mount: `acl` implementation is probably wrong

[AkihiroSuda]

runc/libcontainer/specconv/spec_linux.go

Line 69 in 32d7413

"acl": {false, unix.MS_POSIXACL},

Currently, runc maps the acl mount option strings to MS_POSIXACL, but this option doesn't seem used by util-linux: https://github.com/util-linux/util-linux/search?q=MS_POSIXACL

Surprisingly, MS_POSIXACL isn't even referred by the kernel, except in security/apparmor/mount.c:audit_mnt_flags(): https://elixir.bootlin.com/linux/v6.1.11/C/ident/MS_POSIXACL

Probably the acl string should be just passed to mount(2) via void *data.

[AkihiroSuda]

From torvalds/linux@caaef1b (kernel 5.8) :

vfs: don't parse "posixacl" option

Unlike the others, this is not a standard option accepted by mount(8).

In fact SB_POSIXACL is an internal flag, and accepting MS_POSIXACL on the
mount(2) interface is possibly a bug.

The only filesystem that apparently wants to handle the "posixacl" option
is 9p, but it has special handling of that option besides setting
SB_POSIXACL.