Re: [PATCH v4 0/3] fs: introduce getfsxattrat and setfsxattrat syscalls

["Pali Rohár" <pali@kernel.org>]

On Thursday 27 March 2025 12:47:02 Amir Goldstein wrote:
> On Sun, Mar 23, 2025 at 11:32 AM Pali Rohár <pali@kernel.org> wrote:
> >
> > On Sunday 23 March 2025 09:45:06 Amir Goldstein wrote:
> > > On Fri, Mar 21, 2025 at 8:50 PM Andrey Albershteyn <aalbersh@redhat.com> wrote:
> > > >
> > > > This patchset introduced two new syscalls getfsxattrat() and
> > > > setfsxattrat(). 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 an 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.
> > > >
> > > > Christian, if this get in some mergeable state, please don't merge it
> > > > yet. Amir suggested these syscalls better to use updated struct fsxattr
> > > > with masking from Pali Rohár patchset, so, let's see how it goes.
> > >
> > > Andrey,
> > >
> > > To be honest I don't think it would be fair to delay your syscalls more
> > > than needed.
> >
> > I agree.
> >
> > > If Pali can follow through and post patches on top of your syscalls for
> > > next merge window that would be great, but otherwise, I think the
> > > minimum requirement is that the syscalls return EINVAL if fsx_pad
> > > is not zero. we can take it from there later.
> >
> > IMHO SYS_getfsxattrat is fine in this form.
> >
> > For SYS_setfsxattrat I think there are needed some modifications
> > otherwise we would have problem again with backward compatibility as
> > is with ioctl if the syscall wants to be extended in future.
> >
> > I would suggest for following modifications for SYS_setfsxattrat:
> >
> > - return EINVAL if fsx_xflags contains some reserved or unsupported flag
> >
> > - add some flag to completely ignore fsx_extsize, fsx_projid, and
> >   fsx_cowextsize fields, so SYS_setfsxattrat could be used just to
> >   change fsx_xflags, and so could be used without the preceding
> >   SYS_getfsxattrat call.
> >
> > What do you think about it?
> 
> I think all Andrey needs to do now is return -EINVAL if fsx_pad is not zero.
> 
> You can use this later to extend for the semantics of flags/fields mask
> and we can have a long discussion later on what this semantics should be.
> 
> Right?
> 
> Amir.

It is really enough? All new extensions later would have to be added
into fsx_pad fields, and currently unused bits in fsx_xflags would be
unusable for extensions.