Re: git: a4e4132fa3bf - main - swapoff(2): replace special device name argument with a structure

On Sat, Dec 04, 2021 at 10:21:07PM +0000, Konstantin Belousov wrote:
> The branch main has been updated by kib:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=a4e4132fa3bfadb6047fc0fa5f399f4640460300
> 
> commit a4e4132fa3bfadb6047fc0fa5f399f4640460300
> Author:     Konstantin Belousov <kib@FreeBSD.org>
> AuthorDate: 2021-11-29 16:26:31 +0000
> Commit:     Konstantin Belousov <kib@FreeBSD.org>
> CommitDate: 2021-12-04 22:20:58 +0000
> 
>     swapoff(2): replace special device name argument with a structure
>     
>     For compatibility, add a placeholder pointer to the start of the
>     added struct swapoff_new_args, and use it to distinguish old vs. new
>     style of syscall invocation.

I agree with Jess that this should be a new syscall.  The entry in
sycalls.master now fails to describe the memory footprint of the name
argument.  No system call should be created or altered to have a memory
footprint not describable with SAL annotations unless an applicable
standard such as POSIX requires it.

> diff --git a/sys/vm/swap_pager.h b/sys/vm/swap_pager.h
> index 395fbc9957c4..469de3e8eaf4 100644
> --- a/sys/vm/swap_pager.h
> +++ b/sys/vm/swap_pager.h
> @@ -69,6 +69,14 @@ struct swdevt {
>  #define	SW_UNMAPPED	0x01
>  #define	SW_CLOSING	0x04
>  
> +struct swapoff_new_args {
> +	const char *name_old_syscall;
> +	const char *name;
> +	u_int flags;
> +	u_int pad0;
> +	uintptr_t pad1[8];
> +};

If you're going to attempt to add future-proofing, please pad with the
assumption that pointers are 128-bit sized and aligned.  In this
case, that would mean an uint64_t pad before pad1.  If there were done
in place, adding the pad and dropping pad1 to 6 elements would be safe.

-- Brooks