Message-ID: <CAOQ4uxiTKhGs1H-w1Hv-+MqY284m92Pvxfem0iWO+8THdzGvuA@mail.gmail.com>
Date: Thu, 27 Mar 2025 12:47:02 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Pali Rohár <pali@...nel.org>
Cc: Andrey Albershteyn <aalbersh@...hat.com>, Richard Henderson <richard.henderson@...aro.org>, 
	Matt Turner <mattst88@...il.com>, Russell King <linux@...linux.org.uk>, 
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	Andrey Albershteyn <aalbersh@...nel.org>, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v4 0/3] fs: introduce getfsxattrat and setfsxattrat syscalls

On Sun, Mar 23, 2025 at 11:32 AM Pali Rohár <pali@...nel.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@...hat.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.

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