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

[Alejandro Colomar]

Hi Jack,

On 3/1/23 00:42, Jack Pearson wrote:
> 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(+)

Patch applied.  Thanks!

Alex

> 
> diff --git a/man2/clone.2 b/man2/clone.2
> index d63895189..be802a280 100644
> --- a/man2/clone.2
> +++ b/man2/clone.2
> @@ -1436,6 +1436,16 @@ One of the PIDs specified in
>  .I set_tid
>  was an invalid.
>  .TP
> +.BR EINVAL " (" clone3 "() only)"
> +.\" commit 7f192e3cd316ba58c88dfa26796cf77789dd9872
> +.B CLONE_THREAD
> +or
> +.B CLONE_PARENT
> +was specified in the
> +.I flags
> +mask, but a signal was specified in
> +.I exit_signal.
> +.TP
>  .BR EINVAL " (AArch64 only, Linux 4.6 and earlier)"
>  .I stack
>  was not aligned to a 128-bit boundary.

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