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

[Pali Rohár]

On Thursday 27 March 2025 21:57:34 Amir Goldstein wrote:
> On Thu, Mar 27, 2025 at 8:26 PM Pali Rohár <pali@xxxxxxxxxx> wrote:
> >
> > On Thursday 27 March 2025 12:47:02 Amir Goldstein wrote:
> > > On Sun, Mar 23, 2025 at 11:32 AM Pali Rohár <pali@xxxxxxxxxx> wrote:
> > > >
> > > > On Sunday 23 March 2025 09:45:06 Amir Goldstein wrote:
> > > > > On Fri, Mar 21, 2025 at 8:50 PM Andrey Albershteyn <aalbersh@xxxxxxxxxx> 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?
> 
> I don't know. Let's see...
> 
> > 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.
> 
> I am working under the assumption that the first extension would be
> to support fsx_xflags_mask and from there, you could add filesystem
> flags support checks and then new flags. Am I wrong?
> 
> Obviously, fsx_xflags_mask would be taken from fsx_pad space.
> After that extension is implemented, calling SYS_setfsxattrat() with
> a zero fsx_xflags_mask would be silly for programs that do not do
> the legacy get+set.
> 
> So when we introduce  fsx_xflags_mask, we could say that a value
> of zero means that the mask is not being checked at all and unknown
> flags in set syscall are ignored (a.k.a legacy ioctl behavior).
> 
> Programs that actually want to try and set without get will have to set
> a non zero fsx_xflags_mask to do something useful.

Here we need to also solve the problem that without GET call we do not
have valid values for fsx_extsize, fsx_projid, and fsx_cowextsize. So
maybe we would need some flag in fsx_pad that fsx_extsize, fsx_projid,
or fsx_cowextsize are ignored/masked.

> I don't think this is great.
> I would rather that the first version of syscalls will require the mask
> and will always enforce filesystems supported flags.

It is not great... But what about this? In a first step (part of this
syscall patch series) would be just a check that fsx_pad is zero.
Non-zero will return -EINVAL.

In next changes would added fsx_filter bit field, which for each
fsx_xflags and also for fsx_extsize, fsx_projid, and fsx_cowextsize
fields would add a new bit flag which would say (when SET) that the
particular thing has to be ignored.

So when fsx_pad is all-zeros then fsx_filter (first field in fsx_pad)
would say that nothing in fsx_xflags, fsx_extsize, fsx_projid, and
fsx_cowextsize is ignored, and hence behave like before.

And when something in fsx_pad/fsx_filter is set then it says which
fields are ignored/filtered-out.

> If you can get those patches (on top of current series) posted and
> reviewed in time for the next merge window, including consensus
> on the actual semantics, that would be the best IMO.

I think that this starting to be more complicated to rebase my patches
in a way that they do not affect IOCTL path but implement it properly
for new syscall path. It does not sounds like a trivial thing which I
would finish in merge window time and having proper review and consensus
on this.

> But I am just preparing a plan B in case you do not have time to
> work on the patches or if consensus on the API extensions is not
> reached on time.
> 
> I think that for plan B, the minimum is to verify zero pad field and
> that is something that this syscall has to do anyway, because this
> is the way that backward compact APIs work.
> 
> If you want the syscall to always return -EINVAL for setting xflags
> that are currently undefined I agree that would be nice as well.
> 
> Thanks,
> Amir.