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fuse: propagate SB_POSIXACL to submounts

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fuse_fill_super_common() always sets SB_POSIXACL on the superblock, but
fuse_fill_super_submount() never does. Submount superblocks are created
from an fs_context with no sb_flags, so the superblock of an automounted
FUSE_ATTR_SUBMOUNT directory (e.g. a virtiofs export containing another
host mount) lacks SB_POSIXACL, even though it shares the fuse_conn, and
thus fc->posix_acl, with its parent.

Since IS_POSIXACL() is false on such a submount, the VFS:

 - fails setting POSIX ACLs with -EOPNOTSUPP and hides existing ACLs,
   without sending any request to the server;
 - applies the umask itself on create, so default ACLs on the server are
   not honoured for new files;
 - skips ACL checks in acl_permission_check(), so with
   default_permissions (implied by FUSE_POSIX_ACL) access is decided by
   the mode bits alone and restrictive ACL entries are not enforced.

Copy SB_POSIXACL from the parent superblock, as is already done for the
other superblock fields inherited from it. It cannot be set in
fuse_sb_defaults(), because fuse_fill_super_common() checks whether the
flag was passed at mount time to decide fc->dont_mask before setting it.

Reproducer, with virtiofsd 1.10.0 started with --posix-acl and an export
containing a nested mount:

  host# mount -t tmpfs none /srv/share
  host# mkdir /srv/share/child
  host# mount -t tmpfs none /srv/share/child
  host# touch /srv/share/rootfile /srv/share/child/childfile
  guest# mount -t virtiofs <tag> /mnt
  guest# setfacl -m u:12345:rw /mnt/rootfile           # works
  guest# setfacl -m u:12345:rw /mnt/child/childfile
  setfacl: /mnt/child/childfile: Operation not supported

With this patch the second setfacl succeeds, ACLs set on the host are
reported by getfacl on the submount, and default ACLs and ACL-based
access checks behave the same as on the root mount. Tested on v7.3-rc5
in a QEMU guest.

Signed-off-by: Tarun Khandelwal <tarun@supabase.io>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
Tarun Khandelwal 2026-09-29 15:18:12 +05:30 committed by Miklos Szeredi
parent b776cbe24c
commit ee8f6dfccb
1 changed files with 2 additions and 0 deletions
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2
fs/fuse/inode.c
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@ -1656,6 +1656,8 @@ static int fuse_fill_super_submount(struct super_block *sb,
WARN_ON(sb->s_bdi != &noop_backing_dev_info);
sb->s_bdi = bdi_get(parent_sb->s_bdi);
sb->s_flags |= parent_sb->s_flags & SB_POSIXACL;
sb->s_xattr = parent_sb->s_xattr;
sb->s_export_op = parent_sb->s_export_op;
sb->s_time_gran = parent_sb->s_time_gran;