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

On Friday 28 February 2025 09:30:38 Andrey Albershteyn wrote:
> On 2025-02-21 20:15:24, Amir Goldstein wrote:
> > On Fri, Feb 21, 2025 at 7:13 PM Darrick J. Wong <[email protected]> wrote:
> > >
> > > On Tue, Feb 11, 2025 at 06:22:47PM +0100, Andrey Albershteyn wrote:
> > > > From: Andrey Albershteyn <[email protected]>
> > > >
> > > > 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: [email protected]
> > > > CC: [email protected]
> > > > CC: [email protected]
> > > > Signed-off-by: Andrey Albershteyn <[email protected]>
> > > > ---
> > > > v1:
> > > > https://lore.kernel.org/linuxppc-dev/[email protected]/
> > > >
> > > > Previous discussion:
> > > > https://lore.kernel.org/linux-xfs/[email protected]/
> > > >
> > > > 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/[email protected]
> > > > ---
> > > >  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/[email protected]/
> > [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
> 

Hello Andrey, if I understand correctly then it is needed for new
setfsxattrat() call to return EINVAL on any unsupported flags since
beginning.

Then I could extend it for new flags without breaking backward
or forward compatibility of the setfsxattrat() call.