Re: [PATCH v5 0/7] fs: introduce file_getattr and file_setattr syscalls

[Amir Goldstein <amir73il@gmail.com>]

On Thu, May 15, 2025 at 11:02 AM Christian Brauner <brauner@kernel.org> wrote:
>
> On Tue, May 13, 2025 at 11:53:23AM +0200, Arnd Bergmann wrote:
> > On Tue, May 13, 2025, at 11:17, Andrey Albershteyn wrote:
> >
> > >
> > >     long syscall(SYS_file_getattr, int dirfd, const char *pathname,
> > >             struct fsxattr *fsx, size_t size, unsigned int at_flags);
> > >     long syscall(SYS_file_setattr, int dirfd, const char *pathname,
> > >             struct fsxattr *fsx, size_t size, unsigned int at_flags);
> >
> > I don't think we can have both the "struct fsxattr" from the uapi
> > headers, and a variable size as an additional argument. I would
> > still prefer not having the extensible structure at all and just
>
> We're not going to add new interfaces that are fixed size unless for the
> very basic cases. I don't care if we're doing that somewhere else in the
> kernel but we're not doing that for vfs apis.
>
> > use fsxattr, but if you want to make it extensible in this way,
> > it should use a different structure (name). Otherwise adding
> > fields after fsx_pad[] would break the ioctl interface.
>
> Would that really be a problem? Just along the syscall simply add
> something like:
>
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index c91fd2b46a77..d3943805c4be 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -868,12 +868,6 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
>         case FS_IOC_SETFLAGS:
>                 return ioctl_setflags(filp, argp);
>
> -       case FS_IOC_FSGETXATTR:
> -               return ioctl_fsgetxattr(filp, argp);
> -
> -       case FS_IOC_FSSETXATTR:
> -               return ioctl_fssetxattr(filp, argp);
> -
>         case FS_IOC_GETFSUUID:
>                 return ioctl_getfsuuid(filp, argp);
>
> @@ -886,6 +880,20 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
>                 break;
>         }
>
> +       switch (_IOC_NR(cmd)) {
> +       case _IOC_NR(FS_IOC_FSGETXATTR):
> +               if (WARN_ON_ONCE(_IOC_TYPE(cmd) != _IOC_TYPE(FS_IOC_FSGETXATTR)))
> +                       return SOMETHING_SOMETHING;
> +               /* Only handle original size. */
> +               return ioctl_fsgetxattr(filp, argp);
> +
> +       case _IOC_NR(FFS_IOC_FSSETXATTR):
> +               if (WARN_ON_ONCE(_IOC_TYPE(cmd) != _IOC_TYPE(FFS_IOC_FSSETXATTR)))
> +                       return SOMETHING_SOMETHING;
> +               /* Only handle original size. */
> +               return ioctl_fssetxattr(filp, argp);
> +       }
> +

I think what Arnd means is that we will not be able to change struct
sfxattr in uapi
going forward, because we are not going to deprecate the ioctls and
certainly not
the XFS specific ioctl XFS_IOC_FSGETXATTRA.

This struct is part of XFS uapi:
https://man7.org/linux/man-pages/man2/ioctl_xfs_fsgetxattr.2.html

Should we will need to depart from this struct definition and we might
as well do it for the initial release of the syscall rather than later on, e.g.:

--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -148,6 +148,17 @@ struct fsxattr {
        unsigned char   fsx_pad[8];
 };

+/*
+ * Variable size structure for file_[sg]et_attr().
+ */
+struct fsx_fileattr {
+       __u32           fsx_xflags;     /* xflags field value (get/set) */
+       __u32           fsx_extsize;    /* extsize field value (get/set)*/
+       __u32           fsx_nextents;   /* nextents field value (get)   */
+       __u32           fsx_projid;     /* project identifier (get/set) */
+       __u32           fsx_cowextsize; /* CoW extsize field value (get/set)*/
+};
+
+#define FSXATTR_SIZE_VER0 20
+#define FSXATTR_SIZE_LATEST FSXATTR_SIZE_VER0
+

Right?

Thanks,
Amir.