Re: [PATCH v2 19/21] fuse: add passthrough setattr

On Fri, May 15, 2026 at 5:53 PM Joanne Koong <[email protected]> wrote:
>
> Add passthrough setattr which sets attributes directly on the backing
> inode through backing_inode_setattr() instead of sending FUSE_SETATTR to
> the server.
>
> Passthrough setattr is checked before the
> handle_killpriv/handle_killpriv_v2 suid/sgid stripping because the
> stripping is handled natively by notify_change() on the backing inode.
>
> Signed-off-by: Joanne Koong <[email protected]>
> ---
>  fs/fuse/dir.c             |  3 +++
>  fs/fuse/fuse_i.h          |  3 ++-
>  fs/fuse/passthrough.c     | 28 ++++++++++++++++++++++++++++
>  include/uapi/linux/fuse.h |  1 +
>  4 files changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index 4c7e3e1604af..b67b3b334e69 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -2506,6 +2506,9 @@ static int fuse_setattr(struct mnt_idmap *idmap, struct dentry *entry,
>         if (!fuse_allow_current_process(get_fuse_conn(inode)))
>                 return -EACCES;
>
> +       if (fuse_passthrough_op(inode, FUSE_SETATTR))
> +               return fuse_passthrough_setattr(entry, attr);

In the v1 discussion [1], there was a suggestion about gating
FUSE_PASSTHROUGH_INO on FUSE_HANDLE_KILLPRIV_V2 to avoid the
non-atomic killpriv path, but I think we were only encountering the
non-atomic killpriv path because I was calling
fuse_passthrough_setattr() in the wrong place. I think we can avoid
this altogether by just having the backing filesystem handle all of
the suid/sgid stripping atomically through its own notify_change()
path, which will let us skip all the fuse killpriv handling logic in
this function.

Thanks,
Joanne

[1] https://lore.kernel.org/fuse-devel/CAOQ4uxipQJt5zskb_THxueGi_MXpFdywFiGpd_nmWeY_sMHwzQ@mail.gmail.com/

> +
>         if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) {
>                 attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID |
>                                     ATTR_MODE);