[v2] clone.2: note EINVAL when exit_signal + bad flags

  Document that Linux will report EINVAL when exit_signal is specified and
either CLONE_THREAD or CLONE_PARENT is specified.

From clone3_args_valid in Linux:
```
	if ((kargs->flags & (CLONE_THREAD | CLONE_PARENT)) &&
	    kargs->exit_signal)
		return false;
```

I have verified that this happens on my kernel with a small program:

```
#include <stdio.h>
#include <linux/sched.h>
#include <signal.h>
#include <sys/syscall.h>
#include <unistd.h>

int main() {
	struct clone_args ca = {
		.flags = CLONE_THREAD | CLONE_SIGHAND | CLONE_VM,
		.exit_signal = SIGCHLD, // comment me out to fix error
		.set_tid_size = 0,
	};
	syscall(SYS_clone3, &ca, sizeof(struct clone_args));
	perror("");
}
```

And I have verified that this doesn't happen with normal `clone` through
the glibc helper:

```
#define _GNU_SOURCE

#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <sys/mman.h>

int do_nothing(void *_) { return 0; }

int main() {
        void *map = mmap(NULL, 0x10000, PROT_READ | PROT_WRITE,
	                 MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
	void *stack_top = map + 0x10000 - 1;
	clone(do_nothing, stack_top,
	      CLONE_THREAD | CLONE_VM | CLONE_SIGHAND | SIGCHLD, NULL);
	perror("");
}
```

Signed-off-by: Jack Pearson <jack@pearson.onl>
---
 man2/clone.2 | 10 ++++++++++
 1 file changed, 10 insertions(+)
  

Hello,

In this patch, I included my test program for the absence of this behavior with
normal `clone` per Alex's request:

https://lore.kernel.org/linux-man/fba3de52-91cc-6cbe-b4ae-7140564e9ad2@pearson.onl/T/#mde63a642e9c8d0b4e367b0a2817248e8e0b29a50

Let me know if there's anything else I should do.

Thanks,
Jack

On 12/14/22 13:28, Jack Pearson wrote:
> Document that Linux will report EINVAL when exit_signal is specified and
> either CLONE_THREAD or CLONE_PARENT is specified.
> 
...
  

Hi Jack,

On 2/22/23 03:28, Jack Pearson wrote:
> Hello,
> 
> In this patch, I included my test program for the absence of this 
> behavior with
> normal `clone` per Alex's request:
> 
> https://lore.kernel.org/linux-man/fba3de52-91cc-6cbe-b4ae-7140564e9ad2@pearson.onl/T/#mde63a642e9c8d0b4e367b0a2817248e8e0b29a50
> 
> Let me know if there's anything else I should do.

Could you please resend the patch (keeping all CCs), and I'll have a 
look at it again?

Thanks for the ping,

Alex

> 
> Thanks,
> Jack
> 
> On 12/14/22 13:28, Jack Pearson wrote:
>> Document that Linux will report EINVAL when exit_signal is specified and
>> either CLONE_THREAD or CLONE_PARENT is specified.
>>
> ...

-- 
<http://www.alejandro-colomar.es/>
GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
  

Alright, done!

Thanks,

Jack

On 2/24/23 17:04, Alex Colomar wrote:
> Hi Jack,
> 
> On 2/22/23 03:28, Jack Pearson wrote:
>> Hello,
>>
>> In this patch, I included my test program for the absence of this behavior with
>> normal `clone` per Alex's request:
>>
>> https://lore.kernel.org/linux-man/fba3de52-91cc-6cbe-b4ae-7140564e9ad2@pearson.onl/T/#mde63a642e9c8d0b4e367b0a2817248e8e0b29a50
>>
>> Let me know if there's anything else I should do.
> 
> Could you please resend the patch (keeping all CCs), and I'll have a look at it again?
> 
> Thanks for the ping,
> 
> Alex
> 
>>
>> Thanks,
>> Jack
>>
>> On 12/14/22 13:28, Jack Pearson wrote:
>>> Document that Linux will report EINVAL when exit_signal is specified and
>>> either CLONE_THREAD or CLONE_PARENT is specified.
>>>
>> ...
>