> +
> +	while ((opt = getopt(argc, argv, "adlr"))) {
> +		switch (opt) {
> +		case 'a':
> +			params.at_flags |= AT_NO_AUTOMOUNT;
> +			continue;
> +		case 'd':
> +			debug = true;
> +			continue;
> +		case 'l':
> +			params.at_flags &= ~AT_SYMLINK_NOFOLLOW;
> +			continue;
> +		case 'r':
> +			raw = 1;
> +			continue;
> +		}
> +		break;
> +	}
> +
> +	argc -= optind;
> +	argv += optind;
> +
> +	if (argc != 1) {
> +		printf("Format: test-fsinfo [-alr] <file>\n");
> +		exit(2);
> +	}
> +
> +	for (attr = 0; attr <= FSINFO_ATTR__NR; attr++) {
> +		Nth = 0;
> +		do {
> +			Mth = 0;
> +			do {
> +				params.request = attr;
> +				params.Nth = Nth;
> +				params.Mth = Mth;
> +
> +				switch (try_one(argv[0], &params, raw)) {
> +				case 0:
> +					continue;
> +				case 1:
> +					goto done_M;
> +				case 2:
> +					goto done_N;
> +				}
> +			} while (++Mth < 100);
> +
> +		done_M:
> +			if (Mth >= 100) {
> +				fprintf(stderr, "Fishy: Mth == %u\n", Mth);
> +				break;
> +			}
> +
> +		} while (++Nth < 100);
> +
> +	done_N:
> +		if (Nth >= 100) {
> +			fprintf(stderr, "Fishy: Nth == %u\n", Nth);
> +			break;
> +		}
> +	}
> +
> +	return 0;
> +}
>