Re: [PATCH v6 5/6] fs: prepare for extending file_get/setattr()

Amir Goldstein
2025-07-01T19:27:52Z

On Tue, Jul 1, 2025 at 8:31 PM Darrick J. Wong <djwong@kernel.org> wrote:
>
> On Mon, Jun 30, 2025 at 06:20:15PM +0200, Andrey Albershteyn wrote:
> > From: Amir Goldstein <amir73il@gmail.com>
> >
> > We intend to add support for more xflags to selective filesystems and
> > We cannot rely on copy_struct_from_user() to detect this extension.
> >
> > In preparation of extending the API, do not allow setting xflags unknown
> > by this kernel version.
> >
> > Also do not pass the read-only flags and read-only field fsx_nextents to
> > filesystem.
> >
> > These changes should not affect existing chattr programs that use the
> > ioctl to get fsxattr before setting the new values.
> >
> > Link: https://lore.kernel.org/linux-fsdevel/20250216164029.20673-4-pali@kernel.org/
> > Cc: Pali Rohár <pali@kernel.org>
> > Cc: Andrey Albershteyn <aalbersh@redhat.com>
> > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
> > ---
> > fs/file_attr.c | 8 +++++++-
> > include/linux/fileattr.h | 20 ++++++++++++++++++++
> > 2 files changed, 27 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/file_attr.c b/fs/file_attr.c
> > index 4e85fa00c092..62f08872d4ad 100644
> > --- a/fs/file_attr.c
> > +++ b/fs/file_attr.c
> > @@ -99,9 +99,10 @@ EXPORT_SYMBOL(vfs_fileattr_get);
> > int copy_fsxattr_to_user(const struct fileattr *fa, struct fsxattr __user *ufa)
> > {
> > struct fsxattr xfa;
> > + __u32 mask = FS_XFLAGS_MASK;
> >
> > memset(&xfa, 0, sizeof(xfa));
> > - xfa.fsx_xflags = fa->fsx_xflags;
> > + xfa.fsx_xflags = fa->fsx_xflags & mask;
>
> I wonder, should it be an error if a filesystem sets an fsx_xflags bit
> outside of FS_XFLAGS_MASK? I guess that's one way to prevent
> filesystems from overriding the VFS bits. ;)

I think Pali has a plan on how to ensure that later
when the mask is provided via the API.

>
> Though couldn't that be:
>
> xfa.fsx_xflags = fa->fsx_xflags & FS_XFLAGS_MASK;
>
> instead? And same below?
>

Indeed. There is a reason for the var, because the next series
by Pali will use a user provided mask, which defaults to FS_XFLAGS_MASK,
so I left it this way.

I don't see a problem with it keeping as is, but if it bothers you
I guess we can re-add the var later.

> > xfa.fsx_extsize = fa->fsx_extsize;
> > xfa.fsx_nextents = fa->fsx_nextents;
> > xfa.fsx_projid = fa->fsx_projid;
> > @@ -118,11 +119,16 @@ static int copy_fsxattr_from_user(struct fileattr *fa,
> > struct fsxattr __user *ufa)
> > {
> > struct fsxattr xfa;
> > + __u32 mask = FS_XFLAGS_MASK;
> >
> > if (copy_from_user(&xfa, ufa, sizeof(xfa)))
> > return -EFAULT;
> >
> > + if (xfa.fsx_xflags & ~mask)
> > + return -EINVAL;
>
> I wonder if you want EOPNOTSUPP here? We don't know how to support
> unknown xflags. OTOH if you all have beaten this to death while I was
> out then don't start another round just for me. :P

We have beaten this API almost to death for sure ;)
I don't remember if we discussed this specific aspect,
but I am personally in favor of
EOPNOTSUPP := the fs does not support the set/get operation
EINVAL := some flags provided as value is invalid

For example, if the get API provides you with a mask of the
valid flags that you can set, if you try to set flags outside of
that mask you get EINVAL.

That's my interpretation, but I agree that EOPNOTSUPP can also
make sense in this situation.

Thanks,
Amir.