Re: [PATCH] [GLIBC RFC] clone3: add CLONE3_RESET_SIGHAND

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Oct 09, 2019 at 01:04:03PM +0200, Florian Weimer wrote:
> * Christian Brauner:
> 
> > I've been thinking about two things how to do this:
> > - mask the flags that the kernel does not support
> 
> That doesn't look fully backwards-compatible to me.  The argument isn't
> currently read/write, is it?  It would work for us though.
> 
> > - add another argument to struct clone_args that is "known_flags"
> >   when the syscall returns it'll be set to all the flags this kernel
> >   knows about
> 
> This needs some sort of protocol to detect whether the argument was
> updated.  I suppose we could define CLONE3_INITIALLY_SUPPORTED_FLAGS
> with all the flag bits currently supported and tell developers to
> initialize struct clone_args with:
> 
>   .known_flags = CLONE3_INITIALLY_SUPPORTED_FLAGS,

That won't work. Older kernels will verify that parts of the struct that
are not known are set to 0.
I wonder, what is stopping you from

struct clone args args = {
	.known_flags = 0,
};

pid_t pid = clone3(&args, sizeof(args));
if (pid < 0)
	return -1;

######### kernel code ############
/* on a kernel that is aware of known_flags */
kargs->known_flags = CLONE3_SUPPORTED_FLAGS;
##################################

if (!args.known_flags)
	/* kernel doesn't not support the known_flags extension */

if (args.known_flags & NEW_FLAG_I_CARE_ABOUT)
	/* 
	 * kernel does support the known_flags extension and does
	 * support the feature I care about 
	 */

> 
> Then the result would be correct whether or not known_flags is supported
> by the kernel or not.  This too would work fine for glibc internal use.
> 
> By theway, I don't think we have a good userspace API story for
> extensible *output* arguments yet.  Every system call does things a
> little bit differently there.

Yeah, I'm slowly pushing in this direction but you're right it is nasty
currently... Traditionally syscalls mix and match output arguments in a
single struct. I've honestly have seen now need for a separate output
struct for clone3(). But yeah, we should probably standardize this. We
should have a session somewhere or a mailing list discussion at least.
Though I'd prefer the in-person discussion.

Thanks!
Christian