Re: [PATCH v3] fs: introduce getfsxattrat and setfsxattrat syscalls

Andrey Albershteyn
2025-02-28T08:30:45Z

On 2025-02-21 20:15:24, Amir Goldstein wrote:
> On Fri, Feb 21, 2025 at 7:13 PM Darrick J. Wong <djwong@kernel.org> wrote:
> >
> > On Tue, Feb 11, 2025 at 06:22:47PM +0100, Andrey Albershteyn wrote:
> > > From: Andrey Albershteyn <aalbersh@redhat.com>
> > >
> > > Introduce getfsxattrat and setfsxattrat syscalls to manipulate inode
> > > extended attributes/flags. The syscalls take parent directory fd and
> > > path to the child together with struct fsxattr.
> > >
> > > This is an alternative to FS_IOC_FSSETXATTR ioctl with a difference
> > > that file don't need to be open as we can reference it with a path
> > > instead of fd. By having this we can manipulated inode extended
> > > attributes not only on regular files but also on special ones. This
> > > is not possible with FS_IOC_FSSETXATTR ioctl as with special files
> > > we can not call ioctl() directly on the filesystem inode using fd.
> > >
> > > This patch adds two new syscalls which allows userspace to get/set
> > > extended inode attributes on special files by using parent directory
> > > and a path - *at() like syscall.
> > >
> > > Also, as vfs_fileattr_set() is now will be called on special files
> > > too, let's forbid any other attributes except projid and nextents
> > > (symlink can have an extent).
> > >
> > > CC: linux-api@vger.kernel.org
> > > CC: linux-fsdevel@vger.kernel.org
> > > CC: linux-xfs@vger.kernel.org
> > > Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
> > > ---
> > > 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/
> > >
> > > 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. Moreover, in the case
> > > when special files are created in the directory with already
> > > existing project quota, these inode inherit extended attributes.
> > > This than leaves them with these attributes without the possibility
> > > to clear them out. This, in turn, prevents userspace from
> > > re-creating quota project on these existing files.
> > > ---
> > > 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
> > > ---
> > > 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/inode.c | 75 +++++++++++++++++++++++++++++
> > > fs/ioctl.c | 16 +++++-
> > > include/linux/fileattr.h | 1 +
> > > include/linux/syscalls.h | 4 ++
> > > include/uapi/asm-generic/unistd.h | 8 ++-
> > > 21 files changed, 133 insertions(+), 3 deletions(-)
> > >
> >
> > <cut to the syscall definitions>
> >
> > > diff --git a/fs/inode.c b/fs/inode.c
> > > index 6b4c77268fc0ecace4ac78a9ca777fbffc277f4a..b2dddd9db4fabaf67a6cbf541a86978b290411ec 100644
> > > --- a/fs/inode.c
> > > +++ b/fs/inode.c
> > > @@ -23,6 +23,9 @@
> > > #include <linux/rw_hint.h>
> > > #include <linux/seq_file.h>
> > > #include <linux/debugfs.h>
> > > +#include <linux/syscalls.h>
> > > +#include <linux/fileattr.h>
> > > +#include <linux/namei.h>
> > > #include <trace/events/writeback.h>
> > > #define CREATE_TRACE_POINTS
> > > #include <trace/events/timestamp.h>
> > > @@ -2953,3 +2956,75 @@ umode_t mode_strip_sgid(struct mnt_idmap *idmap,
> > > return mode & ~S_ISGID;
> > > }
> > > EXPORT_SYMBOL(mode_strip_sgid);
> > > +
> > > +SYSCALL_DEFINE4(getfsxattrat, int, dfd, const char __user *, filename,
> > > + struct fsxattr __user *, fsx, unsigned int, at_flags)
> >
> > Should the kernel require userspace to pass the size of the fsx buffer?
> > That way we avoid needing to rev the interface when we decide to grow
> > the structure.
> >
>
> This makes sense to me, but I see that Andreas proposed other ways,
> as long as we have a plan on how to extend the struct if we need more space.
>
> Andrey, I am sorry to bring this up in v3, but I would like to request
> two small changes before merging this API.
>
> This patch by Pali [1] adds fsx_xflags_mask for the filesystem to
> report the supported set of xflags.
>
> It was argued that we can make this change with the existing ioctl,
> because it is not going to break xfs_io -c lsattr/chattr, which is fine,
> but I think that we should merge the fsx_xflags_mask change along
> with getfsxattrat() which is a new UAPI.
>
> The second request is related to setfsxattrat().
> With current FS_IOC_FSSETXATTR, IIUC, xfs ignores unsupported
> fsx_xflags. I think this needs to be fixed before merging setfsxattrat().
> It's ok that a program calling FS_IOC_FSSETXATTR will not know
> if unsupported flags will be ignored, because that's the way it is,
> but I think that setfsxattrat() must return -EINVAL for trying to
> set unsupported xflags.
>
> As I explained in [2] I think it is fine if FS_IOC_FSSETXATTR
> will also start returning -EINVAL for unsupported flags, but I would
> like setfsxattrat() to make that a guarantee.
>
> There was an open question, what does fsx_xflags_mask mean
> for setfsxattrat() - it is a mask like in inode_set_flags() as Andreas
> suggested? I think that would be a good idea.
>
> Thanks,
> Amir.
>
> [1] https://lore.kernel.org/linux-fsdevel/20250216164029.20673-4-pali@kernel.org/
> [2] https://lore.kernel.org/linux-fsdevel/CAOQ4uxjwQJiKAqyjEmKUnq-VihyeSsxyEy2F+J38NXwrAXurFQ@mail.gmail.com/
>

I'm fine with making Pali's patchset a dependency for this syscall,
as if vfs_fileattr_set() will start returning EINVAL on unsupported
flags this syscall will pass it through (ioctls will need to ignore
it). And as these syscalls use fsxattr anyway the fsx_xflags_mask
field will be here.

--
- Andrey