Re: [PATCH v12 11/12] open: openat2(2) syscall

[Aleksa Sarai <cyphar@xxxxxxxxxx>]

On 2019-09-07, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Thu, 2019-09-05 at 06:19 +1000, Aleksa Sarai wrote:
> > + * @flags: O_* flags.
> > + * @mode: O_CREAT/O_TMPFILE file mode.
> > + * @upgrade_mask: UPGRADE_* flags (to restrict O_PATH re-opening).
> > + * @resolve: RESOLVE_* flags.
> > + */
> > +struct open_how {
> > +	__u32 flags;
> > +	union {
> > +		__u16 mode;
> > +		__u16 upgrade_mask;
> > +	};
> > +	__u16 resolve;
> > +};
> > +
> > +#define OPEN_HOW_SIZE_VER0	8 /* sizeof first published struct */
> > +
> 
> Hmm, there is no version field. When you want to expand this in the
> future, what is the plan? Add a new flag to indicate that it's some
> length?

The "version number" is the size of the struct. Any extensions we make
are appended to the struct (openat2 now takes a size_t argument), and
the new copy_struct_{to,from}_user() helpers handle all of the
permutations of {old,new} kernel and {old,new} user space.

This is how clone3(), sched_[gs]etattr() and perf_event_open() all
operate (all of the sigset syscalls operate similarly but don't
gracefully handle different kernel vintages -- you just get -EINVAL).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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