[LTP] [bug?] clone(CLONE_IO) failing after kernel commit commit ef2c41cf38a7

[Florian Weimer <fw@deneb.enyo.de>]

* Christian Brauner:

> On Tue, May 05, 2020 at 11:36:36AM +0200, Florian Weimer wrote:
>> * Christian Brauner:
>> >> Have any flags been added recently?
>> >
>> > /* Flags for the clone3() syscall. */
>> > #define CLONE_CLEAR_SIGHAND 0x100000000ULL /* Clear any signal handler and reset to SIG_DFL. */
>> > #define CLONE_INTO_CGROUP 0x200000000ULL /* Clone into a specific cgroup given the right permissions. */
>> 
>> Are those flags expected to be compatible with the legacy clone
>> interface on 64-bit architectures?
>
> No, they are clone3() only. clone() is deprecated wrt to new features.

But without masking the clone_flags argument, won't it be passed down
to the implementation which is also used to back clone3?

> If I understood the original mail correctly, then the issue is caused by
> an interaction with sign extension and a the new flag value
> CLONE_INTO_CGROUP being defined.

Could be, but for that to happen, the flag would have to be passed
down, no?

> So from what I gather from Jan's initial mail is that when clone() is
> called on ppc64le with the CLONE_IO|SIGCHLD flag:
> clone(do_child, stack+1024*1024, CLONE_IO|SIGCHLD, NULL, NULL, NULL, NULL);
> that the sign extension causes bits to be set that raise the
> CLONE_INTO_CGROUP flag. And since the do_fork() codepath is the same for
> legacy clone() and clone3() the kernel will think that someone requested
> CLONE_INTO_CGROUP but hasn't passed a valid fd to a cgroup. If that is
> the only issue here then couldn't we just do:
>
> clone_flags &= ~CLONE3_ONLY_FLAGS?
>
> and move on, i.e. all future clone3() flags we'll just remove since we
> can assume that they have been accidently set. Even if they have been
> intentionally set we can just ignore them since that's in line with
> legacy clone()'s (questionable) tradition of ignoring unknown flags.
> Thoughts? Or am I missing some subtlety here?

What's the difference between

  clone_flags &= ~CLONE3_ONLY_FLAGS;

and

  clone_flags &= (unsigned) -1;

in this context?