Re: [PATCH v2 1/2] fork: add clone3

On Tue, Jun 04, 2019 at 10:28:12AM +0100, David Howells wrote:

Christian Brauner [off-list ref] wrote:

quoted

+#include <linux/compiler_types.h>

I suspect you don't want to include that directly.

Also, to avoid bloating linux/sched/task.h yet further, maybe put this in
linux/sched/clone.h?

Yeah, not the worst idea.
Though I'd leave the flags where they are and just add struct
kernel_clone_args in there. But I assume that's what you meant anyway.

quoted

-extern long _do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *, unsigned long);
+extern long _do_fork(struct kernel_clone_args *kargs);
 extern long do_fork(unsigned long, unsigned long, unsigned long, int __user *, int __user *);

Maybe these could move into linux/sched/clone.h too.

Meh, that could be a separate cleanup patch after clone3() has been
merged.

quoted

+#define CLONE_MAX ~0U

Can you add a comment summarising the meaning?

Yes, can do.

quoted

+	u64 clone_flags = args->flags;
+	int __user *child_tidptr = args->child_tid;
+	unsigned long tls = args->tls;
+	unsigned long stack_start = args->stack;
+	unsigned long stack_size = args->stack_size;

Some of these are only used once, so it's probably not worth sticking them in
local variables.

[1]:
Ok, will double check.
This was just to minimize copy-paste erros for variables which were used
multiple times.

quoted

-		if (clone_flags &
-		    (CLONE_DETACHED | CLONE_PARENT_SETTID | CLONE_THREAD))
-			return ERR_PTR(-EINVAL);

Did this error check get lost?  I can see part of it further on, but the check
on CLONE_PARENT_SETTID is absent.

No, it's only relevant for legacy clone() since it uses the
parent_tidptr argument to return the pidfd. clone3() has a dedicated
return argument for that in clone_args.
The check for legacy clone() is now done in legacy clone() directly.
copy_process() should only do generic checks for all version of
clone(),fork(),vfork(), etc.

quoted

+	int __user *parent_tidptr = args->parent_tid;

There's only one usage remaining after this patch, so a local var doesn't gain
a lot.

Yes, that leads back to [1].

quoted

 pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
 {
-	return _do_fork(flags|CLONE_VM|CLONE_UNTRACED, (unsigned long)fn,
-		(unsigned long)arg, NULL, NULL, 0);
+	struct kernel_clone_args args = {
+		.flags = ((flags | CLONE_VM | CLONE_UNTRACED) & ~CSIGNAL),
+		.exit_signal = (flags & CSIGNAL),

Kernel threads can have exit signals?

Yes,

kernel/kthread.c:       pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
kernel/umh.c:   pid = kernel_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD);

And even if they couldn't have. This is just to make sure that if they
ever would we'd be prepared.

quoted

+static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
+				     struct clone_args __user *uargs,
+				     size_t size)

I would make this "noinline".  If it gets inlined, local variable "args" may
still be on the stack when _do_fork() gets called.

Hm, can do.

Thanks!
Christian