[PATCH v6 0/6] fs: introduce file_getattr and file_setattr syscalls

Andrey Albershteyn
2025-06-30T16:20:35Z

This patchset introduced two new syscalls file_getattr() and
file_setattr(). These syscalls are similar to FS_IOC_FSSETXATTR ioctl()
except they use *at() semantics. Therefore, there's no need to open the
file to get a fd.

These syscalls allow userspace to set filesystem inode attributes on
special files. One of the usage examples is XFS quota projects.

XFS has project quotas which could be attached to a directory. All
new inodes in these directories inherit project ID set on parent
directory.

The project is created from userspace by opening and calling
FS_IOC_FSSETXATTR on each inode. This is not possible for special
files such as FIFO, SOCK, BLK etc. Therefore, some inodes are left
with empty project ID. Those inodes then are not shown in the quota
accounting but still exist in the directory. This is not critical but in
the case when special files are created in the directory with already
existing project quota, these new inodes inherit extended attributes.
This creates a mix of special files with and without attributes.
Moreover, special files with attributes don't have a possibility to
become clear or change the attributes. This, in turn, prevents userspace
from re-creating quota project on these existing files.

An xfstests test generic/766 with basic coverage is at:
https://github.com/alberand/xfstests/commits/b4/file-attr/

NAME

file_getattr/file_setattr - get/set filesystem inode attributes

SYNOPSIS

#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>

long syscall(SYS_file_getattr, int dirfd, const char *pathname,
struct fsx_fileattr *fsx, size_t size,
unsigned int at_flags);
long syscall(SYS_file_setattr, int dirfd, const char *pathname,
struct fsx_fileattr *fsx, size_t size,
unsigned int at_flags);

Note: glibc doesn't provide for file_getattr()/file_setattr(),
use syscall(2) instead.

DESCRIPTION

The file_getattr()/file_setattr() are used to set extended file
attributes. These syscalls take dirfd in conjunction with the
pathname argument. The syscall then operates on inode opened
according to openat(2) semantics.

This is an alternative to FS_IOC_FSGETXATTR/FS_IOC_FSSETXATTR
ioctl with a difference that file don't need to be open as file
can be referenced with a path instead of fd. By having this one
can manipulated filesystem inode attributes not only on regular
files but also on special ones. This is not possible with
FS_IOC_FSSETXATTR ioctl as ioctl() can not be called on special
files directly for the filesystem inode.

at_flags can be set to AT_SYMLINK_NOFOLLOW or AT_EMPTY_PATH.

RETURN VALUE

On success, 0 is returned. On error, -1 is returned, and errno
is set to indicate the error.

ERRORS

EINVAL Invalid at_flag specified (only
AT_SYMLINK_NOFOLLOW and AT_EMPTY_PATH is
supported).

EINVAL Size was smaller than any known version of
struct fsx_fileattr.

EINVAL Invalid combination of parameters provided in
fsx_fileattr for this type of file.

E2BIG Size of input argument struct fsx_fileattr
is too big.

EBADF Invalid file descriptor was provided.

EPERM No permission to change this file.

EOPNOTSUPP Filesystem does not support setting attributes
on this type of inode

HISTORY

Added in Linux 6.16.

EXAMPLE

Create directory and file "mkdir ./dir && touch ./dir/foo" and then
execute the following program:

#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <linux/fs.h>
#include <stdio.h>
#include <sys/syscall.h>
#include <unistd.h>

#if !defined(SYS_file_getattr) && defined(__x86_64__)
#define SYS_file_getattr 468
#define SYS_file_setattr 469

struct fsx_fileattr {
__u32 fsx_xflags;
__u32 fsx_extsize;
__u32 fsx_nextents;
__u32 fsx_projid;
__u32 fsx_cowextsize;
};
#endif

int
main(int argc, char **argv) {
int dfd;
int error;
struct fsx_fileattr fsx;

dfd = open("./dir", O_RDONLY);
if (dfd == -1) {
printf("can not open ./dir");
return dfd;
}

error = syscall(SYS_file_getattr, dfd, "./foo", &fsx,
sizeof(struct fsx_fileattr), 0);
if (error) {
printf("can not call SYS_file_getattr: %s",
strerror(errno));
return error;
}

printf("./dir/foo flags: %d\n", fsx.fsx_xflags);

fsx.fsx_xflags |= FS_XFLAG_NODUMP;
error = syscall(SYS_file_setattr, dfd, "./foo", &fsx,
sizeof(struct fsx_fileattr), 0);
if (error) {
printf("can not call SYS_file_setattr: %s",
strerror(errno));
return error;
}

printf("./dir/foo flags: %d\n", fsx.fsx_xflags);

return error;
}

SEE ALSO

ioctl(2), ioctl_iflags(2), ioctl_xfs_fsgetxattr(2), openat(2)

