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Message-ID: <20250327192629.ivnarhlkfbhbzjcl@pali>
Date: Thu, 27 Mar 2025 20:26:29 +0100
From: Pali Rohár <pali@...nel.org>
To: Amir Goldstein <amir73il@...il.com>
Cc: Andrey Albershteyn <aalbersh@...hat.com>,
	Richard Henderson <richard.henderson@...aro.org>,
	Matt Turner <mattst88@...il.com>,
	Russell King <linux@...linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Michal Simek <monstr@...str.eu>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
	Helge Deller <deller@....de>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Naveen N Rao <naveen@...nel.org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Rich Felker <dalias@...c.org>,
	John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
	"David S. Miller" <davem@...emloft.net>,
	Andreas Larsson <andreas@...sler.com>,
	Andy Lutomirski <luto@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Mickaël Salaün <mic@...ikod.net>,
	Günther Noack <gnoack@...gle.com>,
	Arnd Bergmann <arnd@...db.de>, Paul Moore <paul@...l-moore.com>,
	James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>, linux-alpha@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
	linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
	linux-arch@...r.kernel.org, selinux@...r.kernel.org,
	Andrey Albershteyn <aalbersh@...nel.org>, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v4 0/3] fs: introduce getfsxattrat and setfsxattrat
 syscalls

On Thursday 27 March 2025 12:47:02 Amir Goldstein wrote:
> 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.

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.

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