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Message-ID: <faqun3wrpvwrhwukql3niqvvauy5ngrpytx5bxbrv5xkounez3@m7j2znjuzapu>
Date: Thu, 27 Mar 2025 10:33:47 +0100
From: Andrey Albershteyn <aalbersh@...hat.com>
To: Amir Goldstein <amir73il@...il.com>
Cc: 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>, Pali Rohár <pali@...nel.org>, 
	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, 
	linux-xfs@...r.kernel.org
Subject: Re: [PATCH v4 3/3] fs: introduce getfsxattrat and setfsxattrat
 syscalls

On 2025-03-23 09:56:25, Amir Goldstein wrote:
> On Fri, Mar 21, 2025 at 8:49 PM Andrey Albershteyn <aalbersh@...hat.com> wrote:
> >
> > From: Andrey Albershteyn <aalbersh@...hat.com>
> >
> > Introduce getfsxattrat and setfsxattrat syscalls to manipulate inode
> > extended attributes/flags. The syscalls take parent directory fd and
> > path to the child together with struct fsxattr.
> >
> > This is an alternative to FS_IOC_FSSETXATTR ioctl with a difference
> > that file don't need to be open as we can reference it with a path
> > instead of fd. By having this we can manipulated inode extended
> > attributes not only on regular files but also on special ones. This
> > is not possible with FS_IOC_FSSETXATTR ioctl as with special files
> > we can not call ioctl() directly on the filesystem inode using fd.
> >
> > This patch adds two new syscalls which allows userspace to get/set
> > extended inode attributes on special files by using parent directory
> > and a path - *at() like syscall.
> >
> > CC: linux-api@...r.kernel.org
> > CC: linux-fsdevel@...r.kernel.org
> > CC: linux-xfs@...r.kernel.org
> > Signed-off-by: Andrey Albershteyn <aalbersh@...hat.com>
> > Acked-by: Arnd Bergmann <arnd@...db.de>
> > ---
> ...
> > +SYSCALL_DEFINE5(setfsxattrat, int, dfd, const char __user *, filename,
> > +               struct fsxattr __user *, ufsx, size_t, usize,
> > +               unsigned int, at_flags)
> > +{
> > +       struct fileattr fa;
> > +       struct path filepath;
> > +       int error;
> > +       unsigned int lookup_flags = 0;
> > +       struct filename *name;
> > +       struct mnt_idmap *idmap;.
> 
> > +       struct dentry *dentry;
> > +       struct vfsmount *mnt;
> > +       struct fsxattr fsx = {};
> > +
> > +       BUILD_BUG_ON(sizeof(struct fsxattr) < FSXATTR_SIZE_VER0);
> > +       BUILD_BUG_ON(sizeof(struct fsxattr) != FSXATTR_SIZE_LATEST);
> > +
> > +       if ((at_flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
> > +               return -EINVAL;
> > +
> > +       if (!(at_flags & AT_SYMLINK_NOFOLLOW))
> > +               lookup_flags |= LOOKUP_FOLLOW;
> > +
> > +       if (at_flags & AT_EMPTY_PATH)
> > +               lookup_flags |= LOOKUP_EMPTY;
> > +
> > +       if (usize > PAGE_SIZE)
> > +               return -E2BIG;
> > +
> > +       if (usize < FSXATTR_SIZE_VER0)
> > +               return -EINVAL;
> > +
> > +       error = copy_struct_from_user(&fsx, sizeof(struct fsxattr), ufsx, usize);
> > +       if (error)
> > +               return error;
> > +
> > +       fsxattr_to_fileattr(&fsx, &fa);
> > +
> > +       name = getname_maybe_null(filename, at_flags);
> > +       if (!name) {
> > +               CLASS(fd, f)(dfd);
> > +
> > +               if (fd_empty(f))
> > +                       return -EBADF;
> > +
> > +               idmap = file_mnt_idmap(fd_file(f));
> > +               dentry = file_dentry(fd_file(f));
> > +               mnt = fd_file(f)->f_path.mnt;
> > +       } else {
> > +               error = filename_lookup(dfd, name, lookup_flags, &filepath,
> > +                                       NULL);
> > +               if (error)
> > +                       return error;
> > +
> > +               idmap = mnt_idmap(filepath.mnt);
> > +               dentry = filepath.dentry;
> > +               mnt = filepath.mnt;
> > +       }
> > +
> > +       error = mnt_want_write(mnt);
> > +       if (!error) {
> > +               error = vfs_fileattr_set(idmap, dentry, &fa);
> > +               if (error == -ENOIOCTLCMD)
> > +                       error = -EOPNOTSUPP;
> 
> This is awkward.
> vfs_fileattr_set() should return -EOPNOTSUPP.
> ioctl_setflags() could maybe convert it to -ENOIOCTLCMD,
> but looking at similar cases ioctl_fiemap(), ioctl_fsfreeze() the
> ioctl returns -EOPNOTSUPP.
> 
> I don't think it is necessarily a bad idea to start returning
>  -EOPNOTSUPP instead of -ENOIOCTLCMD for the ioctl
> because that really reflects the fact that the ioctl is now implemented
> in vfs and not in the specific fs.
> 
> and I think it would not be a bad idea at all to make that change
> together with the merge of the syscalls as a sort of hint to userspace
> that uses the ioctl, that the sycalls API exists.
> 
> Thanks,
> Amir.
> 

Hmm, not sure what you're suggesting here. I see it as:
- get/setfsxattrat should return EOPNOTSUPP as it make more sense
  than ENOIOCTLCMD
- ioctl_setflags returns ENOIOCTLCMD which also expected

Don't really see a reason to change what vfs_fileattr_set() returns
and then copying this if() to other places or start returning
EOPNOTSUPP.

-- 
- Andrey


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