---
Changes in v6:
- Update cover letter example and docs
- Applied __free() attribute for syscall stack objects
- Introduced struct fsx_fileattr
- Replace 'struct fsxattr' with 'struct fsx_fileattr'
- Add helper to fill in fsx_fileattr from fileattr
- Dropped copy_fsx_to_user() header declaration
- Link to v5: https://lore.kernel.org/r/20250513-xattrat-syscall-v5-0-22bb9c6c767f@kernel.org

Changes in v5:
- Remove setting of LOOKUP_EMPTY flags which does not have any effect
- Return -ENOSUPP from vfs_fileattr_set()
- Add fsxattr masking (by Amir)
- Fix UAF issue dentry
- Fix getname_maybe_null() issue with NULL path
- Implement file_getattr/file_setattr hooks
- Return LSM return code from file_setattr
- Rename from getfsxattrat/setfsxattrat to file_getattr/file_setattr
- Link to v4: https://lore.kernel.org/r/20250321-xattrat-syscall-v4-0-3e82e6fb3264@kernel.org

Changes in v4:
- Use getname_maybe_null() for correct handling of dfd + path semantic
- Remove restriction for special files on which flags are allowed
- Utilize copy_struct_from_user() for better future compatibility
- Add draft man page to cover letter
- Convert -ENOIOCTLCMD to -EOPNOSUPP as more appropriate for syscall
- Add missing __user to header declaration of syscalls
- Link to v3: https://lore.kernel.org/r/20250211-xattrat-syscall-v3-1-a07d15f898b2@kernel.org

Changes in v3:
- Remove unnecessary "dfd is dir" check as it checked in user_path_at()
- Remove unnecessary "same filesystem" check
- Use CLASS() instead of directly calling fdget/fdput
- Link to v2: https://lore.kernel.org/r/20250122-xattrat-syscall-v2-1-5b360d4fbcb2@kernel.org

v1:
https://lore.kernel.org/linuxppc-dev/20250109174540.893098-1-aalbersh@kernel.org/

Previous discussion:
https://lore.kernel.org/linux-xfs/20240520164624.665269-2-aalbersh@redhat.com/

---
Amir Goldstein (1):
fs: prepare for extending file_get/setattr()

Andrey Albershteyn (5):
fs: split fileattr related helpers into separate file
lsm: introduce new hooks for setting/getting inode fsxattr
selinux: implement inode_file_[g|s]etattr hooks
fs: make vfs_fileattr_[get|set] return -EOPNOSUPP
fs: introduce file_getattr and file_setattr syscalls

arch/alpha/kernel/syscalls/syscall.tbl | 2 +
arch/arm/tools/syscall.tbl | 2 +
arch/arm64/tools/syscall_32.tbl | 2 +
arch/m68k/kernel/syscalls/syscall.tbl | 2 +
arch/microblaze/kernel/syscalls/syscall.tbl | 2 +
arch/mips/kernel/syscalls/syscall_n32.tbl | 2 +
arch/mips/kernel/syscalls/syscall_n64.tbl | 2 +
arch/mips/kernel/syscalls/syscall_o32.tbl | 2 +
arch/parisc/kernel/syscalls/syscall.tbl | 2 +
arch/powerpc/kernel/syscalls/syscall.tbl | 2 +
arch/s390/kernel/syscalls/syscall.tbl | 2 +
arch/sh/kernel/syscalls/syscall.tbl | 2 +
arch/sparc/kernel/syscalls/syscall.tbl | 2 +
arch/x86/entry/syscalls/syscall_32.tbl | 2 +
arch/x86/entry/syscalls/syscall_64.tbl | 2 +
arch/xtensa/kernel/syscalls/syscall.tbl | 2 +
fs/Makefile | 3 +-
fs/ecryptfs/inode.c | 8 +-
fs/file_attr.c | 493 ++++++++++++++++++++++++++++
fs/ioctl.c | 309 -----------------
fs/overlayfs/inode.c | 2 +-
include/linux/fileattr.h | 24 ++
include/linux/lsm_hook_defs.h | 2 +
include/linux/security.h | 16 +
include/linux/syscalls.h | 6 +
include/uapi/asm-generic/unistd.h | 8 +-
include/uapi/linux/fs.h | 18 +
scripts/syscall.tbl | 2 +
security/security.c | 30 ++
security/selinux/hooks.c | 14 +
30 files changed, 654 insertions(+), 313 deletions(-)
---
base-commit: d0b3b7b22dfa1f4b515fd3a295b3fd958f9e81af
change-id: 20250114-xattrat-syscall-6a1136d2db59

Best regards,
--
Andrey Albershteyn <aalbersh@kernel.org>